Loading...
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 2 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 3 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 4 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 5 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 6 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 7 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 8 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: 9 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 10 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) 11 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 o r o p h y l l -a + P h e o ( u g / L ) 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 o r o p h y l l -a + P h e o ( u g / L ) 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 l o r o p h y l l -a + P h e o ( u g / L ) 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 e n t r a t i o n ( m g / 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 r o p h y l l -a + P h e o ( u g / L ) 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