HomeMy WebLinkAboutNantucket WQ 2020 Report_Final revised 1
Technical Memorandum
FINAL
Water Quality Monitoring and Assessment of the
Nantucket Island-Wide Estuaries and Salt Ponds
Update 2020
To:
Town of Nantucket
Marine and Natural Resources Department
2 Bathing Beach Road
Nantucket, MA 02554
From:
Brian Howes Ph.D. and Roland Samimy Ph.D.
Coastal Systems Program
School of Marine Science and Technology (SMAST)
University of Massachusetts-Dartmouth
706 South Rodney French Blvd.
New Bedford, MA 02744
April 2021
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Executive Summary
Water Quality Monitoring and Assessment of the
Nantucket Island-Wide Estuaries and Salt Ponds
Update 2020
As in previous years, this annual Technical Memorandum summarizes the water quality
monitoring results of the estuarine sampling undertaken through the ongoing Nantucket Island-
wide Water Quality Monitoring Program. The program is a multi-year collaboration between the
Town of Nantucket Natural Resources Department and the Coastal Systems Program within the
University of Massachusetts-Dartmouth, School for Marine Science and Technology. Water
sample collection and analysis has been undertaken according to specific protocols developed at
the outset of the monitoring first started in 2010 such that year to year results would be cross
comparable from one Nantucket estuary to another but also, so that Nantucket results would
also be cross comparable to water quality data collected from other programs managed by the
Coastal Systems Program (e.g. Cape Cod monitoring programs as well as Martha's Vineyard
Island-wide monitoring program). The 2020 summary memo is focused specifically on the
following: 1) Water Quality Results for Nantucket Harbor, Madaket Harbor, Long Pond,
Hummock Pond, Miacomet Pond, Sesachacha Pond, Polpis Harbor Streams and Oyster
Aquaculture Potential Sites and 2) Trophic State - Water Quality/Eutrophication Status and
trends.
The goals of the monitoring program remain unchanged from previous years, primarily to:
1. determine the present (2020) ecological health of each of the main salt ponds and
estuaries within the Town of Nantucket,
2. gauge (as historical data allows, 2010-2020) the decline or recovery of various salt ponds
and embayments over the long-term (also part of TMDL compliance), and
3. provide the foundation (and context) for development of potential alternatives for nutrient
and resource management and quantitative measures of success.
Point (3) is critical for restoration planning should a system be found to be impaired or
trending toward impairment, which requires targeted management actions for restoration
or a system is improving due to Town actions which then should be improved/enhanced.
As was the case in 2010, 2012 - 2019 sampling efforts, the 2020 sampling program focused on
the summer/early fall months (June - September, few systems sampled in October) as this time
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frame is typically representative of worst case water quality conditions and the basis for habitat
management decisions. Samples collected in 2020 were obtained from the same sampling
station locations and the same depths as in previous years to maximize cross comparability and
to gauge temporal changes. It should be noted that the Town of Nantucket did undertake water
quality monitoring in 2011, however, those samples were analyzed by a lab other than the
Coastal Systems Analytical Facility at the UMASS School for Marine Science and Technology
and data was presented in the report on the 2012 monitoring effort.
The Town of Nantucket has been working for decades to protect and more recently restore its
estuaries and their aquatic resources. At present and based on the 2010 - 2020 water quality
database developed under the Island-wide unified water quality monitoring program, activities to
lower nitrogen enrichment and its negative impacts to water and habitat quality have been
planned and implementation is ongoing in 4 estuarine systems: Nantucket Harbor (jetties and
sewers), Madaket/Long Pond (landfill and possible dredging), Sesachacha Pond (openings),
Hummock Pond (refined opening protocol). In addition, all of the Town’s estuaries should benefit
from the recent fertilizer application by-law. As a result, it is anticipated that the efficacy of these
management activities should be seen in the on-going monitoring results.
Summer 2020 appears to have sustained high water quality for the Nantucket Harbor and
Madaket Harbor systems. Hummock Pond water quality appears to continue to be poor in the
upper portions of the system, however, 2020 data indicates slightly lower nutrient concentrations
and overall system health compared to 2019 (possibly due to a more effective opening).
Generally, the decline in water quality started in 2016 which showed a decrease in overall water
quality compared to system reset for the 2015 summer season. The reset was accomplished
through a highly successful opening, which has not been fully repeated at that level since 2016,
hence the decline in water quality over the past 5 years. The variations in TN levels results
primarily from opening success. For instance, in spring 2017 the opening was only 1.5 days with
high precipitation, although the spring opening in 2016 was comparable to the duration of the
opening in 2015 (18 days and 15 days respectively). In contrast, Long Pond has continued to
show declines in TN concentrations over historic levels with variable parallel improvements in
water and habitat quality, although the inter-annual reductions appear to be stabilizing.
Miacomet Pond, which is now functionally a eutrophic freshwater pond, again in 2020 showed
poor water and habitat quality throughout.. After a slight improvement in 2017, as seen in lower
TN concentrations and lower total pigments (CHLA + Pheophytin), water quality worsened in
2018 and continued to show a nutrient degradation in 2019 and 2020. Miacomet Pond from
2014-2018 appeared to have phytoplankton production (e.g. chlorophyll-a) stimulated by both N
and P inputs as pond salinities declined to present freshwater levels. The 2020 monitoring
indicated that Miacomet Pond still has phytoplankton production stimulated by both N and P as
DIN/DIP ratios at certain stations exceed the Redfield Ratio. The longer term record indicates
both N and P as key nutrients from 2014-2020, with nitrogen being the key nutrient only in 2019.
Miacomet Pond exceeds by several fold both known N and P levels needed to maintain a high
quality aquatic system.
Water Quality in Sesachacha Pond appears to be directly related to the efficacy of its seasonal
openings. Water quality in 2010, 2012, 2013 was significantly improved over the levels observed
previously in the MEP analysis (November 2006). The lower TN levels in Sesachacha Pond
(2010-2013) versus historic levels documented by the MEP indicated significant improvement of
pond resources. Since there was no major shift in nitrogen loading within the Sesachacha Pond
watershed during that period of improvement, it is almost certain that the amount of tidal flushing
during a given artificial breaching is driving the variability in the observed summer TN level. The
rise in TN observed during the 2014 sampling suggests that a poor inlet opening may have
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occurred in spring 2014, as watershed loading was unchanged. Short openings in subsequent
years (2015 {5 days}, 2016 {3 days}, 2017 {7 days}) suggests that inlet opening efficacy has a
significant effect on pond water quality as TN concentrations from 2014 to 2017 have steadily
increased to nearly the same level as when the MEP analysis was completed. Since a focused
opening management plan has evolved over the past decade, TN levels have generally declined
in recent years and averaged 0.784 mg TN/L over 2018 and 2019 field seasons (Table 4). In
2020, TN concentrations were even lower and chlorophyll levels appear to be as low
observations in 2013. Although Sesachacha Pond is still above its N threshold the increased
attention to pond openings through 2018 have lowered TN from historic levels (1.2 mg N/L) to
0.7 mg N/L in 2020, only slightly above the threshold TN level. Since a fall opening in 2020 did
occur its impact will be seen in the 2021 monitoring results. This underscores that if opening can
be maintained the threshold N level should be attainable for this estuary.
Trophic State of Nantucket Estuaries: The Trophic State of an estuary is a quantitative
indicator of its nutrient related ecological health and is based on key ecological metrics:
concentrations of inorganic and organic Nitrogen, water clarity (Secchi Depth), lowest measured
concentrations of Dissolved Oxygen (average of lowest 20% of summer measurements), and
Chlorophyll-a pigments (surrogate for phytoplankton biomass/blooms). Nutrient related trophic
health scales generally range from Oligotrophic (healthy-low nutrient) to Mesotrophic (showing
some signs of deterioration of health due to nutrient enrichment) to Eutrophic (habitats
significantly impaired and degraded, high levels of nutrients and organic matter and community
shifts).
Nantucket Harbor (with Madaket Harbor) is presently supporting the highest water
quality of Nantucket's estuaries. In 2018 and 2019, all of the Nantucket and Polpis
Harbor monitoring stations were showing high water quality, slightly better than in 2017.
However, in 2020 there appears to have been further decline in Polpis Harbor East (Nan-
5) possibly due to inflow from the headwater stream. Over the past 5 years, the main
basin of Nantucket Harbor continues to support high quality waters, with only a periodic
small decline in the uppermost basin, Wauwinet basin. Summer 2019 saw a slight
decline in water quality that continued into 2020. Previous summers 2016-2018 generally
showed similar high water quality in this basin as 2014 which also showed improved
chlorophyll-a and TN levels versus prior years. Nutrient management activities
associated with the Harbor (additional sewer hookups, jetty improvement and oyster
aquaculture) should result in even lower TN and chlorophyll levels and reduce the
likelihood of large phytoplankton blooms in the future.
Polpis Harbor basins which after showing moderate impairment in 2010 and 2012 have been
showing only low to no impairment in recent years, suggesting some improvement over historic
conditions. Polpis Harbor showed 2018 & 2017 TN levels similar to 2010, 2012 and 2015,
slightly higher than 2013 and 2014. However, Polpis Harbor West Basin showed elevated TN in
2020 and a decline to moderate water quality, while the East Basin maintained its low TN and
high water quality. This variation makes continued monitoring essential to clarify any trends in
water quality and linkages to stream nitrogen discharges. While the overall Nantucket Harbor
System is generally supporting high quality waters, the variability in the index in Wauwinet and
Polpis basins should be monitored to ascertain their long-term health and to determine the
effectiveness of restoration effort by the Town as it continues to move forward to meet the
MassDEP TMDL for this system. Overall, Nantucket Harbor appears to be relatively stable from
year to year with a continuing improvement into 2020 with high index scores and higher level
metrics that support the contention that it is approaching its TMDL threshold, as is generally also
the case for Polpis Harbor. The role of stream inputs to Polpis Harbor West on its recent water
quality may suggest that some mitigation is needed. Planned sewering within the Nantucket
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watershed should complete the restoration on nitrogen mitigated impairment within this estuarine
system.
Madaket Harbor main basin in 2020 continued to support a high level of nutrient related water
quality, with High Quality waters at all sites over the past 3 years. It has been the more enclosed
basins of Hither Creek and Long Pond with their reduced tidal flushing that have had nitrogen
impairment problems. Over the 10 years of monitoring, Hither Creek (Station 1), which receives
discharge from Long Pond via Madaket Ditch, has consistently supported the poorest “health”
status within the Madaket Estuary. Hither Creek is clearly nitrogen enriched and showing
continuing impairment based on a variety of parameters, but has clearly improved since 2010
and has shown only moderately impaired water quality over the past 3 years. Over the past 10
years the Trophic Index indicates that this basin has been improving gradually, going from fair-
poor water quality and improving to moderate water quality since 2014 with the exception of
2017 where TN levels rose, but stayed within the range that has developed over the past 4-5
years. The main basin of Madaket Harbor is showing relatively high water quality in each year
but also shows a possible improvement from the 2010 to 2012 period and has been generally
stable with high quality unimpaired waters in more recent samplings to 2018-2020.
Long Pond is a large tributary basin to Madaket Harbor, which receives tidal flow
through the artificial connection of Madaket Ditch. Given the structure of the basin and its
watershed, Long Pond operates semi-independently from Madaket Harbor. Long Pond’s
Trophic Index scores for both monitoring stations (5 & 6) in the 10 years of monitoring
(2010, 2012-2020) clearly indicate poor nutrient related water quality. However, based
upon the 2015-2020 results and the overall 10 year time-series, it appears that the
Town’s management of the landfill, has reduced the nitrogen load from this source with
the associated observed lowering of TN levels. The water quality Index for the most
recen 6 years is showing improvement at both Long Pond Stations and the receiving
waters in Hither Creek in response to the lower TN levels. However, the basin remains
moderately impaired. While continued monitoring will determine the final level of
improvement, it does appear that the reduction in N loading is occurring with beneficial
effects. It should be noted that the lack of major change in the Health Index for Long
Pond results in part from the relative coarseness of the Index, where sometimes large
index score changes are required to change the Index value. The analysis of key metrics
(Chlorophyll-a, water clarity-Secchi and total nitrogen) individually do show improving
water quality at stations 5 and 6 (particularly at Station 6 nearest the landfill) in 2012-
2020 compared to 2010, particularly striking are the declining TN levels. A recent Town
supported benthic animal analysis parallels and underscores the water quality
improvements.
Sesachacha Pond is a closed coastal salt pond that has its water quality managed by
periodically breaching the barrier beach to open the basin to tidal exchange with the
adjacent Atlantic Ocean waters. Sesachacha Pond was evaluated under the
Massachusetts Estuaries Project (MEP) which recommended an additional mid-
summertime opening (if logistically possible) as part of the pond management strategy to
enhance flushing of the pond and improve water quality to reach the threshold without
any need for infrastructure. The water quality monitoring program in 2010, 2012 and
2013 showed that the pond nitrogen levels were converging on the MEP Threshold (0.60
mg/L total nitrogen). Improved openings resulted in total nitrogen (TN) levels dropping
significantly from historical levels in 2010 and 2012 with associated improvements in
water clarity and the level of phytoplankton biomass as chlorophyll-a. However, with
limited openings in 2014-2017, TN has risen and has returned to near historic levels
(~0.9 mg/L), with an improvement in 2018 (0.752 mg N/L), and with continuing impaired
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conditions in 2019 but with a significant drop in 2020 (<0.68 mg N/L). It appears that
these changes relate primarily to the quality of the pre-summer opening. Given the 2010-
2013 period and recently when robust openings occurred, it appears that a solid opening
program has the capability to improve the water quality metrics pond-wide to levels near
the TMDL nitrogen threshold.
Based upon the Index alone, changes in water quality in Sesachacha Pond over the 2010-2013
period were stabilized at moderate impairment of this estuary, with more recent monitoring 2015-
2017 seeing a trend toward poor water quality conditions. However, in 2020 water quality was
less impaired than in the prior 10 years, but will likely show continued interannual variation, until
an opening protocol can be perfected (which the Town if working on). It appears that like other
periodically opened ponds, the quality of the opening (amount of water exchanged) controls the
level of water quality in the following months. A closer examination of the opening protocol and
the linkage to resultant water quality is needed for management of this system.
Hummock Pond is a closed coastal salt pond that is only periodically opened to the
ocean to flush out nutrients and organic matter on the ebb tide and receive low nutrient
saline waters on the flood tide. Creating sustained openings that are sufficient to allow
exchange of tidal waters for more than 4-5 days has been difficult for this system due to
its location on the coast and the large amount of sand migration in the coastal zone which
can rapidly reseal the inlet.
The present non-tidal state and level of watershed nutrient inputs have resulted in
moderate to poor nutrient related water quality throughout the pond, with poor water
quality conditions the present norm (2005-2007, 2010, 2012, 2013, 2014, 2015).
Unfortunately, in 2016 the pond appears to have had lower water quality in its upper and
mid reaches than in previous years, although the lower basin did not show this inter-
annual variation. Similarly, in 2017 and 2018 water quality declined even further
throughout most of the estuary, including the lower basin. This resulted from the poor
spring 2017 and 2018 openings. There is generally a small gradient in water quality with
moderate to poor conditions near the ocean and poor conditions in the uppermost basins,
but this collapsed to generally poor water quality throughout the system in 2017, 2018
and 2019. However, in 2020, while still moderately impaired water quality was found, the
Health Index showed the best water quality in all but the upper basin over the monitoring
period. None-the-less, all of the metrics are consistent with a nutrient impaired basin in
all years. Based upon the monitoring results it is clear that the nutrient related health of
Hummock Pond is significantly related to the success of its periodic openings and that
the pond is presently significantly impaired by nitrogen enrichment.
Miacomet Pond is a closed coastal salt/fresh pond that has not experienced significant tidal
exchange for over a decade. The result has been a build-up of nutrients and organic matter
resulting in a highly eutrophic system. As a result of the lack of tidal flow and continuous
groundwater inputs, the pond is presently freshwater, with salinity levels in each of the past 8
years of monitoring of <0.5 ppt, reaching a low of 0.1 ppt, 2015-2017. The present non-tidal
state and extent of watershed nutrient inputs has resulted in a decline in nutrient related water
and habitat quality throughout the pond for both nitrogen and phosphorus, with poor water quality
and habitat impairment the norm as again seen in in 2020. All of the metrics for Miacomet Pond
are consistent with a highly nutrient impaired basin. However, as the freshening of this basin
has become complete and sustained, it likely will have to be managed as a transitional
freshwater system and will need to be reassessed as such. Since Miacomet Pond may have
storm over-wash in the future due to climate change related storm intensification and sea level
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rise, it may be necessary for management to create both a nitrogen and a phosphorus budget for
this system and to conduct short-term incubations to determine which nutrient is controlling pond
health under present and varying salinity conditions. It is likely that regions of the pond may be
sensitive to both nitrogen and phosphorus, such that overall both nutrients need to be monitored
and considered for management of Miacomet Pond, although phosphorus management is
clearly needed at this time. There is no evidence that Miacomet Pond will improve without
management action(s).
Table ES-1. Summary of present status and trends of water/habitat quality of estuaries of
Nantucket based on present (2020) water quality monitoring data reconciled against historic data
collected from 2010-2019 and MEP nutrient threshold analyses undertaken for each system
except Miacomet Pond. A TMDL for Hummock Pond has not yet been released by MassDEP.
Overall, the water quality trends in Nantucket’s major estuaries have been positive over the past
5-10 years. Nantucket Harbor, Long Pond and Sesachacha Pond are showing improvements
and are approaching their TMDL targets to meet restoration goals. Madaket Harbor is
supporting high water and habitat quality and is stable. Only Hummock and Miacomet Ponds
are having significant ecological health issues. Hummock Pond has been very difficult to flush
significantly with annual openings. As a result water quality is impaired and also the salinity is
low, in 2014-2016 salinities in the lower basin (HUM-1,3,5) were 6-7 ppt. But in 2017 salinity
dropped to <4 ppt due a low flushing opening. The salinity as increased slightly ~5 ppt from
2018-2020 with <4 ppt in the upper reaches (HUM-7 & 8). This is still low for maintaining
Hummock Pond as a brackish water estuary. Historically levels have been ~8 ppt, so there is
some concern about potential species changes, however salinity decline is relatively slow so
there is still time to reset the salinity. Miacomet Pond provides evidence of the need to maintain
salinity in Hummock Pond. Miacomet Pond has not received significant tidal flushing for more
than a decade and the pond is now freshwater and has been for several years. Miacomet Pond
is currently highly eutrophic and supports degraded water and habitat quality. Miacomet appears
to need phosphorus management with periodic needs for nitrogen and phosphorus management
in most years including 2020. Given the temporal record, it does not appear that Miacomet Pond
will improve without implementation of management actions.
Estuary Type TMDL Status Trend Remedial Actions
Madaket Harbor Marine In Place Approaching N Target Improving Landfill Remediation
Long Pond Brackish In Place Approaching N Target Improving Landfill Remediation
Nantucket Harbor -
Polpis Harbor Marine In Place Approaching N Target Improving New Jetties, Planned
Sewers
Sesachacha Pond Brackish In Place Moderate Improvement Variable Targeted opening
Hummock Pond Brackish/Fresh In Place??Significantly Impaired Variable Targeted opening
Miacomet Pond Fresh None Significantly Impaired Unchanged Unknown
Summary of Present Status and Trend of Water Quality in Nantucket's Estuaries
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Monitoring and Specific Results: The Technical Memorandum on the 2020 Nantucket Water
Quality Monitoring Program is organized consistent with previous SMAST water quality
monitoring summaries (2010 and 2012 - 2018) but as in 2019, the summary memo is now
focused more on temporal trends while still allowing comparisons to data from the previous years
of monitoring. However, the bulk of the prior data tables that were summarized in previous
reports on the 2012 to 2018 field seasons are not repeated again here. Similarly, as in prior
annual reports, the 2020 summary does not include an overview of the program or the summary
of the sampling approach as these have not changed from previous years, instead they are
included by reference. This tech memo does include the 2020 summary tables and synthesis of
results in tabular and graphical form which includes all data collected under this program from its
inception (2010 and 2012 - 2020). The 2020 summary is focused specifically on the following:
1. Results of Sampling: Summary of Water Quality Results
Nantucket Harbor
Madaket Harbor
Long Pond
Hummock Pond
Miacomet Pond
Sesachacha Pond
Polpis Harbor Streams
Oyster Aquaculture Potential Sites
2. Trophic State: Water Quality/Eutrophication Status
3. Recommendations for Future Monitoring (see last page of report)
As in previous years, the 2020 water quality monitoring of Nantucket's fresh and saltwater
systems was focused on summer-time conditions, as the warmer months typically have the
lowest water quality conditions, hence are the critical period for resource management. As in
previous years (2010, 2012-2019), the 2020 approach utilized the same sample collection and
analysis approach for assessing the water quality of each of the estuaries of Nantucket. This
consistency is intended to maximize the value of the results by making the data perfectly cross-
comparable to water quality monitoring data collected across the Island of Nantucket from
previous years and to previous Massachusetts Estuaries Project results for Nantucket estuaries
as well as more broadly to estuaries throughout the region (Cape Cod, Martha's Vineyard). In
this manner, inter-ecosystem comparisons can be made to better assess system
health/impairment and function and to formulate appropriate nutrient management strategies.
This allows individual towns such as Nantucket to directly benefit from lessons learned
throughout the wider region. It should be noted that in 2020 (as was the case in 2019),
compliance monitoring samples were collected from sentinel locations in Nantucket Harbor,
Sesachacha Pond and Madaket Harbor as established under the Massachusetts Estuaries
Project.
As in past years, UMD-SMAST Coastal Systems Program (CSP) scientists focused primarily on
the analysis of samples collected from the field effort (led by Thaïs Fournier, Nantucket NRD),
the associated data analysis, and overall program coordination. The Nantucket Natural
Resources Department staff primarily focused on coordination of field efforts, field sampling and
data collection on physical parameters and water quality improvement efforts.
The goals of the monitoring program remain unchanged from previous years, primarily to:
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1. determine the present (2020) ecological health of each of the main salt ponds and
estuaries within the Town of Nantucket,
2. gauge (as historical data allows) the decline or recovery of various salt ponds and
embayments over the long-term (also part of TMDL compliance), and
3. provide the foundation (and context) for development of potential alternatives for nutrient
and resource management and quantitative measures of success.
This latter point (3) is critical for restoration planning should a system be found to be impaired or
trending toward impairment, which requires targeted management actions for restoration.
As was the case in 2010, 2012 - 2019 sampling efforts, the 2020 sampling program focused on
the summer/early fall months (June - September, a few samples collected in October).
Additional monitoring of the openings of Sesachacha Pond and Hummock Pond were NOT
continued in 2020 as openings did not occur in spring but in fall after the monitoring season.
However, a detailed plan for refining pond openings was developed for future implementation.
Water samples were collected from each of Nantucket’s 6 estuarine systems (Figures 1-6) on
multiple dates (“events”) in 2020 following the schedule presented in Table 1. Samples were
obtained from the same sampling station locations and the same depths as in previous years to
maximize cross comparability and to gauge temporal changes. It should be noted that the Town
of Nantucket did undertake water quality monitoring in 2011, however, those samples were
analyzed by a lab other than the Coastal Systems Analytical Facility at the UMASS School for
Marine Science and Technology. The 2011 water quality data were presented in tabular form in
Appendix A in Annual Technical Memoranda of 2012 and 2013 and are not being reproduced
again herein.
As in all previous years, the physical/environmental parameters measured in the estuaries during
the 2020 sampling season included: total depth, Secchi depth (light penetration), temperature,
conductivity/salinity (YSI meter), general weather (rain, cloudiness, etc.), wind force and
direction, dissolved oxygen levels and observations of moorings, birds, shell fishing and unusual
events (fish kills, algal blooms, etc.). Laboratory analyses of estuarine waters included: salinity,
nitrate + nitrite, ammonium, dissolved organic nitrogen, particulate organic carbon and nitrogen,
chlorophyll-a and pheophytin-a and orthophosphate. As initiated in the summer of 2015, the
estuarine water quality monitoring undertaken in 2016-2020 included an additional 3 stream
locations. During the summer 2016 season, stream station STA-3 was dropped due to no flow
and a station STA-4a was added to clarify water quality conditions in the stream outflow
associated with cranberry bogs up-gradient of STA-4. In 2017-2018 a fourth stream station was
added (WPH outlet) to directly measure flow and nutrient load into Western basin of Polpis
Harbor. In 2019 & 2020 stream sampling took place at 3 stations: STA-4, STA-6B and WPH
outlet.
In 2020, 27 field duplicates (10% of the total number of samples collected {n=284}) were taken
as part of the field sampling protocol for QA analysis. Data were compiled and reviewed by the
laboratory for accuracy and evaluated to discern any possible artifacts caused by improper
sampling technique, physical disturbance, etc. In addition, some samples were rerun to confirm
initial results.
The Town of Nantucket has been working for decades to protect and more recently restore its
estuaries and their aquatic resources. At present, activities to lower nitrogen enrichment and its
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negative impacts to water and habitat quality are ongoing in 4 estuarine systems: Nantucket
Harbor (jetties and sewers), Madaket/Long Pond (landfill and possible dredging), Sesachacha
Pond (openings), Hummock Pond (refined opening protocol). In addition, all of the Town’s
estuaries should benefit from the recent fertilizer application by-law. As a result, it is anticipated
that the efficacy of these management activities should be seen in the on-going monitoring
results.
Summer 2020, like 2019, appears to have sustained high water quality for the Nantucket Harbor
system (2020 embayment wide TN average of 0.34 mg/L, 2019 = 0.34 mg/L, 2018 = 0.31 mg/L,
2017 = 0.35 mg/L, 2016 = 0.35 mg/L, 2015 = 0.37 mg/L). Madaket Harbor is similar to
Nantucket Harbor in that it generally supports high water quality and is flushed with high quality
low nitrogen offshore waters. There is typically a gradual nitrogen gradient from Hither Creek
(MH-1) into the main basin (MH-2 and MH-3) with lowest levels near the offshore boundary (MH-
4). In 2020 only the levels in Hither Creek exceeded the TMDL target level (0.45 mg TN/L),
which was also the case in 2019. However, there has been a reduction in TN levels, as 2 of the
last 5 years met the TN threshold for Hither Creek. The Long Pond tributary embayment to
Madaket Harbor remains moderately impaired but improved over the past decade as seem in the
2020 results. It appears that the Town’s management of the landfill, has reduced the nitrogen
load from this source with the associated observed lowering of TN levels. The connection is
strengthened by the parallel timing of the landfill work and the lowering of TN in the adjacent
waters, which are only a short travel time from the landfill. While still small, the water quality
Index for Long Pond is showing improvement in response to the lower TN levels although the
basin remains impaired. TN levels in 2015-2017 and 2018-2020 were almost half that of
historical and 2010 measurements although they still show inter-annual variations as 2019 TN
values were slightly higher (0.63 mg/L TN) compared to 2018 (0.59 mg/L TN) and 2020 TN
levels were 0.502 – 0.55 mg N/L a marked improvement. While continued monitoring will
determine the final level of improvement as the system reaches a new equilibrium, it does
appear that the reduction in N loading is occurring with beneficial effects. If TN levels stabilize at
2018-2020 levels, the TN target in the TMDL may be achieved. In contrast, Hummock Pond
water quality appears to remain poor in years 2014-2016 and 2017-2020 after an initial
improvement after 2010-2013. Overall water quality was highest in 2015 (TN = 0.58 mg/L) but
became poor again and has remained poor (2020, 2019, 2018, 2017 and 2016 embayment wide
average TN levels remain high at 0.83, 0.88, 0.83, 1.01 and 0.80 mg/L, respectively). The
variations in TN levels result primarily from opening success, for instance in spring 2017 the
opening was only 1.5 days with high precipitation, although the spring opening in 2016 was
comparable to the duration of the opening in 2015 (18 days and 15 days respectively). Long
Pond estuary wide TN concentrations have improved over historic levels although the reduction
appears to be stabilizing (0.528 mg/L, 0.630 mg/L and 0.587 mg/L in 2020, 2019 and 2018,
respectively). Miacomet Pond, which is now functionally a eutrophic freshwater pond, is
eutrophic with 2020 chlorophyll pigment levels averaging 15.1 ug/L, TN = 1.30 mg/L, (2019: 23.9
ug/L, TN = 0.935 mg/L) and TP = 94 ug/L (2019: TP = 127 ug/L) 1 (TP many fold higher than in
high quality freshwater systems). Water Quality in Sesachacha Pond appears to be directly
related to the efficacy of its seasonal openings. Water quality in 2010, 2012, 2013 (mean TN =
0.671 mg/L, Chla = 5.5 ug/L) was significantly improved over the levels observed previously in
1 Total Phosphorus levels in freshwater ponds/lakes on Cape Cod with high water and habitat quality general have
TP <20 ug/L)
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the MEP analysis. The lower TN levels in Sesachacha Pond (2010-2013) versus historic levels
documented by the MEP indicated improvement of pond resources most likely due to modified
breaching of the pond as recommended by the MEP analysis. Since there was no major shift in
nitrogen loading within the Sesachacha Pond watershed during that period of improvement, it is
almost certain that the amount of tidal flushing during a given artificial breaching drives the
variability in the observed summer TN level. The rise in TN observed during the 2014 sampling
suggests that a poor inlet opening may have occurred in spring 2014, as it is unlikely a
significant change in the watershed load occurred to drive the increase in TN from one summer
to the next. Short openings in subsequent years (2015 {5 days}, 2016 {3 days}, 2017 {7 days})
suggests that inlet opening efficacy has a significant effect on pond water quality as TN
concentrations from 2014 to 2017 have steadily increased to nearly the same level as when the
MEP analysis was completed. In 2014-2017 water quality has declined (mean TN = 0.933 mg/L,
Chla = 8.4 ug/L) and is again approaching MEP levels of enrichment. The amount of flushing
with the openings is reflected in the significantly higher salinities in 2010-2013 versus 2014-
2017, 17.9 PSU and 11.6 PSU, respectively. Higher salinities reflect more flushing which is
correlated with lower TN and total chlorophyll a levels. The 2018 and 2019 water quality analysis
confirmed that the pond remains impaired but did show a mild improvement (TN = 0.752 and
0.840 mg/L and Chla = 13.9 and 8.9 ug/L, respectively) likely due to improved openings. The
2020 water quality analysis confirmed that the pond appears to be improving compared to
previous years (TN = 0.673 and Chla = 3.4 ug/L) and is approaching the MEP Threshold of
<0.60 mg/L TN.
The important role of the pond openings is further supported by the fact that average TN
concentration in Sesachacha Pond was higher (1.00 mg/L TN) in 2016 which received a 3-day
opening compared to average TN concentrations observed in 2017 (0.88 mg/L TN) given a 7-
day opening. These results indicate that the level of water quality observed in Sesachacha Pond
since 2014 does respond positively if sufficient flushing can be maintained. The effect of
improved pond flushing is discussed in detail in the MEP Sesachacha Pond Nutrient Threshold
Report submitted to the Town of Nantucket in 2006.
Table 1. Sampling Schedule for 2020 Nantucket Water Quality Monitoring Program. It should be
noted that in some instance not all stations were sampled on all dates.
Nantucket
Harbor
Madaket
Harbor Long Pond Sesachacha
Pond
Miacomet
Pond
Hummock
Pond
Polpis
Streams
Oyster
Sites
January
February
March
April
May
June 23 25 25 16 17 24 17,30 23
July 8,22 9 7 1 1 7 11 8
August 4,24 6,24 20 12 12 20 6
September 3,28 25 24 9 9 24 2 25
October 1,15 22
November
December
Total Events 7 5 4 6 4 5 4 4
SAMPLE DATES (summer 2020)
Month
12
Figure 1. Madaket Harbor and Long Pond sampling stations 2010, 2012, 2013, 2014, 2015,
2016, 2017, 2018, 2019 and 2020.
13
Figure 2. Nantucket Harbor sampling stations 2020. Station NAN-8 (the cut) was only sampled in 2010 and location changed in 2011 -
2020. Nantucket Harbor and Polpis Harbor each have nitrogen thresholds in the MassDEP/USEPA TMDL for this system.
14
Figure 2a. Sampling locations associated with potential oyster aquaculture deployments in Nantucket Harbor (ORS-2,3,4,5,6) sampled in
2015. ORS-2,4,6 sampled in 2016, 2017, 2018, 2019 and 2020. Sites are associated with possible oyster aquaculture areas (yellow
pins). ORS4 was the selected reef location and reef construction began in June 2017. Spat on shell was deployed in the Fall 2017.
15
Figure 3. Sesachacha Pond sampling stations 2010, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019 and 2020. This system has a
nitrogen threshold set in its site-specific MassDEP/USEPA TMDL.
16
Figure 4. Hummock Pond sampling stations 2010, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019 and 2020. Station 7 is in Head of
Hummock, a kettle pond connected by an artificial channel to the estuary with limited exchange from Station 7 to Station 8.
17
Figure 5. Miacomet Pond sampling stations 2010, 2012, 2013, 2014, 2015, 2016, 2017, 2018,
2019 and 2020. Miacomet Pond is currently functioning as a fresh pond.
Station 3
Station 1
Station 2
Station 3
Station 1
Station 2
18
Figure 6. Polpis Harbor Stream Sampling locations shown in yellow pins (ST-4, 4A, 6B)
sampled in 2016-2017. WPH-outlet stream site sampled in 2017-2018. Sampling in 2019 and
2020 was of ST-4, 6B and WPH. Water samples from mid depth in water exiting culverts.
Figure 6a. Polpis Harbor Stream Sampling locations (ST-4, 4A, 6B and WPH-outlet) relative to
estimated direction of flow as determined by the Town of Nantucket.
19
Table 2. Summary of Water Quality Parameters, 2020 Nantucket Sampling Program. Values are Station Averages of all sampling
events, June-September for sampling sites. Station NAN-2A represent an MEP sentinel location for monitoring in Nantucket Harbor.
NAN-2A is a new station that was first sampled in 2016 and is in a different location than NAN-2. It should be noted that in 2019 and
2018, TP was only evaluated at sites dominated by freshwater: Sesachacha Pond, Hummock Pond, Long Pond, Miacomet Pond and
stream because of the low salinity values in those closed ponds and the possibility that the Miacomet system maybe phosphorous
limited rather than nitrogen limited. Further study should investigate the possibility of P-limitation and TP paired with salinity should
continue to be monitored during the summer 2021 field season.
20% Low 20% Low
2020 Secchi Secchi Field Field Avg.Avg.Avg.Avg.Avg.Avg.Avg.Avg.Avg.Avg.Avg.Avg.Chla/T-Pig Avg.Avg.
average Depth DO DO Salinity PO4 NH4 Nox DIN DON TDN POC PON TON TN Chla Phaeo Ratio Total Pig TP
Embayment Sample ID (meters)% of WC Sat Sat (ppt)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(ug/L)(ug/L)(ug/L)(mg/L)
(mg/L)(% sat)
HUMMOCK POND HUM7 1.175 0.341 4.179 0.435 3.573 0.029 0.081 0.073 0.154 0.377 0.531 5.893 0.929 1.306 1.460 13.664 12.997 0.535 26.661 0.150
HUMMOCK POND HUM8 1.006 0.686 6.237 0.728 3.900 0.015 0.006 0.002 0.008 0.369 0.378 2.756 0.380 0.749 0.758 7.227 4.512 0.577 11.739 0.097
HUMMOCK POND HUM5 1.125 0.621 6.188 0.751 4.550 0.015 0.003 0.004 0.007 0.391 0.397 1.695 0.278 0.669 0.675 6.752 6.364 0.543 13.115 0.097
HUMMOCK POND HUM3 1.240 0.670 7.179 0.866 5.442 0.017 0.005 0.003 0.008 0.375 0.383 1.280 0.216 0.591 0.599 4.454 3.943 0.537 8.398 0.085
HUMMOCK POND HUM1 1.417 0.570 6.873 0.816 5.800 0.019 0.008 0.005 0.013 0.417 0.430 1.250 0.208 0.626 0.639 4.245 2.713 0.577 6.958 0.076
LONG POND LONG5 1.063 0.865 3.184 0.436 16.713 0.040 0.004 0.006 0.010 0.328 0.338 1.453 0.215 0.543 0.553 4.687 2.402 0.615 7.089 0.130
LONG POND LONG6 1.133 0.955 4.144 0.560 18.413 0.032 0.013 0.007 0.020 0.323 0.343 1.059 0.159 0.482 0.502 3.552 1.178 0.657 4.729 0.119
MADAKET HARBOR MH1 1.625 0.929 4.418 0.631 29.064 0.021 0.029 0.005 0.034 0.330 0.365 0.796 0.144 0.475 0.509 6.936 1.310 0.799 8.246 NS
MADAKET HARBOR MH2 2.120 0.988 5.398 0.786 31.263 0.006 0.012 0.008 0.019 0.247 0.266 0.547 0.092 0.339 0.358 3.106 1.021 0.732 4.127 NS
MADAKET HARBOR MH3 2.342 1.000 5.159 0.706 31.633 0.010 0.007 0.003 0.011 0.194 0.204 0.580 0.091 0.284 0.295 2.676 1.205 0.671 3.882 NS
MADAKET HARBOR MH4 3.300 0.930 5.620 0.807 31.729 0.014 0.010 0.003 0.013 0.203 0.216 0.362 0.055 0.258 0.271 2.203 0.817 0.727 3.020 NS
MIACOMET POND MP3 1.167 0.781 2.073 0.240 0.100 0.041 0.240 0.038 0.277 0.496 0.774 2.002 0.367 0.864 1.141 11.925 7.800 0.576 19.725 0.127
MIACOMET POND MP1 1.458 0.836 2.753 0.323 0.100 0.010 0.153 0.005 0.158 0.676 0.834 4.545 0.664 1.340 1.498 12.591 7.054 0.465 19.645 0.079
MIACOMET POND MP2 1.713 0.593 5.046 0.606 0.100 0.004 0.106 0.005 0.111 0.510 0.621 3.541 0.637 1.147 1.258 20.775 3.554 0.613 24.329 0.077
NANTUCKET HARBOR NAN3 2.478 0.420 4.916 0.701 31.838 0.024 0.009 0.003 0.012 0.269 0.281 0.635 0.102 0.371 0.383 4.599 0.817 0.826 5.416 NS
NANTUCKET HARBOR NAN6 2.370 0.864 4.764 0.697 31.621 0.024 0.016 0.003 0.019 0.245 0.264 0.642 0.097 0.343 0.362 3.346 0.999 0.774 4.345 NS
NANTUCKET HARBOR NAN5 1.675 0.925 3.943 0.573 31.263 0.021 0.012 0.002 0.015 0.236 0.251 1.083 0.165 0.401 0.416 5.577 2.185 0.786 7.762 NS
NANTUCKET HARBOR NAN2 2.650 0.552 4.847 0.726 31.838 0.021 0.012 0.004 0.016 0.252 0.268 0.493 0.081 0.333 0.349 4.069 1.167 0.767 5.236 NS
NANTUCKET HARBOR NAN2A 2.670 0.476 4.136 0.596 31.907 0.028 0.021 0.003 0.024 0.249 0.272 0.590 0.098 0.347 0.370 4.389 1.233 0.750 5.622 NS
NANTUCKET HARBOR NAN7 1.740 0.909 5.483 0.769 31.413 0.020 0.011 0.003 0.014 0.198 0.212 0.603 0.095 0.292 0.306 3.089 1.109 0.731 4.198 NS
NANTUCKET HARBOR NAN1 3.225 0.606 5.098 0.712 31.575 0.017 0.006 0.003 0.009 0.188 0.197 0.386 0.063 0.251 0.260 4.263 0.982 0.797 5.245 NS
NANTUCKET HARBOR NAN8N 1.275 1.000 5.751 0.783 31.275 0.013 0.004 0.002 0.006 0.189 0.195 0.508 0.085 0.274 0.280 3.192 1.319 0.730 4.511 NS
NANTUCKET HARBOR NAN4 3.167 0.657 5.648 0.769 31.600 0.016 0.005 0.003 0.009 0.269 0.277 0.466 0.073 0.342 0.351 3.599 0.787 0.831 4.386 NS
Nantucket Hrb. Stream Site ST4 ND ND ND ND 0.000 0.059 0.013 0.002 0.015 0.880 0.895 0.619 0.060 0.940 0.955 NS NS NS NS 0.099
Nantucket Hrb. Stream Site ST6 0.100 0.333 ND ND 0.100 0.016 0.016 0.007 0.023 0.226 0.249 5.522 0.243 0.470 0.493 NS NS NS NS 0.215
Nantucket Hrb. Stream Site ST6B 0.250 0.625 ND ND 0.050 0.118 0.013 0.005 0.017 0.608 0.625 7.854 0.397 1.005 1.022 NS NS NS NS 0.606
Nantucket Hrb. Stream Site WPH Outlet ND ND ND ND 0.100 0.055 0.016 0.003 0.019 0.929 0.948 1.827 0.159 1.087 1.106 NS NS NS NS 0.102
SESACHACHA POND SESA3 1.579 0.480 6.668 0.847 6.858 0.158 0.002 0.005 0.007 0.463 0.470 1.793 0.216 0.679 0.686 2.803 0.768 0.702 3.571 0.214
SESACHACHA POND SESA2 1.686 0.347 5.673 0.714 6.863 0.158 0.003 0.005 0.009 0.449 0.457 1.709 0.207 0.656 0.664 3.801 1.349 0.709 5.150 0.210
SESACHACHA POND SESA4 1.313 0.357 6.279 0.788 6.875 0.158 0.002 0.003 0.005 0.468 0.473 1.629 0.201 0.669 0.674 3.034 0.898 0.746 3.932 0.216
SESACHACHA POND SESA1 1.640 0.303 6.218 0.783 6.871 0.155 0.004 0.012 0.016 0.442 0.458 1.707 0.209 0.651 0.667 3.945 0.953 0.605 4.898 0.211
OLD NORTH WHARF - Oyster ORS2 1.200 1.000 4.966 0.723 31.650 0.019 0.018 0.005 0.023 0.228 0.251 0.470 0.072 0.300 0.323 2.730 0.701 0.814 3.431 NS
SHIMMO Oyster ORS4 0.850 1.000 2.898 0.419 31.000 0.017 0.029 0.008 0.037 0.254 0.291 0.558 0.099 0.353 0.389 5.153 4.221 0.615 9.374 NS
POLPIS Oyster ORS6 1.100 1.000 2.868 0.421 30.630 0.026 0.019 0.004 0.024 0.317 0.340 0.643 0.103 0.420 0.444 3.880 1.038 0.767 4.918 NS
20
Table 3. Summary of Stream Water Quality Parameters (ST4, ST6, WPH OUTLET {Polpis Harbor}) and stations associated
with potential oyster aquaculture locations (ORS,2,4,6), 2020 Nantucket Sampling Program..
Lab PO4 TP NH4 NOx DIN DON TDN POC PON TON TN Avg.Avg.Avg.
Embayment Date Sample ID Salinity (mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)Chla Phaeo Total Pig
ppt (ug/L)(ug/L)(ug/L)
Nantucket Stream 6/30/2020 ST4 0.0 0.0590 0.0986 0.0131 0.0016 0.0147 0.8798 0.8945 0.6187 0.0605 0.9403 0.9550 ND ND ND
Nantucket Stream 9/2/2020 ST6 0.1 0.0068 NS 0.0102 0.0031 0.0133 0.1741 0.1874 3.6649 0.1786 0.3527 0.3660 ND ND ND
Nantucket Stream 8/11/2020 ST6 0.1 0.0244 0.2153 0.0228 0.0099 0.0327 0.2787 0.3114 7.3781 0.3078 0.5865 0.6192 ND ND ND
Nantucket Stream 6/17/2020 ST6B 0.1 0.1739 0.3036 0.0105 0.0060 0.0165 0.5903 0.6068 9.8370 0.4473 1.0375 1.0540 ND ND ND
Nantucket Stream 6/30/2020 ST6B 0.0 0.0623 0.9084 0.0148 0.0037 0.0184 0.6256 0.6440 5.8710 0.3461 0.9717 0.9901 ND ND ND
Nantucket Stream 6/30/2020 WPH 0.1 0.0549 0.1016 0.0159 0.0032 0.0191 0.9288 0.9479 1.8272 0.1585 1.0873 1.1064 ND ND ND
Lab PO4 TP NH4 NOx DIN DON TDN POC PON TON TN Avg.Avg.Avg.
Embayment Date Sample ID Salinity (mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)Chla Phaeo Total Pig
ppt (ug/L)(ug/L)(ug/L)
NANTUCKET HARBOR 6/23/2020 ORS2 31.6 0.0141 NS 0.0260 0.0064 0.0324 0.2415 0.2739 0.5258 0.0919 0.3335 0.3658 4.224 0.355 4.579
NANTUCKET HARBOR 7/9/2020 ORS2 31.4 0.0213 NS 0.0257 0.0079 0.0336 0.3201 0.3537 0.4384 0.0692 0.3893 0.4229 1.552 0.030 1.582
NANTUCKET HARBOR 8/6/2020 ORS2 31.8 0.0242 NS 0.0196 0.0036 0.0231 0.2349 0.2580 0.4873 0.0801 0.3150 0.3381 3.331 1.234 4.565
NANTUCKET HARBOR 9/25/2020 ORS2 31.8 0.0154 NS 0.0015 0.0020 0.0036 0.1156 0.1192 0.4271 0.0475 0.1631 0.1667 1.813 1.186 2.998
NANTUCKET HARBOR 6/23/2020 ORS4 31.1 0.0107 NS 0.0212 0.0050 0.0261 0.2922 0.3184 0.4965 0.0933 0.3855 0.4117 3.324 1.202 4.526
NANTUCKET HARBOR 7/8/2020 ORS4 30.8 0.0157 NS 0.0136 0.0156 0.0292 0.3696 0.3988 0.6264 0.1077 0.4773 0.5065 9.630 10.776 20.406
NANTUCKET HARBOR 8/6/2020 ORS4 30.9 0.0226 NS 0.0234 0.0042 0.0276 0.2386 0.2662 0.7130 0.1312 0.3698 0.3974 4.441 1.180 5.621
NANTUCKET HARBOR 9/25/2020 ORS4 31.2 0.0179 NS 0.0568 0.0071 0.0639 0.1153 0.1793 0.3944 0.0620 0.1774 0.2413 3.217 3.728 6.945
NANTUCKET HARBOR 6/23/2020 ORS6 28.5 0.0253 NS 0.0004 0.0093 0.0096 0.5241 0.5338 0.9684 0.1514 0.6755 0.6852 6.933 0.720 7.654
NANTUCKET HARBOR 7/9/2020 ORS6 30.7 0.0205 NS 0.0094 0.0009 0.0103 0.2701 0.2804 0.5564 0.0905 0.3606 0.3709 2.504 1.837 4.341
NANTUCKET HARBOR 7/9/2020 ORS6 NS NS NS NS NS NS NS NS NS NS NS NS NS NS NS
NANTUCKET HARBOR 8/6/2020 ORS6 31.4 0.0340 NS 0.0286 0.0048 0.0333 0.3643 0.3976 0.6903 0.1181 0.4824 0.5157 3.682 0.972 4.654
NANTUCKET HARBOR 9/25/2020 ORS6 31.9 0.0229 NS 0.0388 0.0020 0.0408 0.1081 0.1489 0.3556 0.0535 0.1616 0.2024 2.402 0.623 3.025
21
Status and Trends in Water Quality of Nantucket’s Estuaries:
Based on the water quality data collected by the Nantucket Island-wide Monitoring program
since 2010, it is now possible to conduct an initial trend analysis. While trends have been noted
in previous reports, due to the now available 10 years of data, trends are becoming clearer and
coming through the noise of interannual natural variations. For this analysis temporal trends are
assessed in each of the 6 estuaries by combining data from 2010, 2012 and 2013; 2014, 2015
and 2016; 2017, 2018 and 2019. Results from 2020 where compared to these three year
groupings. Key water quality parameters were examined to determine if changes in estuarine
health have occurred over the past decade.
Nantucket Harbor has had a number of nitrogen mitigations: a) raising the jetties, b) new
sewering, c) fertilizer by-law. From the time course of Total Nitrogen (TN) it is clear that the
most recent 3 years of monitoring at each harbor station show lower TN levels than in the prior
2 three year groupings going back to 2010 with the 2014-2016 grouping showing intermediate
levels and the 2017-2019 and 2020 generally showing the lowest TN levels (note 2020 is a
single year so is more variable). This indicates a lowering of TN harbor-wide over the past
decade (Figure 7). Similarly when examined by basin, again there is a consistent decline in TN
and the rate of decline is consistent among the basins (Figure 8). This pattern is consistent with
the Town’s mitigation efforts, which may not have fully impinged upon Nantucket Harbor at this
time.
Phytoplankton biomass as total chlorophyll a pigments, shows a more variable pattern as the
levels are controlled by a variety of factors in addition to nitrogen levels. Overall, chlorophyll
levels within the main basin of Nantucket Harbor have been indicative of a modest level of
impairment (averaging <10 ug/L), with slightly higher levels in Polpis Harbor. However, it is
clear that chlorophyll a has also declined from the early period (2010,2012,2013) to recent
sampling, with 2020, 2019, 2017, 2016 and 2014 being relatively low (<6 ug/L). These modest
summertime chlorophyll a levels throughout much of the Harbor are indicative of a high quality
habitat (Figure 9). Trends in both chlorophyll a and TN are consistent with improving estuarine
conditions. TN levels throughout the Harbor are approaching the TMDL targets for this system.
22
Figure 7. Temporal trends in Nantucket Total Nitrogen levels at all stations. Note that in all
cases the 2017-2019 levels are lower than the earliest grouping (2010-2013) with 2014-2016
being in the middle. Clearly TN levels have declined harbor-wide over the past 10 years due to
nitrogen mitigation strategies implemented by the Town. 2020 data generally confirms the trend
(with exception Nan-4), lower Harbor still showing TN decline)
Figure 8. Temporal trends in Nantucket Total Nitrogen levels within the key basins. Years were
combined as 2010, 2012 and 2013; 2014, 2015 and 2016; 2017, 2018 and 2019. 2020 is
presented relative to the three year groupings in appears slightly above the trend but likely
indicative of inter-annual variation.
23
Figure 9. Average Chlorophyll-a concentrations by station in the well flushed Nantucket Harbor
system. Stations Nan-5 and 6 are in Polpis Harbor the rest relate to the main basin. Note that
2019 levels were slightly lower in the upper basins (Nan-5 slightly lower in 2019 vs. 2020, 2018
and 2017, Nan-6 slightly lower in 2020 & 2019 vs. 2018 & 2017) and low and relatively constant
throughout the Harbor. 2020 levels and previous 5 years were indicative of high water quality
and Polpis Harbor periodically shows levels indicative of moderate impairment.
Figure 10. Components of Total Nitrogen pool in Nantucket Harbor waters by station (2020). At
all stations TN is almost entirely in organic forms due to phytoplankton uptake of inorganic
nitrogen entering from the watershed and recycled from the sediments and conversion to
organic forms. Note that the TN levels remain very close to the MEP TN Threshold level
codified in the present TMDL.
MEP TN
Threshold
NAN3 = 0.350 mg/l
NAN6 = 0.355 mg/l
24
Figure 12. Total nitrogen levels at station 2 and 2A over time in Nantucket Harbor. Station 2A is
a sentinel station in the head of the harbor established in 2016. The current 3 year average is
0.366 mg N/L close to the threshold of 0.350 mg N/L.
Table 3. TN levels by station and year in Nantucket Harbor for comparison to TMDL targets of
0.35 mg/L in Wauwinet (Stations 3 and 2A) and 0.355 mg/L in Polpis Harbor (Station 6). 2020
nearing threshold levels.
[1] It is almost certain that this does not represent the TN level in the inflow to Nantucket Harbor
on the flood tide, but rather the 2012 data is influenced by mixing with TN enriched out-flowing
waters. An attempt to control for this issue was implemented in the 2013 monitoring program. It
should be noted that the 2019 concentration may also be high due to influence from out-flowing
waters. Concentration at station 4 is very sensitive to timing of sampling.
0.00
0.25
0.50
0.75
1.00
1.25
1.50
NAN2 NAN2A
N
C
o
n
c
e
n
t
r
a
t
i
o
n
(
m
g
/
L
)
Station Number
Nantucket Harbor Total Nitrogen Gradient
(2016, 2017, 2018, 2019, 2020)
Sentinel Station 2A
TN 2016 TN 2017 TN 2018 TN 2019 TN 2020
Sentinel StationMid Harbor
0.302
0.3770.314
0.399
0.256
0.3370.292
0.3920.349 0.370
Historical 2010 2010 2012 2013 2014 2015 2016 2017 2018 2019 2020
MEP Town Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN
Mean TN ID
(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)
Head of the Harbor - Upper 2 0.408 0.188 NA NS NS NS NS NS NS NS NS NS NS
Head of the Harbor - Mid Town 3 0.401 0.115 3 0.392 0.411 0.415 0.345 0.436 0.355 0.366 0.325 0.336 0.383
Head of the Harbor - Lwr 2A 0.339 0.07 2A NS NS NS NS NS 0.415 0.377 0.337 0.392 0.370
Pocomo Head 3 0.335 0.081 NA NS NS NS NS NS NS NS NS NS NS
Quaise Basin 3A+Town 2 0.336 0.112 2 0.297 0.364 0.345 0.314 0.374 0.314 0.302 0.256 0.292 0.349
East Polpis Harbor 4+Town 6 0.362 0.105 6 0.438 0.484 0.401 0.378 0.404 0.371 0.405 0.327 0.378 0.362
West Polpis Harbor 4A+Town 5 0.388 0.119 5 0.431 0.419 0.385 0.389 0.422 0.42 0.429 0.341 0.416 0.416
Abrams Point 5 0.335 0.06 NA NS NS NS NS NS NS NS NS NS NS
Monomoy 6 0.297 0.086 NA NS NS NS NS NS NS NS NS NS NS
Mooring Area 7+Town 1, 1A 0.326 0.106 1, 7 0.332,
0.377
0.335,
0.379
0.323,
0.323
0.294,
0.284
0.390,
0.330
0.304,
0.319
0.317,
0.333
0.212,
0.253
0.262,
0.274
0.260,
0.306
Nantucket Sound OS+Town 4 0.239 0.041 4 0.283 0.344[1]0.3171 0.277 0.297 0.283 0.288 0.192 0.307 0.351
Sub-Embayment Monitoring
Station s.d.
25
Madaket Harbor is similar to Nantucket Harbor in that it generally supports high water quality
and is flushed with high quality low nitrogen offshore waters. There is typically a gradual
nitrogen gradient from Hither Creek (MH-1) into the main basin (MH-2 and MH-3) with lowest
levels near the offshore boundary (MH-4). In 2020 only the levels in Hither Creek exceeded the
TMDL target level (0.45 mg TN/L), which was also the case in 2019 (Figure 11). However,
there has been a reduction in TN levels, as 2 of the last 5 years met the TN threshold for Hither
Creek. Similar to Nantucket Harbor, inorganic nitrogen levels are very low and represent only a
minor fraction of the total N pool. This is due to the rapid uptake of this plant available form of
nitrogen by phytoplankton and supports the contention that nitrogen is the key nutrient for
managing phytoplankton blooms and maintaining clear waters. After uptake by phytoplankton
the inorganic forms are converted to phytoplankton biomass, seen as PON, and can be
released upon phytoplankton death and decay as DON.
There appears to be a trend towards lower TN levels at the Hither Creek Stations in recent
years compared to 2010-2014 which may be related to changes in Long Pond (Figure 12 and
see below). Phytoplankton biomass (as total chlorophyll a) follows the TN gradient from inshore
to offshore quite well in each year (Figure 13). This is due to the mixing of higher nitrogen
higher chlorophyll a waters discharging from Hither Creek with the lower nitrogen lower
chlorophyll a waters of Madaket Harbor. The variable temporal decline in TN in recent years,
therefore, may be resulting from lower TN discharges from Hither Creek which is significantly
fed from outflowing waters from Long Pond and direct watershed inputs to Hither Creek itself.
Examining Hither Creek in combination with Long Pond suggest that there may have been a
slight decline in TN levels over the past decade, although the trend is not statistically significant.
However, the large continuing decline in TN levels in Long Pond through 2020 is significant
(Figures 14 and 15) and is likely due at least in part to modifications of the Town Landfill
resulting in a lessening of nitrogen emanating from this source. At present the TN level in Long
Pond is approaching the target in the TMDL for this basin.
The effects of this lower TN can be seen in the generally lower chlorophyll a levels at station 6
in Long Pond adjacent the Landfill. Although variable, the values tend to be lower in recent
years and significantly lower than in 2003 when the Massachusetts Estuaries Project (MEP)
found average summer chlorophyll a levels of >40 ug/L compared to >15 ug/L in summer 2019.
Overall, the Nantucket Monitoring Program will continue to closely track improvements in Long
Pond relative to its regulatory TMDL, but it seems at present that this estuarine basin has
improved since the MEP and during the past decade of consistent monitoring.
26
Figure 11. TN, dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and
particulate organic nitrogen at stations within Madaket Harbor 2020.
Figure 12. Total nitrogen levels by station over time in Madaket Harbor, horizontal line it TMDL
TN level. 2020 data remains above threshold, but the threshold was met in 2 of the past 5
years an improvement over the historic record.
MEP TN
Threshold
MH1 (aka. M11)
= 0.45 mg/l
MEP TN
Threshold
MH1 (aka. M11)
= 0.45 mg/l
27
Figure 13. Average Total Chlorophyll-a concentrations by station in the well flushed Madaket
Harbor system during the 2010-2020 summer sampling seasons. Stations MH-2,3,4 are in the
main open basin and consistently show moderate to low chlorophyll a pigments, MH-1 is the
MEP sentinel station in Hither Creek, which after blooms in 2017-2018 has again become <10
ug/L in 2019-2020.
Figure 14. TN levels in Madaket Harbor main basin (MH-2,3-4), Hither Creek (MH-1) and Long
Pond (stations Long 5 & 6) over the past decade. Note that in all cases the 2017-2020 levels
are similar to (but slightly lower than) the earlier grouping in Hither Creek and Madaket Harbor.
In contrast Long Pond shows continuous significant reductions in TN from 2010,2012, 2013 to
2014-2016 to 2017-2019 and again in 2020 Clearly TN levels have declined in Long Pond and
possibly Hither Creek over the past 10 years.
0
2
4
6
8
10
12
14
16
MH-1 MH-2 MH-3 MH-4
Ch
l
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Madaket Harbor Total Pigment Level
2010 2012 2013 2014 2015
2016 2017 2018 2019 2020
0.000
0.200
0.400
0.600
0.800
1.000
1.200
1.400
Madaket Harbor / Long Pond
TN concentration (mg/L)
TN (avg. 2010,12,13)TN (avg. 2014,15,16)TN (avg. 2017,18,19)TN 2020
28
Figure 15. Change in TN level in Madaket Harbor main basin (MH-2,3-4), Hither Creek (MH-1)
and Long Pond (stations Long 5 & 6) over the past decade. While TN levels have been stable
in Madaket Harbor, Long Pond has seen a significant decline, such that 2017-2020 levels are
approaching the TN level target in the TMDL for this estuary, likely linked to Town efforts to
remediate the Town Landfill in the watershed to the upper basin.
Figure 16. Average Total Pigment (Chlorophyll-a +Pheophytin) concentrations by station in the
Long Pond portion of the Madaket Harbor system during the summer 2020 sampling season
compared to 2010, 2012, 2013, 2014, 2015, 2016, 2017, 2018 and 2019. Total Chlorophyll a
pigment levels were slightly lower in 2020 compared to summers 2018-2019.
y = -0.03x + 0.5975
R² = 0.3454
y = -0.0129x + 0.345
R² = 0.5324
y = -0.2014x + 1.2945
R² = 0.9494
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
Madaket Harbor / Long Pond
TN concentration (mg/L)
TN (MH1)TN (avg. MH2,3,4)TN (avg. LONG5,6)
Linear (TN (MH1))Linear (TN (avg. MH2,3,4))Linear (TN (avg. LONG5,6))
0
5
10
15
20
25
30
2010 2012 2013 2014 2015 2016 2017 2018 2019 2020
Long Pond Total Chlorophyll a (ug/L)
Long-6 Long-5
29
Sesachacha Pond is a periodically opened salt water kettle pond that is managed by the
Nantucket Natural Resources Department. Due to coastal processes it is not possible to
maintain an open tidal inlet to this system or the other major coastal pond of Hummock Pond
(brackish) and Miacomet Pond (once brackish now freshwater). Given the general lack of
flushing of these ponds makes them particularly sensitive to nutrient inputs and the negative
effects of eutrophication.
Sesachacha Pond has only a moderate level of watershed nitrogen inputs, but they have a
magnified effect as the system only periodically receives tidal flushing (1-2 times/yr). Prior
monitoring results have determined that successful openings (those of >5 days) have a positive
effect on water and habitat quality. In fact, it appears that Sesachacha Pond can nearly achieve
or even achieve its regulatory TMDL level for TN if openings can be prolonged. At present,
Sesachacha Pond’s total nitrogen levels (2020= 0.667 mg N/L) are above its TMDL target level
at all stations (Figure 17). Although Sesachacha Pond remains above its TMDL target of 0.60
mg/L, the summer average TN levels have changed. While the openings 2010-2020 have not
achieved the TMDL TN level, this salt pond has generally had higher water quality than
historically (MEP). The baseline TN level for this pond found by the MEP was 1.197 mg TN/L,
which supported degraded water quality and periodic chlorophyll a blooms of 100 ug/L. Since a
focused opening management plan has evolved over the past decade, TN levels have generally
declined and averaged 0.784 mg TN/L over 2018 and 2019 and even lower in the 2020 field
season (Table 4). This underscores that even at present levels of success, openings play a
critical role in lowering eutrophication in this estuary.
Although the system is brackish due to its reduced salt water input, its nitrogen components are
similar to the tidally flushed estuaries in that, entering dissolved inorganic N (DIN) is rapidly
taken up by phytoplankton and converted to organic forms. The result is that almost all water
column nitrogen can be found in growing or senescent phytoplankton (PON) or as dissolved
organic nitrogen after release from senescent or decaying phytoplankton in the water column or
after being deposited to the sediments (Figure 17). Sesachacha Pond waters do contrast with
Nantucket and Madaket Harbor waters in the fraction of particulate organic nitrogen (PON) held
in suspended phytoplankton, consistent with the elevated phytoplankton biomass indicated by
the elevated total chlorophyll a levels in 2020 (see below).
The variation in pond openings can be seen in the variation in summer TN levels between years
(Figure 18). The variation in TN causes large interannual variations in total chlorophyll a, but
the elevated TN supports periodic blooms supporting the contention that the system is still
eutrophic (Figure 19). Chlorophyll a levels >10ug/L typically indicate eutrophic conditions
resulting from elevated nitrogen (which is also observed, Figure 17). However, as a variety of
factors modify phytoplankton levels established by nitrogen inputs, the variation in
phytoplankton levels show more variation than the TN levels from year to year. However, the
general pattern of openings to TN levels to overall water quality is clear.
30
Figure 17. TN, dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and
particulate organic nitrogen at stations throughout Sesachacha Pond 2020. Note that at all
stations the TN level (0.667 mg N/L) are nearly at the regulatory TMDL, indicating that this
system can achieve the threshold if annual flushing regime can be enhanced slightly.
Table 4. Summer average TN levels for Sesachacha Pond for comparison to its TMDL target of
0.60 mg/L. Note that while the openings 2010-2019 have not achieved the TMDL TN level, the
salt pond has generally had higher water quality than historically (MEP). 2020 TN levels are
approaching the threshold. Pond opening duration in 2020 maybe the driver of lower levels in
2020. Top value is concentration at Station 1. Value in () is average concentration (Stations
1,2,3,4).
MEP TN Threshold
SESA1 = <0.60 mg/l
Historical 2010 2012 2013 2014 2015 2016 2017 2018 2019 2020
MEP Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN Mean TN
Mean TN (mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)(mg/L)
Sesachacha Pond 1.197 0.078 0.684
(0.704)
0.678
(0.639)
0.714
(0.669)
0.919
(0.922)
0.918
(0.904)
1.01
(1.003)
0.960
(0.884)
0.734
(0.752)
0.833
(0.845)
0.667
(0.673)
Sampling Station Location s.d.
31
SESA1 = <0.60 mg/l
MEP TN Threshold
32
Figure 18. Total nitrogen levels by station over time in Sesachacha Pond, horizontal line is
TMDL TN level. Longer openings in 2010-2014 resulted in lower TN levels (<0.75 mg/L), recent
opening attempts are again being more successful, but TN levels remain above the target level.
2020 pond opening duration may have been longer than past years resulting in the lowest TN
level on record, half of the MEP 2003 average (Table 4).
Figure 19. Average Total Pigment (Chlorophyll-a +Pheophytin) concentrations by station in the
seasonally opened Sesachacha Pond system during the summer 2020 sampling season
compared to 2010, 2012- 2019. Total chlorophyll a pigment levels in 2020 appeared stable
across stations at ~11 ug/L indicative of moderate impairment, consistent with the TN level.
0
2
4
6
8
10
12
14
16
Ses-3 Ses-2 Ses-4 Ses-1
Ch
l
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+
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(
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)
Sesachacha Pond Total Pigment Level
2010 2012 2013 2014 2015
2016 2017 2018 2019 2020
33
Hummock Pond is a periodically opened drowned river valley pond that is managed by the
Nantucket Natural Resources Department. Due to coastal processes it is not possible to
maintain an open tidal inlet to this system or the other major coastal pond of Sesachacha Pond
(brackish) and Miacomet Pond (once brackish now freshwater). The general lack of flushing
makes these ponds particularly sensitive to nutrient inputs and the negative effects of
eutrophication. Managing openings of Hummock Pond has been particularly difficult due to
longshore transport and the location of the opened inlet relative to dominant wind direction and
storm flows. For this pond to be effectively opened analysis of weather and wave/wind direction
needs to be monitored. The Town NRD and SMAST has developed a plan to establish this
system for the periodically opened ponds program of Nantucket and are working jointly with the
Martha’s Vineyard Commission (who has the same problems) towards implementation. At
present, we have variable openings that generally do not sufficiently lower pond TN levels
before the inlet closes with resulting eutrophic conditions.
Hummock Pond has only a moderate level of watershed nitrogen inputs, but they have a
magnified effect, as the system only periodically receives limited tidal flushing (at most 2
times/yr). At present, Hummock Pond’s total nitrogen levels are well above its TMDL target
level at all stations (Figure 20). This is true for the main drowned river valley basin and its
tributary salt pond (Head of Hummock).
Although Hummock Pond remains above its TMDL target of 0.50 mg/L, the summer average TN
levels have varied over time. While the openings 2010-2020 have not achieved the TMDL TN
level, this salt pond has shown a range of water quality associated with the success of its tidal
openings and sometime storm overwash. Overall, the pond has an increasing level of TN
moving from near the barrier beach toward its headwaters (Figure 20). Head of Hummock does
not have good exchange with the main basin and sometimes functions as a separate eutrophic
basin.
Although the system is brackish due to its reduced salt water input, its nitrogen components are
similar to the tidally flushed estuaries in that, entering dissolved inorganic N (DIN) is rapidly
taken up by phytoplankton and converted to organic forms. The result is that almost all water
column nitrogen can be found in growing or senescent phytoplankton (PON) or as dissolved
organic nitrogen after release from senescent or decaying phytoplankton in the water column or
after being deposited to the sediments (Figure 20). Hummock Pond waters are similar to
Sesachacha Pond and contrast with Nantucket and Madaket Harbor waters in the large fraction
of particulate organic nitrogen (PON) held in suspended phytoplankton. This is consistent with
the elevated phytoplankton biomass indicated by the elevated total chlorophyll a levels in 2019
(see below).
The baseline TN level for this pond, (2010-2014 monitoring) was generally lower than over the
past 3-4 years, except for HUM-7 which have exceedingly levels in the baseline period (Figure
21). It appears that this recent period TN levels have been higher at most stations (HUM-1,3,8)
and at some stations (HUM-7) have been rising consistently from year to year from 2015 to
2019. It appears that after the successful opening prior to the 2015 field season, the tidal
flushing significantly lowered TN throughout Hummock Pond and reset the system to new lower
34
TN levels. However, following years were not as successful and the TN levels rose over time to
their new much higher (than 2015) levels. Apparently, (a) even a single very successful
opening can have a multiyear positive impact on Hummock Pond water quality and (b) it takes
multiple years after a solid opening for the system to be reset to higher TN levels. This
underscores that even a modest improvement in openings of Hummock Pond can play a critical
role in lowering eutrophication in this estuary.
The variation in pond openings can be seen in the variation in summer TN levels between years
(Figure 21). The variation in TN causes large interannual variations in total chlorophyll a, but
the elevated TN supports periodic blooms supporting the contention that the system is still
eutrophic (Figure 20). Chlorophyll a levels >10ug/L typically indicate eutrophic conditions
resulting from elevated nitrogen (which is also observed, Figure 22). Chlorophyll a levels in
Hummock Pond are generally well above 10 ug/L exceeding 30 ug/L over some summers
(2010, 2017-2020), matching the level of nitrogen enrichment throughout this estuary.
However, as a variety of factors modify phytoplankton levels established by nitrogen inputs, the
variation in phytoplankton levels show more variation than the TN levels from year to year. The
general pattern of openings to TN levels to overall water quality is clear with the high TN levels
in 2017-2019 (Figure 23) and highest chlorophyll a levels (Figure 22) and conversely the lowest
TN levels in 2014-2016 and more moderate chlorophyll a levels. 2020 had intermediate
chlorophyll levels consistent with its TN levels.
Overall, Hummock Pond remains eutrophic and is currently declining further. Unless nitrogen
mitigations become possible, it appears that improvement to flushing are the primary
mechanism to improving this ponds water quality and meeting its regulatory nitrogen TMDL.
The positive news is that new approaches are being developed and the Nantucket Monitoring
Program working with Nantucket NRD has demonstrated that periodic flush can make major
improvement to the health of this estuarine system. Monitoring has revealed a potential threat
to Hummock Pond habitats. In 2014-2016 salinities in the lower basin (HUM-1,3,5) were 6-7
ppt. But in 2017 salinity dropped to <4 ppt due a low flushing opening. The salinity as
increased slightl ~5 ppt from 2018-2020 and <4 ppt in the upper reaches (HUM-7 & 8). This is
still low for maintaining Hummock Pond as a brackish water estuary. Historically levels have
been~8 ppt, so there is some concern over species changes, however, salinity decline is
relatively slow so there is still time to reset the salinity.
35
Figure 20. TN, dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and
particulate organic nitrogen at stations along the main axis of Hummock Pond, with HUM-7
being the tributary deeper basin at the head of the estuary. Note that at all stations the TN level
greatly exceeds the regulatory TMDL (0.50 mg TN/L) and in 2020 the average of the lower
stations Hum-1,3,5 was 0.638 mg N/L, indicating that this system remains nitrogen impaired.
MEP TN Threshold
HUM-3 = 0.50 mg/l
MEP TN Threshold
HUM-3 = 0.50 mg/l
36
Figure 21. Summer average total nitrogen levels by station over time in Hummock Pond,
horizontal line is the TMDL TN level (0.50 mg TN/L). The long opening in 2015 lowered TN
levels (<0.65 mg/L), but recent opening attempts have been difficult and TN levels have risen
>0.75 mg/L well above the target level for high water quality. Lower TN in levels in 2020 maybe
the result of a longer opening duration.
Figure 22. Average Total Pigment (Chlorophyll-a +Pheophytin) concentrations by station in the
seasonally opened Hummock Pond system, during the summer 2020 sampling season
compared to 2010, 2012, 2013, 2014, 2015, 2016, 2017, 2018 and 2019. T-pigment levels in
2020 generally lower than 2019 consistent with lower TN levels.
0
10
20
30
40
50
60
70
Hum-7 Hum-8 Hum-5 Hum-3 Hum-1
Ch
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(
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Hummock Pond Total Pigment Level
2010 2012 2013 2014 2015
2016 2017 2018 2019 2020
37
Figure 23. Change in TN level in Hummock Pond main basin (HUM-1,3,5,7) and the Head of
Hummock basin (HUM-7) over the past decade grouped to reduce the variation to see if any
trends existed. Note that the 2014-2016 (best openings) showed the highest water quality
(lowest TN levels) throughout the pond, while the previous period and recent period had higher
TN levels. 2020 results are generally higher than the 2014-2016 period which relate to effective
pond openings.
Miacomet Pond was a periodically opened salt water pond managed by the Nantucket Natural
Resources Department (NRD). However, due to coastal processes it is not possible to maintain
an open tidal inlet to this system or the other major coastal ponds of Sesachacha Pond
(Brackish) and Hummock Pond (brackish). Although Miacomet Pond did have an illegal pond
opening in 2018, that opening did not result in signficiant tidal exchange and had an insignificant
effect on pond salinity. Due to its not experiencing any functional tidal exchange for over a
decade, Miacomet Pond is now functioning as a freshwater pond with freshwater salinities
throughout. As a fresh pond, Miacomet is highly eutrophic with periodic needs for nitrogen and
phosphorus management, with both N and P generally being key nutrients for management in
most years including 2020 (Table 5).
Nitrogen levels in Miacomet Pond in 2020 and 2019 were exceedingly elevated, >0.9 mg N/L,
indicative of a highly eutrophic pond (Figure 24). Target thresholds for estuaries to maintain a
high water and habitat quality are generally in the range of 0.35 – 0.50 mg N/L, less than half of
Miacomet Pond (Figure 24). Although this pond is not opened by the Town and has not had
any effective tidal exchange for over a decade, this salt pond has shown a range of TN levels
from year to year the cause of which is being investigated. Overall the pond has an increasing
level of TN moving from near the barrier beach toward its headwaters (Figure 24), a common
pattern if nutrients enter more into the headwaters than from the margins. Although the system
is fresh due to its lack of salt water input, its nitrogen components are similar to the brackish
water estuaries in that, entering dissolved inorganic N (DIN) is rapidly taken up by
0.000
0.500
1.000
1.500
2.000
2.500
Hummock Pond
TN concentration (mg/L)
TN (avg. 2010,12,13)TN (avg. 2014,15,16)TN (avg. 2017,18,19)TN (2020)
38
phytoplankton and converted to organic forms. The result is that almost all watercolumn
nitrogen can be found in growing or senescent phytoplankton (PON) or as dissolved organic
nitrogen after release from senescent or decaying phytoplankton in the watercolumn or after
being deposited to the sediments (Figure 24). Miacomet Pond waters are similar to Hummock
and Sesachacha Pond in the large fraction of particulate organic nitrogen (PON) held in
suspended phytoplankton. This is consistent with the elevated phytoplankton biomass indicated
by the elevated total chlorophyll a levels in 2019 (see below).
Since Miacomet Pond has not had any effective tidal exchange for over a decade the large
variations that take place from year to year are a bit unclear (Figure 25). One explanation being
investigated is associated with rainfall and groundwater recharge. In periods of high rainfall,
groundwater levels increase and watershed discharge (carrying nitrogen as nitrate) increases,
which would increase nitrogen levels in Miacomet Pond. The region has had variable recharge
over the past decade with wet and drought periods.
As part of the monitoring effort, total phosphorus is measured and in some years phosphorus is
also a nutrient of concern. In 2019 total phosphorus levels were: at MP-3 in the innermost
basin, 120 ug P/L (s.d.=82 ugP/L). At MP-2 in the mid basin, 69 ug P/L (s.d. = 30) and in the
basin nearest the barrier beach, 60 ug P/L (s.d.=27 ug/L). For Cape Cod ponds the target for a
high quality pond is a total phosphorus level very much lower than at present, <20 ug/L, based
upon either TN or TP levels, Miacomet Pond is highly eutrophic. Proper management of
Miacomet Pond must take into account both N and P and would benefit from a bioassay test to
determine which is the predominant limiting nutrient.
Phytoplankton biomass, as total chlorophyll a also varies widely from year to year and generally
exceeds the 10 ug/L level indicative of a nutrient enrich water body (Figure 26). Given the high
nitrogen and phosphorus levels throughout Miacomet Pond and prior indications that both
nutrients may periodically cause phytoplankton or algae blooms, at present management
actions should focus on both nutrients. However, if phosphorus control is implemented, it is
likely that the blooms would decline, since most freshwater systems have an overabundance of
nitrogen (like Miacomet) but low phosphorus levels. This approach requires some simple
feasibility study but may indicate a way forward for this eutrophic pond. Over the past decade
Miacomet Pond has supported degraded water quality and poor habitat health, it’s condition,
though variable is not improving and 2020 recorded the highest TN levels on record..
Table 5a - Nitrogen to phosphorous ratios from Miacomet Pond during the summer 2020
sampling season indicating that in 2020 the nutrient for managing eutrophication was
primarily phosphorus as and TN/TP ratios at all stations were <16, the Redfield ratio,
however DIN/DIP ratios at MP-1 and 2 were >16 indicating nitrogen is also a nutrient to
be managed.
39
Table 5b - Nitrogen to phosphorous ratios from Miacomet Pond during the summer 2019
sampling season indicating that in 2019 the nutrient for managing eutrophication was
primarily phosphorus as DIN/DIP and TN/TP ratios at all stations were <16, the Redfield
ratio.
Station Id.N/P PC/PN DIN/DIP TN/TP
(2020)organic particulate inorganic total
MP-3 21.8 6.4 11.6 2.5
MP-1 41.3 8.4 46.3 4.2
MP-2 37.2 6.2 48.0 9.8
Station Id.N/P PC/PN DIN/DIP TN/TP
(2019)organic particulate inorganic total
MP-3 16.2 9.9 3.7 2.6
MP-1 26.0 10.2 8.7 3.1
MP-2 35.3 9.2 9.4 6.8
0.00
0.50
1.00
1.50
2.00
2.50
MP3 MP1 MP2
N C
o
n
c
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t
r
a
t
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(
m
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/
L
)
Station Number
Miacomet Pond Nitrogen Gradient (2020)
TN
DIN
DON
PON
upstream downstream
40
Figure 24. TN, dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and
particulate organic nitrogen at stations along the main axis of Miacomet Pond, with MP2 in the
basin nearest the barrier beach. Note that at all stations the TN level greatly exceeds the
threshold for brackish and estuarine systems.
41
0.00
0.25
0.50
0.75
1.00
1.25
1.50
MP3 MP1 MP2
N C
o
n
c
e
n
t
r
a
t
i
o
n
(
m
g
/
L
)
Station Number
Miacomet Pond Total Nitrogen Gradient
(Avg 2010-2014, 2015, 2016, 2017, 2018, 2019,2020)
TN 2010-2014 TN 2015 TN 2016 TN 2017 TN 2018 TN 2019 TN 2020
upstream downstream
42
Figure 25. Summer average total nitrogen levels by station over time in freshwater Miacomet
Pond. As a freshwater pond a regulatory TMDL target for nitrogen or phosphorus has yet to be
established. 2020 recorded the highest TN for each station 2010-2020.
Figure 26. Average Total Pigment (Chlorophyll-a +Pheophytin) concentrations by station in the
Miacomet Pond system during the summer 2020 sampling season compared to 2010, 2012,
2013, 2014, 2015, 2016. 2017, 2018 and 2019. Miacomet Pond is not opened to the Atlantic
Ocean and now contains freshwater (<0.2 PSU). Total chlorophyll a pigment levels in 2020 were
close to average conditions but are indicative of continuing eutrophic conditions.
Trophic State of the Estuaries of Nantucket Island future:
0
10
20
30
40
50
60
MP-3 MP-1 MP-2
Ch
l
o
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p
h
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-a
+
P
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(
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Miacomet Pond Total Pigment Level
2010 2012 2013 2014 2015
2016 2017 2018 2019 2020
43
The Trophic State of an estuary is a quantitative indicator of its nutrient related
ecological health and is based on key ecological metrics: concentrations of inorganic
and organic Nitrogen, water clarity (Secchi Depth), lowest measured concentrations of
Dissolved Oxygen (average of lowest 20% of measurements), and Chlorophyll-a
pigments (surrogate for phytoplankton biomass/blooms). Nutrient related trophic health
scales generally range from Oligotrophic (healthy-low nutrient) to Mesotrophic (showing
some signs of deterioration of health due to nutrient enrichment) to Eutrophic (habitats
significantly impaired and degraded, high levels of nutrients and organic matter and
community shifts). The Trophic Health Index Score used here is a standard numerical
scale based on criteria for open water embayments and uses the above mentioned
measured parameters to create a habitat quality scale (Howes et al. 1999,
http://www.savebuzzardsbay.org). For the estuaries within the Town of Nantucket, a
trophic index score was calculated for each sampling location for each year (2010 and
2012- 2020) using the summer monitoring results. The Index scores were calculated in
2 ways, one which included the low dissolved oxygen for each year in the index ("with
DO", Table 6a) and one which excluded the oxygen metric ("without DO", Table 6b).
The reason for this dual approach is that in some estuaries, such as those on Nantucket,
there are only periodic depletions in bottom water dissolved oxygen, generally related to
meteorological events acting on nutrient enriched basins. While these short-term
depletions have important ecological consequences, they are difficult to capture in
programs that sample 4 or 5 dates per summer. In these cases, inclusion of the oxygen
can bias the Index upwards (i.e. higher quality) because of the greater probability of
capturing high versus low oxygen events (i.e. missing periodic low oxygen events). This
bias was found in the previous analysis of the 2010 dataset, as well as for other
estuaries in southeastern Massachusetts. However, this is not always the case and
there was no substantive “biasing high” between the "with DO" and "without DO" Index
scores based on the 2013 and 2014 data and again in 2016 and 2017 none-the-less, the
index analysis by both methods is presented for informational purposes herein (Tables
6a,6b). It should be noted that to the extent the bias does exist in a given year, it relates
only to the oxygen data, the other water quality parameters do not change as rapidly as
dissolved oxygen and therefore the sampling program adequately captures accurate
concentrations of nutrient related metrics (DO changes by the hour). Further analysis of
the with DO and without DO index values for all stations and monitoring years indicates
that there is very little difference in the determined index value between the methods
(RPD <10%; generally <5%). The exception was in Miacomet Pond and Head of
Hummock Pond which have significant DO depletions in some years and therefore
higher variability between the methods (RDP up to 20%). Given that inclusion of oxygen
data did not generally change the bay health rank, the index calculated by including DO
appears to be most appropriate based upon 10 years of data from Nantucket’s systems,
but both approaches are developed each year and presented herein.
For the present analysis the standard Index (with DO) was used for assessment and the
Health Status was determined for each site based on the data collected during the
sampling events. The ranges of Index scores that fall within a particular Health Status
44
determination are given below with the Index values and description for each monitoring
station. Figures 27-31 show the distribution of Health Status throughout each estuary
based on each of the 10 years of monitoring (2010, 2012-2020). For the location maps,
only the “with DO” index is shown as in 5 of the past 6 years the inclusion of DO made
no substantive difference in the index value and in 2015 the difference was minor.
Therefore, the Index maps shown for the “with DO” index are comparable to other
estuaries in the region for assessing Trophic Status. Numerical results in the tables are
color coded in the figures for ease of interpretation. The colors of each triangle
represent the Bay Health Index status of each site and follow the designation scheme
below:
Color Health Status
Blue High Quality
Blue/Yellow High-Moderate
Yellow Moderate
Yellow/Red Moderate/Fair
Red Fair/Poor
The integrated water quality scores, as represented by the Index were generally
consistent among all 10 years of monitoring. Although change at some sites was
observed, change was gradual and large inter-annual shifts were typically related to
major management actions. This relative stability is typical as nutrient related health
does not generally change rapidly unless a significant alteration has occurred to the
watershed nitrogen loading or to tidal flushing of a basin (e.g. Hummock Pond, Long
Pond, Sesachacha Pond). However, 4 systems do appear to show a potential shift in
nitrogen related health over the past 10 years with improvements in Hither Creek,
Sesachacha Pond and Long Pond and slight improvements in Nantucket Harbor seem
to be emerging (see below). However, Hummock Pond and Miacomet Pond are
impaired and stable. Based upon the results it is possible to assess the nutrient related
health of the basins within each of the 5 estuarine systems within the Nantucket Water
Quality Monitoring Program. The following assessments rely mainly on the Index "with
DO" scores as it appears to accurately represent current conditions:
High >69 Blue
High-Moderate 61-69 Blue-Yellow
Moderate 39-61 Yellow
Moderate-Fair 31-39 Yellow-Red
Fair-Poor <31 Red
45
Madaket Harbor
Madaket Harbor main basin in 2020 continued to support a high level of nutrient related water
quality as was observed over the prior 9 years. It has been the more enclosed basins of Hither
Creek and Long Pond with their reduced tidal flushing that have had nitrogen impairment
problems. Water quality generally changes gradually, unless there has been a major change in
loading or flushing. Within the Madaket Harbor/Long Pond watershed there has been a
significant change in the nitrogen sourced at the Town Landfill. The Landfill has recently been
undergoing management actions that reduce nitrogen loading to the groundwater, hence to
upper Long Pond. It appears that the long-term gradual reduction in TN levels within the upper
portions of this complex estuary is consistent with a lowering of watershed nitrogen loading.
Over the 10 years of monitoring, Hither Creek (Station 1), which receives discharge from Long
Pond via Madaket Ditch, has consistently supported the poorest “health” status within the
Madaket Estuary (Table 6a,b, Figure 29). Hither Creek is clearly nitrogen enriched and showing
continuing impairment based on a variety of parameters but has clearly improved since 2010.
The linkage between water quality in Hither Creek and Long Pond can be seen in the parallel
improvements over the past 5 years versus prior 5 years of monitoring. Over the past 10 years
the Index indicates that this basin has improved modestly, and is now supporting moderate
water quality TN level declined to pre-2017 levels in 2018 (0.41 mg/L) but went up slightly in
2019 (0.50 mg/L). It is expected that this variability will be reduced in coming years as the
system reaches a new equilibrium with the new lower landfill TN loading rate. The main basin
of Madaket Harbor is showing relatively high water quality in each year but also shows a
possible improvement from 2010 to 2012 and has been generally stable at high water quality in
more recent samplings to 2019. It appears that Station 2, near the outlet to Hither Creek can
receive low quality waters on the ebb tide from Hither Creek and that can modify water quality at
this nearshore location (e.g. 2010). But with improvements in the water quality in Hither Creek,
its effect on water quality at Station 2 in nearshore Madaket Harbor has been insufficient to
affect the Harbor waters, which have maintained high quality status since 2012. The offshore
sites (3 & 4) support high quality waters resulting from low nitrogen inputs and very high rates of
water exchange. The 9 year positive trend in health index is at least partially the result of the
reduced loading from the landfill to upper Long Pond and an improvement in the ebbing waters
through Madaket Ditch. This trend is consistent with the upper basin feeding Madaket Ditch. In
2014-2018 the previously observed gradient in water quality metrics between the 2 Long Pond
stations is only periodically detectable, lessening concern over a potential tidal restriction,
however, slightly higher TN and CHLA. levels observed in 2019 in Long Pond indicate that the
system does exhibit variability and careful management is still required. While a further analysis
of the mechanism underlying this periodic gradient would be useful, it is not needed at this time,
tracking the phenomenon through the on-going monitoring program should be sufficient. If the
improvement in Hither Creek becomes stabilized at the lower end of its range (0.8 – 0.6 mg/L)
and 2018 was 0.59 mg N/L (2019 TN 0.50 mg/L at station MH1), it is possible that the TMDL for
Madaket Harbor may require somewhat less nitrogen load reduction, making the success of
other nitrogen management actions (other than associated with the landfill and any needed
septic system work in Madaket) more certain. However, at this point it is likely that additional
nitrogen management will be required to meet the TMDL in Hither Creek as it remains of
46
moderate quality, although it appears the reduced load discharging from Madaket Ditch has
resulted in somewhat lower TN levels already. It should be noted that the “fair/poor" quality
water designation in Long Pond relates primarily to Long Pond being not supportive of eelgrass.
But the MEP and TMDL for Long Pond is based on infaunal animals living in the bottom
sediments (i.e. not eelgrass) so the water quality index should not cause concern. However,
additional analysis is required to determine if landfill activities will be sufficient to meet the TMDL
for Long Pond (see below). Additional study to clarify the drivers of the improvement
(controllable such as the landfill improvements or fertilizer management) will further assist in
determining the magnitude of septic system management as posited through the MEP analysis.
This further analytical effort is in progress in parallel with the Town’s actual and potential
management actions.
Long Pond
Long Pond is a large tributary basin to Madaket Harbor, which receives tidal flow through
the artificial connection of Madaket Ditch. Given the structure of the basin and its
watershed, Long Pond operates semi-independently from Madaket Harbor (Figure 29).
Unlike Madaket Harbor which is marine, Long Pond is a brackish water system resulting
from mixing of groundwater inflows and salt water entering through its restricted tidal
channel (2018 and 2019 salinity = 16 PSU). Long Pond’s Bay Health scores for both
stations (5 & 6) in the 8 years of monitoring (2010, 2012-2018) clearly indicate poor
nutrient related water quality and this continues to be the case in 2019. It is nearly
certain that the water quality of Hither Creek is partially dependent on the nitrogen load
from Long Pond via Madaket Ditch during the ebb tide. However, it should be noted that
the TMDL targets restoration of sediment animal communities (infauna) which do not
require the same high water quality as eelgrass.
Based upon the 2020 results and the 10 year time-series, it appears that the Town’s
management of the landfill, has reduced the nitrogen load from this source with the
associated observed lowering of TN levels. The connection is strengthened by the
parallel timing of the landfill work and the lowering of TN in the adjacent waters, which
are only a short travel time from the landfill. While still small, the water quality Index for
Long Pond is starting to improve in response to the lower TN levels although the basin
remains impaired. TN levels in 2015-2017 and 2018-2020 were almost half that of
historical and 2010 measurements although they still show inter-annual variations as
2019 TN values were slightly higher (0.63 mg/L TN) compared to 2018 (0.59 mg/L TN)
and 2020 TN levels were 0.502 – 0.55 mg N/L a marked improvement . While continued
monitoring will determine the final level of improvement as the system reaches a new
equilibrium, it does appear that the reduction in N loading is occurring with beneficial
effects. If TN levels stabilize at 2018-2020 levels, the TN target in the TMDL may be
achieved and in summer 2021 or 2022 a habitat and DO survey should be considered to
evaluate the removal of Long Pond from the MassDEP list of impaired waters It should
be noted that the lack of major change in the Health Index for Long Pond results in part
from the relative coarseness of the Index, where sometimes large index score changes
are required to change the Index value. The analysis of key metrics (Chlorophyll-a,
47
water clarity-Secchi and total nitrogen) individually do show improving water quality at
stations 5 and 6 in 2012-2020 compared to 2010 and in the MEP threshold analysis (see
analysis and figures above), however there was a slight decline at station 5 (fair-poor).
This may just be attributable to inter-annual variability and careful monitoring should
continue.
Nantucket Harbor
Nantucket Harbor with Madaket Harbor are presently supporting the highest water
quality of Nantucket's estuaries. In 2020 and 2019, all of the Nantucket and Polpis
Harbor stations were showing high water quality, slightly better than in 2017. Over the
past 7 years, the main basin of Nantucket Harbor has supported high quality waters, with
only a periodic small level of decline in the uppermost basin, Wauwinet basin (Figure
27). Wauwinet basin (station 3) had the highest average total nitrogen values for the
Harbor System in 2013 (0.415 mg/L) and 2015 (0.436 mg/L) consistent with its
designation as the surrogate for the sentinel station for the main basin and its
documented past eelgrass loss. It should be noted that in summer 2016, SMAST added
station 2A and established it as the official TMDL sentinel station for which the nitrogen
threshold was established (refer to Figure 2, TN 0.399 mg/L), along with station 3 in
order to meet TMDL compliance monitoring criteria. Summer 2017 saw a slightly lower
TN level at station NAN-2A (0.368 mg/L) and 2018 lower still (0.337 mg/L) but a slight
increase in 2019 (0.392 mg/L) and 2020 (0.383 mg N/L). Summers 2016-2019 generally
showed similar water quality in this basin as 2014 which showed improved chlorophyll-a
and TN levels versus prior years. The main driver of the poorer 2015 water quality was
a phytoplankton bloom in the upper Harbor, which was relatively large for Nantucket
Harbor but only moderate for more enriched estuaries in the region. While it is unclear if
blooms will become more commonplace in the future, the levels in 2016 reflected more
historic conditions and 2017/2018 again higher levels but lower in 2019. 2020
monitoring showed slightly elevated chlorophyll levels but still indicative of a productive
high quality system. However, other activities associated with the Harbor (additional
sewer hookups, jetty improvement and oyster aquaculture) should result in even lower
TN and chlorophyll levels and reduce the likelihood of phytoplankton blooms (>10 ug/L)
in the future.
A similar pattern was seen in the enclosed sub-basins of Polpis Harbor (East and West)
as in the main basins of Nantucket Harbor. Polpis Harbor basins which after showing
moderate impairment in 2010 and 2012 have been showing only low to no impairment in
recent years, suggesting some improvement over historic conditions. As in Wauwinet,
Polpis Harbor showed 2020-2015 (except for 2017) TN levels similar to 2010, 2012 and
2015, slightly higher than 2013 and 2014. Over the past 5 of 6 years TN levels have
been very stable in both East and West Polpis, except that in 2020 West Polpis had a
phytoplankton bloom and subsequent low DO event that lowered its Health Index Score
to moderate impairment for the first time in 8 years. This variation makes continued
monitoring essential to clarify any trends in water quality and linkages to stream nitrogen
discharges. Polpis Harbor did not show a phytoplankton bloom in 2015 and supported
48
only moderate-low phytoplankton biomass in 2015-2017, 2018 and 2019 (average <6
ug/L) and appears to have attained generally high water quality status from moderate
status in 2010 and 2012. The 2020 bloom in West Polpis raised the summer average to
8 ug/l, double that of East Polpis. While the cause is not certain, it may be related to the
surface water discharge into the head of this basin.
While the overall Nantucket Harbor System is generally supporting high quality waters,
the variability in the index in Wauwinet and Polpis basins should be monitored to
ascertain their long-term health and to determine the effectiveness of restoration effort
by the Town as it continues to move forward to meet the MassDEP TMDL for this
system. Overall, Nantucket Harbor appears to be relatively stable from year to year with
a gradual slight improvement with high index scores and higher level metrics that
support the contention that it is approaching its TMDL threshold. This is also partially
the case for the Polpis Harbor basins, although the 2020 water quality suggests that
surface water inflows maybe important to this basin. Sesachacha Pond
Sesachacha Pond is a closed coastal salt pond that has its water quality managed by
periodically breaching the barrier beach to open the basin to tidal exchange with the
adjacent Atlantic Ocean waters. This management action serves to flush out nutrients
and organic matter on the ebb tides and receive saline waters on the flood tides.
Sesachacha Pond was evaluated under the Massachusetts Estuaries Project and a
nitrogen threshold (0.60 mg/L) was established for restoration of this system.
Additionally, the MEP analysis recommended an additional mid-summertime opening (if
logistically possible) as part of the pond management strategy to enhance flushing of the
pond and improve water quality to reach the threshold without any need for
infrastructure. The water quality monitoring program in 2010, 2012 and 2013 showed
that the pond nitrogen levels were converging on the 0.60 mg/L total nitrogen threshold
established by the MEP. Total nitrogen (TN) levels dropped significantly from historical
levels of 1.20 mg/L to ~0.68 mg/L in 2010 and 2012 and 0.67 mg/L in 2013, with
associated improvements in the levels of water clarity and chlorophyll-a. However, with
limited openings in 2014-2017, TN rose to near historic levels, ~0.9 mg/L. Re-
evaluation by NRD using monitoring results has resulted in solid openings in recent
years with an improvement in 2018 (0.752 mg N/L) and 2020 (0.673 mg N/L) with
moderate restoration of overall water quality but with moderate continuing impaired
conditions. It appears that these changes relate to the quality of the pre-summer
opening. Given the 2010-2013 period when robust openings occurred, it appears that a
solid opening program continues to have the capability to improve the water quality
metrics pond-wide to levels near the TMDL nitrogen threshold.
Based upon the Index alone, changes in water quality in Sesachacha Pond over the
2010-2013 period were stabilized at moderate impairment of this estuary, with more
recent monitoring 2015-2017 seeing a trend toward poor water quality conditions with a
rebound in 2018 and 2020 where TN levels declined (Figure 28). In 2019, conditions
did worsen slightly compared to 2018 but the significant decline in 2020 is promising.
Additional higher level assessment of Sesachacha Pond initiated based on the 2010
49
monitoring results was conducted and confirmed that the pond was improving by 2013,
but was impaired in 2014 consistent with the monitoring results. 2020 showed the
highest Health Index values in the 10 year record with low-moderate chlorophyll,
unimpaired oxygen levels, and moderate clarity. It appears that like other periodically
opened ponds, the quality of the opening (amount of water exchanged) controls the level
of water quality in the following months. Fortunately, the data indicate that attaining
pond openings of the quality of 2012 and 2013 (done under Town supervision) in the
future may be sufficient to attain the TMDL for this system. In addition, the impaired but
improved conditions in 2018 and 2020, while high in TN (organic N component), did not
show bottom water D.O. depletion, and only moderate chlorophyll-a, consistent with the
MEP threshold target of 0.60 mg N/L. Similarly, while TN was higher in 2019 than 2018
and 2020, DO depletions were not apparent (4.7 to 7.8 mg/L, 20% low DO values). A
closer examination of the opening protocol and the linkage to resultant water quality is
needed for management of this system.
Hummock Pond
Hummock Pond is a closed coastal salt pond that is only periodically opened to the
ocean to flush out nutrients and organic matter on the ebb tide and receive low nutrient
saline waters on the flood tide. Creating sustained openings that are sufficient to allow
exchange of tidal waters for more than 4-5 days has been difficult for this system due to
its location on the coast and the large amount of sand migration in the coastal zone
which can rapidly reseal the inlet.
Hummock Pond is opened at a sufficient frequency to sustain salinity levels in the 4-8
ppt range, with only small inter-annual differences (2012 slightly higher than 2010). The
pond continues to support a small but clear salinity gradient from Station 1 nearest the
ocean to Station 7 in the uppermost basin (Head of Hummock). The present non-tidal
state and watershed nutrient inputs have resulted in moderate to poor nutrient related
water quality throughout the pond, with poor water quality conditions the present norm
(2005-2007, 2010, 2012, 2013, 2014, 2015). Unfortunately, in 2016 the pond appears to
have had lower water quality in its upper and mid reaches than in previous years,
although the lower basin did not show this inter-annual variation. Similarly, in 2017 and
2018 water quality declined further throughout most of the estuary, including the lower
basin. This resulted from the poor spring 2017 and 2018 opening which is reflected in
the very low salinities throughout summer 2017 (3.5 PSU), 2018 (4.8 PSU) and even
more so in 2019 (2.8-4.8 PSU). Fortunately, the 2020 opening was moderately improved
such that the salinity of the lower basin (HUM-1,3,5) again rose to 5.3 ppt. but Hummock
Pond is still slightly below its earlier salinity which is of concern for habitat stability. There
is generally a small gradient in water quality with moderate to poor conditions near the
ocean and poor conditions in the uppermost basins, but this collapsed to generally poor
water quality throughout the system in 2017-2019(Figure 31). However, with the slight
improvement in salinity there was also a modest improvement in water quality in 2020,
with a rise in Index Score. The uppermost basin Head of Hummock, Station 7, is
approaching fresh/brackish conditions (3.5-4.0 ppt in 2020 and recent years and as a
50
result is currently supporting mainly freshwater plant and animal habitats (note salinity
was 2.8 ppt in 2019). This basin is particularly eutrophic with phytoplankton blooms
periodically exceeding 70 ug/L, 26 ug/L in 2020 (offshore waters are ~2 ug/L). It
appears that the upper basin (Station 7), is supporting relatively stable salinities and
phytoplankton biomass with chlorophyll a averages from 2016-2020 ranging from 26 ug/l
to 14 ug/L. Head of Hummock appears to have been artificially connected to the
adjacent estuary and is the recipient of much of the freshwater inflow. It continues to be
one of the most highly eutrophic basins within the Town of Nantucket with oxygen
depletions to 3 mg/L (1.36 mg/L DO in 2019). All of the metrics are consistent with a
nutrient impaired basin in all years. It should be noted that the lower third of the
Hummock Pond Estuary is currently supporting impaired benthic animal habitat even
though conditions are the "best" in the overall impaired system.
Based upon the monitoring results it is clear that the nutrient related health of Hummock
Pond is significantly related to the success of its periodic openings. The individual
metrics and the Health Index for summer 2014 and 2015 appear to support that tidal
flushing was improved as nutrient related health was highest in 2014 and 2015 and
again in 2020 of the years monitored. It also appears that the continued successful inlet
openings from April 2014 into 2015 and 2020 have resulted in additional improvements
in water quality, with 2015 showing the lowest TN levels in records back to 2005,
although it is still above its threshold value to support high quality habitat. Unfortunately,
this pattern was not seen in the 2016-2019 water quality data, which showed further
decline in the upper and mid reaches, and even the lower basin. However, the rapid
improvement in 2020 suggests that water quality response is rapid to improved tidal
flushing. The opening program and associated monitoring around the openings and in
the summer will need to be used to set metrics for a “successful” opening, to produce a
simplified assessment protocol for opening success and to document and further refine
the opening protocol for the Town’s on-going program. To date this joint effort has
shown the potential for significant benefits to Hummock Pond water quality and
associated natural resources at low cost to the Town, but successful openings appear to
vary from year-to-year.
Miacomet Pond
Miacomet Pond is a closed coastal salt pond that is rarely opened to the ocean to flush
out nutrients and organic matter on the ebb tide and receive low nutrient saline waters
on the flood tide. As a result of the lack of tidal flow and continuous groundwater inputs,
the pond is presently freshwater, with salinity levels in each of the past 8 years of
monitoring of <0.5 ppt, and only 0.1 – 0.2 in 2019 and 2020 and 0.1 ppt, 2015-2017.
Clearly, Miacomet remains a very fresh water dominated system. The present non-tidal
state and extent of watershed nutrient inputs has resulted in a decline in nutrient related
water quality throughout the pond for both nitrogen and phosphorus, with poor water
quality and habitat impairment the present norm (Figure 30) and it remained that way in
the summer of 2019. This can be seen, for example, in the high chlorophyll levels
(2010: 12-50 ug/L); 2012: 10-20 ug/L; 2013: 20-26 ug/L; 2014: 23-70 ug/L; 2015: 38-53
51
ug/L) several times greater than the levels found in the high quality basins of Nantucket
and Madaket Harbors with a maximum in 2018 (25 ug/L) following highs in 2017 and
2016 (mean = ~14 ug/L). Summer 2019 chlorophyll levels ranged between 11-16 ug/L
but 2020 again rose to 20-24 ug/L levels. All of the metrics for Miacomet Pond are
consistent with a highly nutrient impaired basin. As previously discussed in previous
years, as the freshening of this basin has become complete and sustained, it likely will
have to be managed as a transitional freshwater system and will need to be reassessed
as such. As salt ponds freshen and become fresh ponds the nutrient causing
eutrophication can shift to phosphorus from nitrogen or become both nitrogen and
phosphorus (seasonally varying nutrient limitation). Since Miacomet Pond may have
storm over-wash in the future due to climate change related storm intensification and
sea level rise, it may be necessary for management to create both a nitrogen and a
phosphorus budget for this system and to conduct short-term incubations to determine
which nutrient is controlling pond health under present and varying salinity conditions.
None-the-less, phosphorus should be a part of pond management, with phosphorus
limitation to phytoplankton growth throughout Miacomet Pond in 2016-2020. While
nitrogen is also supporting eutrophication in this system, it is likely that phosphorus
controls alone may create a significant improvement and should be evaluated as a cost-
effective management action.
Another challenge in managing Miacomet Pond is that it will be difficult for the Pond to
maintain itself as a purely freshwater system as storm over-wash and rising sea level (as
well as increasing storm intensity and frequency related to climate change) will tend to
periodically cause seawater intrusion into its lower basin. An analysis of future
conditions for Miacomet Pond as sea level rises may be in order in the near future, as
remediation is considered. But at present the system is a highly nutrient impaired aquatic
system with poor water quality. It would be prudent for the Town to continue the
development of a management plan that takes into account not only the nutrient related
impairment of Miacomet Pond, but also accommodates the likely shifting between fresh
and salt water over the long term.
Recommendations for Future Monitoring:
Each year after review of a given years monitoring data and comparison to the long-term record,
the Island-Wide Monitoring Program puts forward recommendations to the NRD for the on-going
effort. As this is a long-term program many of the prior issues and additional data collection
efforts have been addressed. In 2020, specific points are raised based on the long-term data
and overall restoration effort :
• Salinity levels in Hummock Pond have slowly declined to <5.5 ppt in the lower basin
(2028-2020) and <4 ppt in the upper reaches in 2020. These levels are of concern as
they may result in species changes. These levels are an improvment over 2017, but
need to be tracked closely with the openings.
52
• Both N and P may be causing the highly eutrophic conditions in Miacomet Pond. It
would be useful for management planning to determine their relative importance. Since
P is much easier to control than N, the results might suggest targeted management to
improve habitat health. It appears that Miacomet Pond will not improve without
management action.
• Improving water quality in Nantucket Harbor and Long Pond due to Town efforts should
be noted to the Town.
• A close examination of Sesachacha Pond opening and improving water quality needs to
be conducted,
• The role of Long Pond water quality changes on Hither Creek could benefit from a re-run
of the MEP models.
Overall, the monitoring program is performing as designed and producing the needed high
quality data for management. Coordination between NRD and SMAST is at a high level due in
no small part to the efforts of NRD Staff.
53
Table 6a. 2020 Trophic Health Index Scores and status for water quality monitoring stations in
Nantucket estuaries based upon open water embayment (not salt marsh) habitat quality scales.
Index calculated with Dissolved Oxygen data (described in Howes et. al., 1999 at
www.savebuzzardsbay.org)
(With DO)
2020 Low20%2020
EMBAYMENT Secchi Oxsat DIN TON T-Pig EUTRO Health Status
SCORE SCORE SCORE SCORE SCORE Index
HUM7 41.8 10.4 0.0 0.0 0.0 10.4 Fair-Poor
HUM8 32.1 73.9 100.0 0.0 0.0 41.2 Moderate
HUM5 39.1 77.6 100.0 0.0 0.0 43.3 Moderate
HUM3 45.1 95.2 100.0 1.9 14.5 51.4 Moderate
HUM1 53.4 87.9 100.0 0.0 30.1 54.3 Moderate
LONG6 39.5 41.6 84.3 28.8 62.2 51.3 Moderate
LONG5 35.5 10.6 100.0 13.1 28.6 37.6 Moderate-Fair
HC1 NA NA NA NA NA NA
HC2 NA NA NA NA NA NA
HC3 NA NA NA NA NA NA
MH1 61.9 56.3 61.2 30.7 16.0 45.2 Moderate
MH2 78.4 83.3 86.6 74.9 73.5 79.3 High
MH3 84.6 70.0 100.0 98.0 78.6 86.2 High
MH4 100.0 86.6 100.0 100.0 99.4 97.2 High
MP3 41.3 0.0 0.0 0.0 0.0 8.3 Fair-Poor
MP1 55.2 0.0 0.0 0.0 0.0 11.0 Fair-Poor
MP2 65.2 51.2 10.1 0.0 0.0 25.3 Fair-Poor
NAN1 100.0 71.1 100.0 100.0 53.6 84.9 High
NAN2 92.3 73.4 93.9 77.4 53.7 78.2 High
NAN2A 92.8 49.2 77.5 72.0 47.8 67.9 High-Moderate
NAN3 88.1 69.2 100.0 63.0 50.9 74.3 High
NAN4 100.0 80.6 100.0 73.8 68.5 84.6 High
NAN5 63.8 44.3 98.1 52.8 21.0 56.0 Moderate
NAN6 85.4 68.4 86.5 73.5 69.2 76.6 High
NAN7 66.2 80.6 99.1 94.5 72.1 82.5 High
NAN8N 46.8 82.9 100.0 100.0 66.1 79.2 High
ST4 100.0 100.0 97.8 0.0 0.0 59.6 Moderate
ST6
ST6B 0.0 100.0 90.4 0.0 0.0 38.1 Moderate-Fair
WPH OUTLET 100.0 100.0 86.5 0.0 0.0 57.3 Moderate
SESA3 60.1 92.6 100.0 0.0 85.5 67.6 High-Moderate
SESA2 64.2 71.4 100.0 0.0 55.1 58.2 Moderate
SESA4 48.6 83.7 100.0 0.0 77.5 62.0 High-Moderate
SESA1 62.5 82.9 95.2 0.0 59.3 60.0 Moderate
ORS2 43.1 73.0 78.1 90.8 88.8 74.8 High
ORS4 21.6 5.7 58.1 69.8 5.4 32.1 Moderate-Fair
ORS6 37.7 6.2 77.5 46.8 58.9 45.4 Moderate
Station Average 2020 (With DO)
54
Table 6b. 2020 Trophic Health Index Scores and status for water quality monitoring stations in Nantucket estuaries based upon
open water embayment (not salt marsh) habitat quality scales. Index calculated with and without Dissolved Oxygen data.
Station Average 2020 (with DO)Station Average 2020 (with DO)Station Average 2020 (with DO)With DO No DO With DO No DO
2020 %Sat 2020 Low20%2020 2020 2020 2020
Salinity EMBAYMENT Secchi m DO DIN TON T-Pig Secchi Oxsat DIN TON T-Pig EUTRO Health Status EUTRO Health Status EUTRO EUTRO
ppt YR All 20%ppm ppm ug/L SCORE SCORE SCORE SCORE SCORE Index Index Colors Colors
3.57 HUM7 2020 1.18 44%0.154 1.306 26.66 41.8 10.4 0.0 0.0 0.0 10.4 Fair-Poor 10.4 Fair-Poor Red Red
3.90 HUM8 2020 1.01 73%0.008 0.749 11.74 32.1 73.9 100.0 0.0 0.0 41.2 Moderate 33.0 Moderate-Fair Yellow Yellow-Red
4.55 HUM5 2020 1.13 75%0.007 0.669 13.12 39.1 77.6 100.0 0.0 0.0 43.3 Moderate 34.8 Moderate-Fair Yellow Yellow-Red
5.44 HUM3 2020 1.24 87%0.008 0.591 8.40 45.1 95.2 100.0 1.9 14.5 51.4 Moderate 40.4 Moderate Yellow Yellow
5.80 HUM1 2020 1.42 82%0.013 0.626 6.96 53.4 87.9 100.0 0.0 30.1 54.3 Moderate 45.9 Moderate Yellow Yellow
18.41 LONG6 2020 1.13 56%0.020 0.482 4.73 39.5 41.6 84.3 28.8 62.2 51.3 Moderate 53.7 Moderate Yellow Yellow
16.71 LONG5 2020 1.06 44%0.010 0.543 7.09 35.5 10.6 100.0 13.1 28.6 37.6 Moderate-Fair 44.3 Moderate Yellow-Red Yellow
HC1 2020 #N/A #N/A #N/A #N/A #N/A #N/A #N/A
HC2 2020 #N/A #N/A #N/A #N/A #N/A #N/A #N/A
HC3 2020 #N/A #N/A #N/A #N/A #N/A #N/A #N/A
29.06 MH1 2020 1.63 63%0.034 0.475 8.25 61.9 56.3 61.2 30.7 16.0 45.2 Moderate 42.5 Moderate Yellow Yellow
31.26 MH2 2020 2.12 79%0.019 0.339 4.13 78.4 83.3 86.6 74.9 73.5 79.3 High 78.4 High Blue Blue
31.63 MH3 2020 2.34 71%0.011 0.284 3.88 84.6 70.0 100.0 98.0 78.6 86.2 High 90.3 High Blue Blue
31.73 MH4 2020 3.30 81%0.013 0.258 3.02 100.0 86.6 100.0 100.0 99.4 97.2 High 99.9 High Blue Blue
0.10 MP3 2020 1.17 24%0.277 0.864 19.73 41.3 0.0 0.0 0.0 0.0 8.3 Fair-Poor 10.3 Fair-Poor Red Red
0.10 MP1 2020 1.46 32%0.158 1.340 19.64 55.2 0.0 0.0 0.0 0.0 11.0 Fair-Poor 13.8 Fair-Poor Red Red
0.10 MP2 2020 1.71 61%0.111 1.147 24.33 65.2 51.2 10.1 0.0 0.0 25.3 Fair-Poor 18.8 Fair-Poor Red Red
31.58 NAN1 2020 3.23 71%0.009 0.251 5.24 100.0 71.1 100.0 100.0 53.6 84.9 High 88.4 High Blue Blue
31.84 NAN2 2020 2.65 73%0.016 0.333 5.24 92.3 73.4 93.9 77.4 53.7 78.2 High 79.3 High Blue Blue
31.91 NAN2A 2020 2.67 60%0.024 0.347 5.62 92.8 49.2 77.5 72.0 47.8 67.9 High-Moderate 72.5 High Blue-Yellow Blue
31.84 NAN3 2020 2.48 70%0.012 0.371 5.42 88.1 69.2 100.0 63.0 50.9 74.3 High 75.5 High Blue Blue
31.60 NAN4 2020 3.17 77%0.009 0.342 4.39 100.0 80.6 100.0 73.8 68.5 84.6 High 85.6 High Blue Blue
31.26 NAN5 2020 1.68 57%0.015 0.401 7.76 63.8 44.3 98.1 52.8 21.0 56.0 Moderate 58.9 Moderate Yellow Yellow
31.62 NAN6 2020 2.37 70%0.019 0.343 4.34 85.4 68.4 86.5 73.5 69.2 76.6 High 78.7 High Blue Blue
31.41 NAN7 2020 1.74 77%0.014 0.292 4.20 66.2 80.6 99.1 94.5 72.1 82.5 High 83.0 High Blue Blue
31.28 NAN8N 2020 1.28 78%0.006 0.274 4.51 46.8 82.9 100.0 100.0 66.1 79.2 High 78.2 High Blue Blue
0.00 ST4 2020 ND ND 0.015 0.940 NS 100.0 100.0 97.8 0.0 0.0 59.6 Moderate 49.5 Moderate Yellow Yellow
0.10 ST6 2020 0.10 ND 0.023 0.470 NS
0.05 ST6B 2020 0.25 ND 0.017 1.005 NS 0.0 100.0 90.4 0.0 0.0 38.1 Moderate-Fair 22.6 Fair-Poor Yellow-Red Red
0.10 WPH OUTLET 2020 ND ND 0.019 1.087 NS 100.0 100.0 86.5 0.0 0.0 57.3 Moderate 46.6 Moderate Yellow Yellow
6.86 SESA3 2020 1.58 85%0.007 0.679 3.57 60.1 92.6 100.0 0.0 85.5 67.6 High-Moderate 61.4 High-Moderate Blue-Yellow Blue-Yellow
6.86 SESA2 2020 1.69 71%0.009 0.656 5.15 64.2 71.4 100.0 0.0 55.1 58.2 Moderate 54.8 Moderate Yellow Yellow
6.88 SESA4 2020 1.31 79%0.005 0.669 3.93 48.6 83.7 100.0 0.0 77.5 62.0 High-Moderate 56.5 Moderate Blue-Yellow Yellow
6.87 SESA1 2020 1.64 78%0.016 0.651 4.90 62.5 82.9 95.2 0.0 59.3 60.0 Moderate 54.2 Moderate Yellow Yellow
31.65 ORS2 2020 1.20 72%0.023 0.300 3.43 43.1 73.0 78.1 90.8 88.8 74.8 High 75.2 High Blue Blue
31.00 ORS4 2020 0.85 42%0.037 0.353 9.37 21.6 5.7 58.1 69.8 5.4 32.1 Moderate-Fair 38.7 Moderate-Fair Yellow-Red Yellow-Red
30.63 ORS6 2020 1.10 42%0.024 0.420 4.92 37.7 6.2 77.5 46.8 58.9 45.4 Moderate 55.2 Moderate Yellow Yellow
55
Figure 27. Nantucket Harbor Eutrophication Index 2010 (top triangle) and 2020 (bottom triangle). Index was calculated with
dissolved oxygen. Colors indicate High (Blue), Moderate (Yellow), Fair/Poor (Red) nutrient related water quality. Station Nan-2A MEP
Sentinel Station.
NAN 5-
NAN 7
(Childrens Beach)
NAN 4
NAN 3
NAN 6
NAN 2
-
NAN 8
The Cut) 2010
NAN 1
Nantucket Harbor
Eutro. Index (2010,12,13,14,15,16,17,18,19, 20)
Color Health Status
Blue High Quality
Blue/Yellow High-Moderate
Yellow Moderate
Yellow/Red Moderate/Fair
Red Fair/Poor
NAN 8N
(2012,13,14,15,16,17,18,19,20)
NAN-2A (2016)
MEP Sentinel Station
Bay Health Index
Top 2010
Mid 2015
Mid 2013
Mid 2012
Mid 2014
Mid 2016
Mid 2017
Mid 2018
Mid 2019
Btm 2020
56
Figure 28. Sesachacha Pond Eutrophication Index 2010 (top triangle) and 2020 (bottom
triangle). Index was calculated with dissolved oxygen. Colors indicate High (Blue), Moderate
(Yellow), Fair/Poor (Red) nutrient related water quality.
Station 2
Station 3
Station 4
Station 1
Sesachacha Pond
Eutro. Index (2010,12,13,14,15,16,17,18,19, 20)
Color Health Status
Blue High Quality
Blue/Yellow High-Moderate
Yellow Moderate
Yellow/Red Moderate/Fair
Red Fair/Poor
Bay Health Index
Top 2010
Mid 2015
Mid 2013
Mid 2012
Mid 2014
Mid 2016
Mid 2017
Mid 2018
Mid 2019
Btm 2020
57
Figure 29. Madaket Harbor Eutrophication Index 2010 (top triangle) and 2020 (bottom triangle). Index was calculated with dissolved
oxygen. Colors indicate High (Blue), Moderate (Yellow), Fair/Poor (Red) nutrient related water quality.
Station 4
Station 3
Station 1
Station 5
Station 6
Station 2
Madaket Harbor and Long Pond
Eutro. Index (2010,12,13,14,15,16,17,18,19,20)
Color Health Status
Blue High Quality
Blue/Yellow High-Moderate
Yellow Moderate
Yellow/Red Moderate/Fair
Red Fair/Poor
Bay Health Index
Top 2010
Mid 2015
Mid 2013
Mid 2012
Mid 2014
Mid 2016
Mid 2017
Mid 2018
Mid 2019
Btm 2020
58
Figure 30. Miacomet Pond Eutrophication Index 2010 (top triangle) and 2020 (bottom triangle). Index was calculated with dissolved
oxygen. Colors indicate High (Blue), Moderate (Yellow), Fair/Poor (Red) nutrient related water quality.
Miacomet Pond
Eutrophication Index
(2010,12,13,14,15,
16,17,18,19,20)
Color Health Status
Blue High Quality
Blue/Yellow High-Moderate
Yellow Moderate
Yellow/Red Moderate/Fair
Red Fair/Poor
MP 3
MP 2
MP 1
Bay Health Index
Top 2010
Mid 2015
Mid 2013
Mid 2012
Mid 2014
Mid 2016
Mid 2017
Mid 2018
Mid 2019
Btm 2020
59
Figure 31. Hummock Pond Eutrophication Index 2010 (top triangle) and 2020 (bottom triangle).
Index was calculated with dissolved oxygen. Colors indicate High (Blue), Moderate (Yellow),
Fair/Poor (Red) nutrient related water quality.
Station 3
Station 1
Station 5
Station 8
Station 7
Station 2
Station 4
Station 6
Hummock Pond
Eutrophication Index
(2010,12,13,14,15,16,17,18,19,20)
Color Health Status
Blue High Quality
Blue/Yellow High-Moderate
Yellow Moderate
Yellow/Red Moderate/Fair
Red Fair/Poor
Station 9
Bay Health Index
Top 2010
Mid 2015
Mid 2013
Mid 2012
Mid 2014
Mid 2016
Mid 2017
Mid 2018
Mid 2019
Btm 2020