Water Quality of Four Estuaries

By CUSH, Inc.


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This report adds monitoring results for 2014 to those contained in our previous report,“Water Quality of Four Estuaries in Coastal Stonington and Mystic Connecticut, 2008-2013.” The purpose of our monitoring program is to assess the current and long-term health of local waters, to identify any ongoing trends, and to track identifiable sources of pollution. Our methods and quality control are those developed by the University of Rhode Island’s Watershed Watch (URIWW) program (http://web.uri.edu/watershedwatch).


In all CUSH sites except those in tide-dominated outer Stonington Harbor, overall Aquatic Health Index scores declined between 2013 and 2014. The decrease was driven primarily by high levels of chlorophyll-a and espe- cially of dissolved inorganic nitrogen. In some sites, oxygen saturation also declined.

NEW IN 2014

As CUSH volunteers continued to monitor local waters in 2014, we took some steps to refine our ability to characterize the water quality of our four local estuaries: Stonington Harbor, Mystic River, Pequotsepos Cove, and Wequetequock Cove. To test the reach of tidal flushing, the Lambert’s Cove site was moved from the east end of the Cove, which receives the full force of strong Harbor flood tides, to the more sheltered west end. After two years of sampling closer to shore at Murphy’s Point (Brewer Yacht Yard), CUSH decided that the original location, near the outfall of the Mystic sewage treatment plant, provides more useful information. In Old Mystic, we added a Whitford Brook site just downstream from its confluence with Haley Brook, to evaluate the major source of fresh water to the Mystic River. And we report on recent conditions in Oxecosset Brook at Route 1, a Wequetequock Cove feeder stream monitored since 2013.


To describe and compare the water quality of each site, we use the Aquatic Health Index (AHI), developed for New England waters by the Buzzards Bay Coalition (MA) and the Salt Ponds Coalition (RI). Based on summer measurements of dissolved oxygen, organic and inorganic nitrogen, and chlorophyll-a as a measure of micro- scopic algae, each site is assigned an overall AHI score between zero and 100, the worst to the best (Callender, 2008). Zero represents severe degradation (very high nitrogen and algae, very low oxygen) that is harmful

to aquatic life, while 100 represents excellent water quality fully supporting abundant and diverse aquatic life. Scores above 65 are considered Good, those below 35 Poor, and those between 35 and 65 are Fair.


Dissolved inorganic nitrogen (DIN) often reaches coastal waters in runoff carried by fast-running tributaries. In the relatively warm, calm waters of coves and inlets, the DIN fertilizes microscopic algae, which are detected by measuring the chlorophyll-a they produce. Organic nitrogen is nitrogen that is, or once was, part of a living organism; it may originate either in the sediments or in runoff containing leaves, grass clippings, dead plants
or animals, animal droppings, or sewage. Algae represent organic nitrogen produced within the embayment in response to DIN entering from the land. Dissolved oxygen is essential for most aquatic life and is a measure of how well the ecosystem can resist the damage done by nitrogen and algae (Buzzard’s Bay report, 1992-1998). Because warmer and saltier water holds less dissolved oxygen than cold, fresh water, the measured oxygen concentration is expressed as percent oxygen saturation, the amount a sample actually contains compared to the most it could hold at its temperature and salinity. We also include water-quality standards for dissolved oxygen itself, and we track fecal bacteria, which can contaminate shellfish and close swimming beaches.


When the upper reaches of narrow, shallow coves are deprived of both fresh and salt water, pollutants and sediments accumulate, water temperatures rise, dissolved oxygen plummets, and aquatic wildlife habitat is destroyed (http://watersgeo.epa.gov/mywaterway/docs). Under these conditions, good water quality can only be restored by limiting the amount of nitrogen that enters the water via surface runoff and by taking steps to increase life-giving tributary flows by limiting upstream water withdrawals and flow restricting structures.