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Cockeast Pond Study

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Health of Cockeast Pond

From 2015 to 2019 WRWA worked with the Coastal Systems Program at UMASS Dartmouth on an in-depth scientific study to determine the health of the pond and factors that may be affecting it. In 2023 monitoring continued to assess nutrient levels in the pond. Results from 2023 are presented in this Technical Memo. 

https://www.westportwatershed.org/_files/ugd/927577_6ee2add9093445d9807140bbd779d657.pdf

WRWA is also worked with SMAST Scientists on a project that used the waters of Cockeast Pond as a natural laboratory to quantify the utility of oyster aquaculture in removing excess nitrogen to promote estuarine restoration. Click this link for more information on this project.

The Importance of Cockeast Pond

Nitrogen enrichment occurs when the rate of nitrogen input to a salt pond is higher than the rate of output through tidal flushing and removals from within the pond itself.  Generally as watersheds become developed the N load from the watershed to the pond is increased and the nitrogen levels within the pond water increases as the tidal flushing remains constant.  This is the case for Cockeast Pond.  There are only 3 options for lowering the nitrogen levels to restore Cockeast Pond: (1) reduce the amount of nitrogen entering from the watershed, (2) increase the rate at which nitrogen leave the pond via tidal flushing, (3) increase the rate of nitrogen removal within the pond through direct removals (plant or animal harvest) or enhanced denitrification.

Click to watch a presentation of results done by Dr. Brain Howes on July 21, 2018

summary

  • Unique and diverse habitat for numerous species of plants and animals.

  • A buffer for flooding, storm surges, nutrient pollution, climate changes.

  • Esthetic, recreational, and economic benefits for those who see and use it.

What Are the Issues?

  • WRWA has examined pond water quality from 2008-2015 to create baseline data.

  • Pollution due to increased nitrogen loading is of particular concern.

  • More and larger algal blooms and increased plant growth degrade the pond ecosystem and affect its use.

  • The pond contributes nitrogen to the Westport River, which is already above its healthy limit for that nutrient.

  • Our goals: to determine how tidal flushing, Hurricane Sandy, surrounding land use, and other factors may be affecting the Pond’s health and develop an action plan to address any adverse changes.

Read the technical document on research done on Westport ponds from 2008-2012

The Scientific Study was a multi-step, phased study to:

  • Determine the extent of the pond’s change in water chemistry and ecological health.

  • Identify causes of the change.

  • Recommend actions for remediating, restoring and maintaining the pond’s health.

  • A cooperative effort between WRWA, UMass Dartmouth, and stakeholders.

A result of the poor tidal exchange in Cockeast Pond, the freshwater inflow through the stream and groundwater discharge have a much larger impact on the pond than in the adjacent well flushed Westport Harbor basin and most estuaries in the region.  This can be seen in the seasonal cycle of water column salinity where high freshwater inflows in winter and spring result in reduced salinities which then rise in late spring and summer as freshwater decreases and tidal inflows remain constant.  Since most of the nitrogen load is carried by freshwater the high inflow, low salinity winter and early spring period sees both a drop in salinity and a significant rise in TN levels which support the large spring bloom in Cockeast Pond.  In contrast, with the dominance of tidal inflows in late spring and summer comes a rise in salinity and drop in TN as the salinity of the inflowing Westport Harbor water is ~30 ppt and the TN is only ~0.3 mg N L-1, compared to the Pond at ~6 ppt and 1.2 mg N L-1.  The seasonality of freshwater inflow under conditions of restricted tidal flow creates a highly variable environment with significant nitrogen enrichment and impaired water and habitat quality.  To restore the impaired conditions nitrogen levels need to be lowered.

At present, Cockeast Pond is a highly nitrogen impaired salt pond, as seen in its high TN levels, large phytoplankton blooms, accumulations of macroalgae and severely impaired benthic animal habitat quality, in low salinity brackish waters.  The community has very low diversity and generally low evenness in addition to very low species numbers.  In the northern region of the pond, the number of individuals is so low that the contribution as a food source for fish is minimal, while the southern region supports similarly impaired habitat but with moderate numbers of individuals.  This trend parallels the distribution of sediment quality with the northern region supporting soft anoxic organic rich muds and the southern region having fine sands with oxic surface layer.  The higher numbers of animals in the southern basin are indicative of a moderate level of productivity, but the other metrics clearly indicate impaired habitat.  While the species were generally similar from the north to the south basins, the number of individuals in the north basin was very low, less than 1/10th of the southern basin.  Based on preliminary field observations, it appears that this difference in benthic animal communities may be related to sediment quality and the distribution of macroalgal accumulations within the Pond, which can smother benthic organisms,  Restoration of benthic habitat is a target for TMDL’s under the Clean Water Act and will require a lowering of the current level of nitrogen enrichment throughout the pond waters and if possible a modest increase in salinity.  As part of the management analysis, water sampling and phytoplankton productivity studies were conducted, which again confirmed that nitrogen is the nutrient controlling phytoplankton blooms and habitat quality in Cockeast Pond and therefore is the correct target nutrient for restoring the impaired habitats within the pond.

Annual field measurements and subsequent water quality modeling of Cockeast Pond clearly show that the extremely tidally restricted nature (e.g. poor tidal flushing) of Cockeast Pond makes this system very sensitive to watershed nitrogen inputs resulting in habitat impairment.  The very low volumetric tidal exchange allows nitrogen levels to become significantly enriched over incoming tidal waters from Westport Harbor and supports the development of large phytoplankton blooms and accumulations of macroalgae. 

The Study - What Was Done

Phase 1:

  • Data collection on water circulation, biology,and other measures

    • Tide gauges (3) circulation measurement component

    • Bathymetry—circulation measurement component

    • Tidal Flux—circulation measurement component

    • Benthic grabs—sampling of sediments to determine species and populations of inhabitants, which is an indicator of pond health

    • Land use data—determine site-specific land uses and the associated nitrogen loads

    • Macroalgal studies—a look at the populations of large algal species within the pond

  • Reporting and project management

Phase 2 :

  • Data collection, modeling, and final report with management recommendations

    • Habitat assessment—determine the health and diversity of flora and fauna of the pond ecosystem

    • Historical ecosystem study—an investigation of historic biological and ecological conditions

    • Time-series water column oxygen measurements— measurement of oxygen levels, an indicator of pond health

    • Benthic nitrogen regeneration in bottom sediments— determine how nitrogen is cycled within the pond and determine sinks and sources of nitrogen

  • Presentation to stakeholders

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