HomeMy WebLinkAboutMadaket Harbor and Long Pond Estuaries Project Executive Summary_201401230916392606Executive Summary 1
Massachusetts Estuaries Project
Linked Watershed-Embayment Model
to Determine Critical Nitrogen Loading
Thresholds for Madaket Harbor and Long Pond
Nantucket, Massachusetts
Executive Summary
1. Background
This report presents the results generated from the implementation of the Massachusetts
Estuaries Project’s Linked Watershed-Embayment Approach to the Madaket Harbor and Long
Pond embayment system, a complex coastal embayment of the Island of Nantucket within the
Town of Nantucket, Massachusetts. Analyses of the Madaket Harbor / Long Pond embayment
system was performed to assist the Town with up-coming nitrogen management decisions
associated with the Towns’ current and future wastewater planning efforts, as well as wetland
restoration, anadromous fish runs, shell fishery, open-space, and harbor maintenance
programs. As part of the MEP approach, habitat assessment was conducted on the
embayment based upon available water quality monitoring data, historical changes in eelgrass
distribution, time-series water column oxygen measurements, and benthic community structure.
Nitrogen loading thresholds for use as goals for watershed nitrogen management are the major
product of the MEP effort. In this way, the MEP offers a science-based management approach
to support the Town of Nantucket resource planning and decision-making process. The primary
products of this effort are: (1) a current quantitative assessment of the nutrient related health of
the Madaket Harbor / Long Pond embayment, (2) identification of all nitrogen sources (and their
respective N loads) to embayment waters, (3) nitrogen threshold levels for maintaining
Massachusetts Water Quality Standards within embayment waters, (4) analysis of watershed
nitrogen loading reduction to achieve the N threshold concentrations in embayment waters, and
(5) a functional calibrated and validated Linked Watershed-Embayment modeling tool that can
be readily used for evaluation of nitrogen management alternatives (to be developed by the
Town) for the protection of Madaket Harbor and restoration of Hither Creek and Long Pond.
Wastewater Planning: As increasing numbers of people occupy coastal watersheds, the
associated coastal waters receive increasing pollutant loads. Coastal embayments throughout
the Commonwealth of Massachusetts (and along the U.S. eastern seaboard) are becoming
nutrient enriched. The elevated nutrients levels are primarily related to the land use impacts
associated with the increasing population within the coastal zone over the past half-century.
Massachusetts
Department of
Environmental
Protection
Executive Summary 2
The regional effects of both nutrient loading and bacterial contamination span the
spectrum from environmental to socio-economic impacts and have direct consequences to the
culture, economy, and tax base of Massachusetts’s coastal communities. The primary nutrient
causing the increasing impairment of our coastal embayments is nitrogen, with its primary
sources being wastewater disposal, and nonpoint source runoff that carries nitrogen (e.g.
fertilizers) from a range of other sources. Nitrogen related water quality decline represents one
of the most serious threats to the ecological health of the nearshore coastal waters. Coastal
embayments, because of their shallow nature and large shoreline area, are generally the first
coastal systems to show the effect of nutrient pollution from terrestrial sources.
In particular, the Madaket Harbor / Long Pond embayment system within the Town of
Nantucket is at risk of eutrophication (over enrichment) from enhanced nitrogen loads entering
through groundwater from the increasingly developed watershed to this coastal system.
Eutrophication is a process that occurs naturally and gradually over a period of tens or hundreds
of years. However, human-related (anthropogenic) sources of nitrogen may be introduced into
ecosystems at an accelerated rate that cannot be easily absorbed, resulting in a phenomenon
known as cultural eutrophication. In both marine and freshwater systems, cultural
eutrophication results in degraded water quality, adverse impacts to ecosystems, and limits on
the use of water resources.
The relatively pristine nature of Nantucket's nearshore and Harbor waters has historically
been a valuable asset to the island. However, concern over the potential degradation of Harbor
water quality began to arise, which resulted in monitoring, scientific investigations and
management planning which continues to this day. Madaket Harbor is one of the largest
enclosed bays in southeastern Massachusetts and one of the few with a relatively high water
quality capable of supporting significant high quality ecological habitats, such as eelgrass beds.
Ironically, it is the pristine nature of this system which may indirectly threaten its ecological
health as the coastal waters throughout Southeastern New England become increasingly
degraded and the pressure for access and development of remaining high quality environments
increases. The Town of Nantucket and work groups have long ago recognized that a rigorous
scientific approach yielding site-specific nitrogen loading targets was required for decision-
making, alternatives analysis and ultimately, habitat protection. The completion of this multi-
step process has taken place under the programmatic umbrella of the Massachusetts Estuaries
Project, which is a partnership effort between all MEP collaborators and the Town. The
modeling tools developed as part of this program provide the quantitative information necessary
for the Towns’ nutrient management groups to predict the impacts on water quality from a
variety of proposed management scenarios.
Nitrogen Loading Thresholds and Watershed Nitrogen Management: Realizing the need
for scientifically defensible management tools has resulted in a focus on determining the aquatic
system’s assimilative capacity for nitrogen. The highest-level approach is to directly link the
watershed nitrogen inputs with embayment hydrodynamics to produce water quality results that
can be validated by water quality monitoring programs. This approach when linked to state-of-
the-art habitat assessments yields accurate determination of the “allowable N concentration
increase” or “threshold nitrogen concentration”. These determined nitrogen concentrations are
then directly relatable to the watershed nitrogen loading, which also accounts for the spatial
distribution of the nitrogen sources, not just the total load. As such, changes in nitrogen load
from differing parts of the embayment watershed can be evaluated relative to the degree to
which those load changes drive embayment water column nitrogen concentrations toward the
“threshold” for the embayment system. To increase certainty, the “Linked” Model is
independently calibrated and validated for each embayment.
Executive Summary 3
Massachusetts Estuaries Project Approach: The Massachusetts Department of
Environmental Protection (DEP), the University of Massachusetts – Dartmouth School of Marine
Science and Technology (SMAST), and others including the Cape Cod Commission (CCC)
have undertaken the task of providing a quantitative tool to communities throughout
southeastern Massachusetts (the Linked Watershed-Embayment Management Model) for
nutrient management in their coastal embayment systems. Ultimately, use of the Linked
Watershed-Embayment Management Model tool by municipalities in the region results in
effective screening of nitrogen reduction approaches and eventual restoration and protection of
valuable coastal resources. The MEP provides technical guidance in support of policies on
nitrogen loading to embayments, wastewater management decisions, and establishment of
nitrogen Total Maximum Daily Loads (TMDLs). A TMDL represents the greatest amount of a
pollutant that a waterbody can accept and still meet water quality standards for protecting public
health and maintaining the designated beneficial uses of those waters for drinking, swimming,
recreation and fishing. The MEP modeling approach assesses available options for meeting
selected nitrogen goals that are protective of embayment health and achieve water quality
standards.
The core of the Massachusetts Estuaries Project analytical method is the Linked
Watershed-Embayment Management Modeling Approach, which links watershed inputs with
embayment circulation and nitrogen characteristics.
The Linked Model builds on well-accepted basic watershed nitrogen loading approaches
such as those used in the Buzzards Bay Project, the CCC models, and other relevant models.
However, the Linked Model differs from other nitrogen management models in that it:
requires site-specific measurements within each watershed and embayment;
uses realistic “best-estimates” of nitrogen loads from each land-use (as opposed to loads
with built-in “safety factors” like Title 5 design loads);
spatially distributes the watershed nitrogen loading to the embayment;
accounts for nitrogen attenuation during transport to the embayment;
includes a 2D or 3D embayment circulation model depending on embayment structure;
accounts for basin structure, tidal variations, and dispersion within the embayment;
includes nitrogen regenerated within the embayment;
is validated by both independent hydrodynamic, nitrogen concentration, and ecological data;
is calibrated and validated with field data prior to generation of “what if” scenarios.
The Linked Model Approach’s greatest assets are its ability to be clearly calibrated and
validated, and its utility as a management tool for testing “what if” scenarios for evaluating
watershed nitrogen management options.
For a comprehensive description of the Linked Model, please refer to the Full Report:
Nitrogen Modeling to Support Watershed Management: Comparison of Approaches and
Sensitivity Analysis, available for download at
http://www.mass.gov/dep/water/resources/coastalr.htm. A more basic discussion of the Linked
Model is also provided in Appendix F of the Massachusetts Estuaries Project Embayment
Restoration Guidance for Implementation Strategies, available for download at
http://www.mass.gov/dep/water/resources/coastalr.htm. The Linked Model suggests which
management solutions will adequately protect or restore embayment water quality by enabling
Executive Summary 4
towns to test specific management scenarios and weigh the resulting water quality impact
against the cost of that approach. In addition to the management scenarios modeled for this
report, the Linked Model can be used to evaluate additional management scenarios and may be
updated to reflect future changes in land-use within an embayment watershed or changing
embayment characteristics. In addition, since the Model uses a holistic approach (the entire
watershed, embayment and tidal source waters), it can be used to evaluate all projects as they
relate directly or indirectly to water quality conditions within its geographic boundaries. Unlike
many approaches, the Linked Model accounts for nutrient sources, attenuation, and recycling
and variations in tidal hydrodynamics and accommodates the spatial distribution of these
processes. For an overview of several management scenarios that may be employed to restore
embayment water quality, see Massachusetts Estuaries Project Embayment Restoration
Guidance for Implementation Strategies, available for download at
http://www.mass.gov/dep/water/resources/coastalr.htm.
Application of MEP Approach: The Linked Model was applied to the Madaket Harbor / Long
Pond embayment system by using site-specific data collected by the MEP and water quality
data from the Water Quality Monitoring Program conducted by the Nantucket Marine
Department, with technical guidance from the Coastal Systems Program at SMAST (see
Section II). Evaluation of upland nitrogen loading was conducted by the MEP. Estuaries
Project staff obtained digital parcel and tax assessors data from the Town of Nantucket
Geographic Information Systems Department, watershed specific water use data from the
Wannacomet Water Company (WWC) and watershed boundaries adopted by the town as the
Harbor Watershed Protection District (http://www.nantucket-ma.gov). During the development
of the Nantucket Water Resources Management Plan, an island-wide groundwater mapping
project, using many of the USGS wells on the Island, was completed to characterize the water
table configuration of Nantucket (Horsley, Whittan, Hegeman, 1990). Estuary watershed
delineations completed in areas with relatively transmissive sand and gravel deposits, like most
of Cape Cod and the Islands, have shown that watershed boundaries are usually better defined
by elevation of the groundwater and its direction of flow, rather than by land surface topography
(Cambareri and Eichner 1998, Millham and Howes 1994a,b). This approach was used by
Horsley, Whittan and Hegeman, Inc. (HWH) to complete a watershed delineation for Madaket
Harbor (Section III); this watershed delineation was been largely confirmed by subsequent water
table characterizations (e.g., Lurbano, 2001, Gardner and Vogel, 2005). MEP staff compared
the HWH Harbor watershed to a 2004 aerial base map. This comparison found some slight
discrepancies likely based on a better characterization of the shoreline; changes were made
based on best professional judgment and watershed/water table characterization experience in
similar geologic settings. The watershed to Madaket Harbor has been adopted in the town
zoning bylaws as the Madaket Harbor Watershed Protection District.
(http://www.nantucket-ma.gov/Pages/NantucketMA_IT/gismapsfolder/madaketharborwpd.pdf).
The land-use data obtained from the Town was used to determine watershed nitrogen
loads within the Madaket Harbor embayment system and each of the systems sub-embayments
as appropriate (current and build-out loads are summarized in Section IV). Water quality within
a sub-embayment is the integration of nitrogen loads with the site-specific estuarine circulation.
Therefore, water quality modeling of this tidally influenced estuary included a thorough
evaluation of the hydrodynamics of the estuarine system. Estuarine hydrodynamics control a
variety of coastal processes including tidal flushing, pollutant dispersion, tidal currents,
sedimentation, erosion, and water levels. Once the hydrodynamics of the system was
quantified, transport of nitrogen was evaluated from tidal current information developed by the
numerical models.
Executive Summary 5
A two-dimensional depth-averaged hydrodynamic model based upon the tidal currents
and water elevations was employed for the Madaket Harbor / Long Pond embayment system.
Once the hydrodynamic properties of the estuarine system were computed, two-dimensional
water quality model simulations were used to predict the dispersion of the nitrogen at current
loading rates. Using standard dispersion relationships for estuarine systems of this type, the
water quality model and the hydrodynamic model was then integrated in order to generate
estimates regarding the spread of total nitrogen from the site-specific hydrodynamic properties.
The distributions of nitrogen loads from watershed sources were determined from land-use
analysis. Boundary nutrient concentrations in Nantucket Sound source waters were taken from
water quality monitoring data. Measurements of current salinity distributions throughout the
estuarine waters of the Madaket Harbor / Long Pond embayment system was used to calibrate
the water quality model, with validation using measured nitrogen concentrations (under existing
loading conditions). The underlying hydrodynamic model was calibrated and validated
independently using water elevations measured in time series throughout the embayments.
MEP Nitrogen Thresholds Analysis: The threshold nitrogen level for an embayment
represents the average water column concentration of nitrogen that will support the habitat
quality being sought. The water column nitrogen level is ultimately controlled by the watershed
nitrogen load and the nitrogen concentration in the inflowing tidal waters (boundary condition).
The water column nitrogen concentration is modified by the extent of sediment regeneration.
Threshold nitrogen levels for the embayment systems in this study were developed to restore or
maintain SA waters or high habitat quality. High habitat quality was defined as supportive of
eelgrass and infaunal communities. Dissolved oxygen and chlorophyll a were also considered
in the assessment.
The nitrogen thresholds developed in Section VIII-2 were used to determine the amount of
total nitrogen mass loading reduction required for restoration of eelgrass and infaunal habitats in
the Madaket Harbor / Long Pond system. Tidally averaged total nitrogen thresholds derived in
Section VIII.1 were used to adjust the calibrated constituent transport model developed in
Section VI. Watershed nitrogen loads were sequentially lowered, using reductions in septic
effluent discharges only, until the nitrogen levels reached the threshold level at the sentinel
station chosen for the Madaket Harbor system. It is important to note that load reductions can
be produced by reduction of any or all sources or by increasing the natural attenuation of
nitrogen within the freshwater systems to the embayment. The load reductions presented below
represent only one of a suite of potential reduction approaches that need to be evaluated by the
community. The presentation is to establish the general degree and spatial pattern of reduction
that will be required for protection/restoration of this nitrogen threatened embayment.
The Massachusetts Estuaries Project’s thresholds analysis, as presented in this technical
report, provides the site-specific nitrogen reduction guidelines for nitrogen management of the
Madaket Harbor / Long Pond embayment system in the Town of Nantucket. Future water
quality modeling scenarios should be run which incorporate the spectrum of strategies that
result in nitrogen loading reduction to the embayment. The MEP analysis has initially focused
upon nitrogen loads from on-site septic systems as a test of the potential for achieving the level
of total nitrogen reduction for restoration of the embayment system. The concept was that since
septic system nitrogen loads generally represent 58% of the controllable watershed load to the
Madaket Harbor embayment system and are more manageable than other of the nitrogen
sources, the ability to achieve needed reductions through this source is a good gauge of the
feasibility for protection/restoration of the system. Additionally, an alternative scenario was
completed which focused on the elimination of nitrogen loads to the Long Pond portion of the
embayment system as that source represents 24% of the controllable watershed load to the
Executive Summary 6
Madaket Harbor embayment system and is also more manageable than other of the nitrogen
sources.
2. Problem Assessment (Current Conditions)
A habitat assessment was conducted throughout the Madaket Harbor / Long Pond system
based upon available water quality monitoring data, historical changes in eelgrass distribution,
time-series water column oxygen measurements, and benthic community structure. The
Madaket Harbor-Long Pond Embayment System is a complex estuary with full tidal marine
basins (Madaket Harbor, Hither Creek) connected via Madaket Ditch to tidally restricted
brackish water basins (Long Pond, North Head Long Pond) that have significant wetland
influence.
Each of type of functional component (salt marsh basin, embayment, tidal river, deep
basin (sometimes drown kettles), shallow basin, etc.) has a different natural sensitivity to
nitrogen enrichment and organic matter loading. Evaluation of eelgrass and infaunal habitat
quality must consider the natural structure of the specific type of basin and the ability to support
eelgrass beds and the types of infaunal communities that they support. At present, some of the
component basins within the Madaket Harbor-Long Pond Estuary are showing nitrogen
enrichment and impairment of both eelgrass and infaunal habitats (Section VII), indicating that
nitrogen management of this system will be for restoration rather than for protection or
maintenance of an unimpaired system.
Overall, the large open water semi-enclosed main basin of Madaket Harbor is presently
supporting high quality eelgrass habitat and productive benthic animal communities. Oxygen
generally shows little depletion and chlorophyll a levels were consistently low. It is clear that the
open nature of this basin and its relatively small watershed have resulted in only a low level of
nitrogen enrichment and high quality habitat. In contrast, the enclosed basin of Hither Creek is
presently nitrogen enriched, with high chlorophyll levels and periodic hypoxia (low oxygen).
Habitat impairment is clear from the loss of previously existing eelgrass beds and the near
absence of benthic animals in the upper reaches. The brackish basins of Long Pond and North
Head of Long Pond are also nitrogen enriched beyond their assimilative capacity, but given the
natural nutrient and organic matter enrichment of wetland influenced tidal basins their level of
impairment is only moderate. There is no evidence that eelgrass habitat has existed previously
in these basins, so the present absence does not indicate impairment of this habitat.
The level of oxygen depletion and the magnitude of daily oxygen excursion and
chlorophyll a levels indicate only slightly nutrient enriched conditions within Madaket Harbor and
moderate to significant impairment of the enclosed component basins. However, the degree of
enrichment and subsequent effect on habitat quality varied widely between these impaired sub-
basins.
Madaket Harbor, which functions as a open marine basin generally has only moderate
declines in oxygen, moderate amounts of phytoplankton biomass (chlorophyll a), and a low level
of nitrogen enrichment (tidally averaged TN <0.33 mg L-1), all factors consistent with its high
quality eelgrass habitat. In contrast, Hither Creek's oxygen and chlorophyll a levels indicate a
nitrogen and organic matter enriched basin with oxygen frequently declining below 4 mg L-1 and
3 mg L-1. Chlorophyll a levels were also significantly elevated. These elevated levels of
phytoplankton are consistent with the observed periodic bottom water hypoxia and organic rich
soft sediments of the basin. The periodic hypoxia, elevated chlorophyll levels and sediment
characteristics are consistent with a nitrogen enriched basin with significantly impaired eelgrass
Executive Summary 7
habitat. The oxygen and chlorophyll a data further support the conclusion that Hither Creek
habitats are likely presently impaired by nitrogen enrichment.
Long Pond is a tidally restricted brackish pond dominated by fringing wetlands. Oxygen
depletion is large and frequent, generally following the diurnal light/dark cycle. Oxygen
frequently declined to <2 mg L-1, with a large daily excursion frequently rising to 2-3 times air
equilibration. Although natural wetland channels periodically are hypoxic/anoxic at night, the
large daily oxygen excursions are atypical and indicate impairment. Consistent with the oxygen
levels, chlorophyll a levels were also very high. The oxygen and chlorophyll a data indicate that
while the middle portion of Long Pond is a wetland dominated basin and therefore naturally
nutrient and organic matter enriched, the large phytoplankton blooms coupled with the large
oxygen excursions suggest that it is currently beyond its nutrient assimilative capacity. The
southern tidal reach of Long Pond is less nutrient enriched and shows a lower degree of habitat
impairment. While Long Pond, overall, has significant wetland influence and therefore is
naturally enriched in nutrients and organic matter the chlorophyll a and to a lesser extent
oxygen records indicate that this lower basin is also beyond its nutrient assimilative capacity.
Overall, the oxygen and chlorophyll a levels within the Madaket Harbor - Long Pond
System indicate little to no impairment of the outer harbor consistent with its low level of
nitrogen enrichment. In contrast, Hither Creek which receives high quality waters on the
flooding tide from Madaket Harbor, but nutrient and organic matter enrichment from its
watershed inputs and from the upper estuarine reaches via Madaket Ditch, has oxygen declines
and chlorophyll levels consistent with its tidally averaged TN of 0.51 mg L-1 (Section VI),
indicating nitrogen related habitat impairment. Long Pond and North Head of Long Pond are
brackish wetland influenced systems that are naturally enriched with nutrients and organic
matter. The North Head of Long Pond supported generally high oxygen conditions and
moderate chlorophyll a levels at a high tidally averaged TN (0.89 mg L-1). Based upon the
function type of this basin, the oxygen and chlorophyll a levels are indicative of high quality to
possibly slightly impaired habitat. In contrast, the wetland dominated Long Pond basin is
presently showing wide oxygen excursions, frequent hypoxia/anoxia and very high chlorophyll
levels indicating that even this naturally enriched system is receiving external nitrogen loading
that is resulting in habitat impairments.
The survey of infauna communities throughout the Madaket Harbor-Long Pond Estuary
indicated a system presently supporting impaired benthic infaunal habitat in its enclosed
component sub-basins (Hither Creek, Long Pond, North Head of Long Pond).
A wide range of benthic animal habitat quality exists within the Madaket Harbor-Long
Pond Embayment System. The highest quality infauna habitat was found throughout the main
basin of Madaket Harbor that also presently supports extensive eelgrass beds and sustains high
oxygen levels and low chlorophyll levels, consistent with its low level of nitrogen enrichment.
In contrast, Hither Creek has low numbers of individuals, species and diversity and is dominated
by organic enrichment tolerant species (Capitellids). The upper reach of Hither Creek (between
water quality monitoring sites MAD 9 & 10) did not support any significant infaunal habitat. The
observed impaired infauna habitat is consistent with the observed oxygen and chlorophyll levels
in this basin. Long Pond and North Head of Long Pond are brackish water basins with
significant wetland influence. As such, these basins are naturally nutrient and organic matter
enriched, and assessment of infaunal habitat accounted for their functional types. Overall,
these brackish basins presently support productive benthic animal communities. Long Pond
supports high numbers of individuals, but low species numbers, diversity and Evenness. The
low numbers of total species and overall diversity indicate an impaired habitat consistent with
Executive Summary 8
the observed hypoxic conditions and elevated chlorophyll levels. The North Head of Long Pond
is similar to Long Pond with lower numbers of individuals, but the community is dominated by
amphipods rather than oligochaeta worms, indicative of a productive organic rich habitat and
consistent with the observed oxygen levels in this basin.
At present, eelgrass coverage is extensive and stable throughout the main portion of
Madaket Harbor. The existing beds have increased significantly relative to the estimate from
1951. The temporal pattern of eelgrass coverage in Hither Creek clearly indicates that the
eelgrass habitat within this basin is presently significantly impaired. In 1951, eelgrass beds
covered much of the main basin of the Creek. However, by 1995 the beds had been
significantly reduced and limited to the margins of the basin and eelgrass was not found in the
2001 and 2006 MassDEP surveys or the MEP 2003 observations. The recent loss of the 1995
beds coupled with measured periodic hypoxia and high chlorophyll a levels supports the
contention that nitrogen enrichment caused the decline in eelgrass habitat. Deepening the
basin does impact the ability to restore eelgrass in this basin to 1951 coverage, since the basin
is now deeper and depositional. In its present basin configuration, restoration of the eelgrass
habitat in Hither Creek, should focus on restoration of the fringing beds in the shallower margins
of the basin to the inland extent of the 1951 coverage (water quality station, M11).
In contrast to Madaket Harbor and Hither Creek, the Long Pond basins do not appear to
have eelgrass habitat, as there is not present or historical evidence of eelgrass within these
basins. Management of nitrogen levels through reduction in watershed nitrogen inputs or
increased tidal flushing, as appropriate, is required for restoration of eelgrass and infaunal
habitats within the Madaket Harbor-Long Pond Embayment System.
3. Conclusions of the Analysis
The threshold nitrogen level for an embayment represents the average watercolumn
concentration of nitrogen that will support the habitat quality being sought. The watercolumn
nitrogen level is ultimately controlled by the integration of the watershed nitrogen load, the
nitrogen concentration in the inflowing tidal waters (boundary condition) and dilution and
flushing via tidal flows. The water column nitrogen concentration is modified by the extent of
sediment regeneration and by direct atmospheric deposition.
Threshold nitrogen levels for this embayment system were developed to restore or
maintain SA waters or high habitat quality. In this system, high habitat quality was defined as
supportive of eelgrass and supportive of diverse benthic animal communities. Dissolved oxygen
and chlorophyll a were also considered in the assessment.
Watershed nitrogen loads (Tables ES-1 and ES-2) for the Town of Nantucket, Madaket
Harbor / Long Pond embayment system was comprised primarily of runoff from impervious
surfaces, fertilizers and wastewater nitrogen. Land-use and wastewater analysis found that
generally about 58% of the controllable watershed nitrogen load to the embayment was from
wastewater.
A major finding of the MEP clearly indicates that a single total nitrogen threshold cannot
be applied to Massachusetts’ estuaries, based upon the results of the Great, Green and
Bournes Pond Systems, Popponesset Bay System, the Hamblin / Jehu Pond / Quashnet River
analysis in eastern Waquoit Bay and the analysis of the adjacent Nantucket Harbor and
Sesechacha Pond systems on the Island of Nantucket. This is almost certainly going to be true
for the other embayments within the MEP area, as well as Madaket Harbor and Long Pond.
Executive Summary 9
The threshold nitrogen levels for the Madaket Harbor / Long Pond embayment system in
Nantucket were determined as follows:
Madaket Harbor / Long Pond Threshold Nitrogen Concentrations:
Following the MEP protocol, the restoration target for the Madaket Harbor / Long Pond
system should reflect both recent pre-degradation habitat quality and be reasonably
achievable. Determination of the critical nitrogen threshold for maintaining high quality
habitat within the Madaket Harbor Estuarine System is based primarily upon the nutrient
and oxygen levels, temporal trends in eelgrass distribution and current benthic
community indicators. Given the information on a variety of key habitat and basin
characteristics, it is possible to develop a site-specific threshold at a sentinel location
within the embayment. The sentinel location is selected such that the restoration of that
one site will necessarily bring the other regions of the system to acceptable habitat
quality levels, which is a refinement upon more generalized threshold analyses
frequently employed. Evaluation of eelgrass and infaunal habitat quality must consider
the natural structure of the specific type of basin and the ability to support eelgrass beds
and the types of infaunal communities that they support. At present, some of the
component basins within the Madaket Harbor-Long Pond Estuary are showing nitrogen
enrichment and impairment of both eelgrass and infaunal habitats (Section VII),
indicating that nitrogen management of this system will be for restoration rather than for
protection or maintenance of an unimpaired system.
Overall, the large open water semi-enclosed main basin of Madaket Harbor is presently
supporting high quality eelgrass habitat and productive benthic animal communities.
Oxygen generally shows little depletion and chlorophyll a levels were consistently low,
with only very sparse macroalgal abundance.
The enclosed basin of Hither Creek is presently nitrogen enriched with a tidally averaged
TN of 0.51 mg N L-1 compared to 0.33 mg N L-1 in Madaket Harbor. The result is high
chlorophyll levels and periodic hypoxia (low oxygen), complete loss of eelgrass habitat
and regions of dense accumulations of drift macroalgae. In addition, the benthic animal
habitat is impaired and nearly absent in much of the northern tidal basin. While nitrogen
management needs to target eelgrass restoration in this basin, it will also restore benthic
animal habitat, as benthic communities are generally more tolerant of nitrogen
enrichment effects than is eelgrass.
The brackish basins of Long Pond and North Head of Long Pond are also nitrogen
enriched beyond their assimilative capacity, but given the natural nutrient and organic
matter enrichment of wetland influenced tidal basins their level of impairment is only
moderate. TN levels are elevated in these basins, 0.85 - 1.05 mg N L-1, typical of
wetland basins and tidal creeks. However, some impairment of habitat presently exists,
seen primarily in the high chlorophyll levels and periodic blooms and structure of the
benthic animal community. There is no evidence that eelgrass habitat has existed
previously in these basins, so the present absence does not indicate impairment of this
habitat.
The decline in eelgrass within Hither Creek makes restoration of eelgrass the target for
TMDL development by MassDEP and the primary focus of threshold development for
Executive Summary 10
this system. Additionally, restoration of the basins with impaired benthic animal habitat is
also required. However, given the level of impairment in the brackish basins and the goal
of restoring eelgrass in Hither Creek, it is certain that nitrogen management to restore
eelgrass habitat within Hither Creek the will also result in restoration of the impaired
infaunal habitat, as nitrogen enrichment will be significantly reduced to the overall
estuary. As such, it appears that the appropriate sentinel station for the Madaket
Harbor-Long Pond Embayment System should be located at the northern most extent of
the 1951 eelgrass coverage in Hither Creek, which coincides with the baseline
Nantucket Water Quality Monitoring Station, M11. To achieve the restoration target of
restoring the fringing eelgrass beds in Hither Creek requires lowering the level of
nitrogen enrichment. Within Madaket Harbor the basin-wide tidally averaged TN is
presently <0.33 mg N L-1, and the basin is supporting high quality eelgrass and benthic
infaunal habitat. However, Madaket Harbor eelgrass coverage includes areas in deeper
water than that of the location of the fringing eelgrass beds to be restored in Hither
Creek (< 1 m) and so a higher level of nitrogen is appropriate for restoration in Hither
Creek.
In shallow systems like the restoration area in Hither Creek, eelgrass beds are
sustainable at higher TN (higher chlorophyll a) levels than in deeper waters, because of
the "thinner" water column that light has to pass through to support eelgrass growth (less
water to penetrate). Therefore to restore eelgrass habitat in Hither Creek the nitrogen
concentration (tidally averaged TN) at the sentinel location needs to be between 0.48
and 0.43 mg TN L-1. A threshold of 0.45 mg TN L-1 was determined to be appropriate for
the Hither Creek sentinel station to restore eelgrass (and infaunal habitat) within this
basin.
It should be noted that as the benthic habitats in the brackish components (Long Pond
and the North Head of Long Pond) of the overall system are naturally nitrogen enriched,
a moderate reduction in nitrogen levels should be sufficient to restore the benthic
habitat. In tidal wetlands the nitrogen levels between 1 and 2 mg N L-1 are associated
with unimpaired habitat. This is consistent with the only slight impairment of the North
Head of Long Pond at TN levels of 0.894 mg L-1 and the moderately impaired benthic
habitat in Long Pond at a basin averaged TN (tidally averaged) of 0.939 mg N L-1. Given
the observed level of impairment in these brackish basins and the frequent association
of high quality benthic habitat in wetland influenced tidal channels at 1 mg N L-1, a
threshold of 0.8 mg N L-1 is appropriate as the average basin TN level to be supportive
of benthic animal habitat. This is a secondary threshold and one that should be met as
nitrogen management options are implemented to meet the nitrogen threshold at the
down-gradient sentinel station in Hither Creek.
It is important to note that the analysis of future nitrogen loading to the Madaket Harbor
/ Long Pond estuarine system focuses upon additional shifts in land-use from
forest/grasslands to residential and commercial development. However, the MEP analysis
indicates that increases in nitrogen loading can occur under present land-uses, due to shifts
in occupancy, shifts from seasonal to year-round usage and increasing use of fertilizers.
Therefore, watershed-estuarine nitrogen management must include management
approaches to prevent increased nitrogen loading from both shifts in land-uses (new
sources) and from loading increases of current land-uses. The overarching conclusion of
the MEP analysis of the Madaket Harbor / Long Pond estuarine system is that
protection/restoration will necessitate a reduction in the present (2009) nitrogen inputs and
management options to negate additional future nitrogen inputs.
Executive Summary 11 Table ES-1. Existing total and sub-embayment nitrogen loads to the estuarine waters of the Madaket Harbor and Long Pond estuary system, observed nitrogen concentrations, and sentinel system threshold nitrogen concentrations. Sub-embayments Natural Background Watershed Load 1 (kg/day) Present Land Use Load 2 (kg/day) Present Septic System Load (kg/day) Present WWTF Load 3 (kg/day) Present Watershed Load 4 (kg/day) Direct Atmospheric Deposition 5 (kg/day) Present Net Benthic Flux (kg/day) Present Total Load 6 (kg/day) Observed TN Conc. 7 (mg/L) Threshold TN Conc. (mg/L) SYSTEMS Madaket Bay 0.238 0.279 0.384 -- 0.663 8.603 17.952 27.218 0.34-0.42 -- Hither Creek 0.425 1.134 2.907 -- 4.041 0.534 -0.583 3.992 0.58-0.78 -- Madaket Ditch 0.507 0.923 1.510 -- 2.433 - 0.061 2.494 -- -- Long Pond 0.142 2.888 0.342 -- 3.230 0.975 3.065 7.270 0.24-0.40 System Total 1.457 5.392 5.214 -- 10.605 10.805 21.490 42.901 -- 0.458 1 assumes entire watershed is forested (i.e., no anthropogenic sources) 2 composed of non-wastewater loads, e.g. fertilizer and runoff and natural surfaces and atmospheric deposition to lakes 3 existing wastewater treatment facility discharges to groundwater 4 composed of combined natural background, fertilizer, runoff, and septic system loadings 5 atmospheric deposition to embayment surface only 6 composed of natural background, fertilizer, runoff, septic system atmospheric deposition and benthic flux loadings 7 average of 2001 – 2008 data, ranges show the upper to lower regions (highest-lowest) of an sub-embayment. Individual yearly means and standard deviations in Table VI-1. 8 Threshold for sentinel site located in Hither Creek at water quality station M-11
Executive Summary 12 Table ES-2. Present Watershed Loads, Thresholds Loads, and the percent reductions necessary to achieve the Thresholds Loads for the Madaket Harbor and Long Pond estuary system, Town of Madaket, Massachusetts. Sub-embayments Present Watershed Load 1 (kg/day) Target Threshold Watershed Load 2 (kg/day) Direct Atmospheric Deposition (kg/day) Benthic Flux Net 3 (kg/day) TMDL 4 (kg/day) Percent watershed reductions needed to achieve threshold load levels SYSTEMS Madaket Bay 0.663 0.663 8.603 17.952 27.22 0.00% Hither Creek 4.041 1.134 0.534 -0.583 1.09 -71.94% Madaket Ditch 2.433 2.433 - 0.061 2.49 0.00% Long Pond 3.230 1.101 0.975 3.065 5.14 -65.91% North Head Long Pond 0.238 0.238 0.693 0.995 1.93 0.00% System Total 10.605 5.570 10.805 21.49 37.86 -47.48% (1) Composed of combined natural background, fertilizer, runoff, and septic system loadings. (2) Target threshold watershed load is the load from the watershed needed to meet the embayment threshold concentration identified in Table ES-1. (3) Projected future flux (present rates reduced approximately proportional to watershed load reductions). (4) Sum of target threshold watershed load, atmospheric deposition load, and benthic flux load.