Shore + Sound

Swimming, Clamming & Heavy Rain               Plovers and Others               
The Water of Long Island Sound               Trends Under the (Rising) Surface

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The Water of Long Island Sound

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The areas with acceptable levels of dissolved oxygen throughout the year are on track to meet the 2035 goal.
Marine life requires oxygen. The percentage of Long Island Sound that has adequate oxygen throughout the year is shown in the chart above. Continue reading about oxygen in Long Island Sound...

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2018 saw the highest nitrogen discharged to the Sound since 2013.

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Connecticut’s investments in nitrogen-removal technology at sewage treatment plants have been successful. Continue reading about the critical role of nitrogen pollution in Long Island Sound...


Quick Summary - x check dash

As Connecticut reduces or increases the amount of nitrogen discharged into the Sound, the level of dissolved nitrogen in the water usually follows suit.

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Top chart (Oxygen): During the summer, some areas of the Sound experience hypoxia, which is a condition in the water where oxygen levels are not adequate to fully support desirable forms of life, including fish and lobsters. Hypoxia occurs when the nitrogen in pollution stimulates excessive growth of aquatic plants, which die and get consumed by oxygen-using bacteria. Hypoxia occurs predominantly in the western portions of the Sound. Weather greatly influences hypoxia, making year-to-year changes less important than long-term trends. Detailed reports that include maps of the extent and duration of hypoxia in Long Island Sound are produced annually by the Department of Energy and Environmental Protection.

Goal for hypoxia:  The goal line on the top chart, set at 86 percent of the Sound, is an approximation of the target adopted in the 2015 edition of the Long Island Sound Study's Comprehensive Conservation and Management Plan. That plan's goal calls for "measurably reducing the area of hypoxia in Long Island Sound from pre-2000 averages." A "measurable reduction" is at least a 28 percent reduction, according to the plan's statistical analysis that accounts for the year-to-year weather-induced fluctuations that bedevil this indicator.

The middle chart tracks the amount of nitrogen discharged by 80 sewage treatment facilities across Connecticut, two large coastal industrial facilities and a small group of industrial sources in the Naugatuck River watershed. The sewage treatment plants include those along the coast and many more that discharge to rivers that flow to the Sound. Connecticut’s investments in nitrogen-removal technology at many of those plants have been successful. DEEP attributes the higher discharges of 2018 to increased precipitation, which can lead to overflows of combined sewers and exacerbate infiltration of groundwater and inflow of surface water into the sewers. The nitrogen discharges of New York, which lags Connecticut in nitrogen control, are not shown.
To reduce the nitrogen inputs that cause hypoxia, Connecticut and New York adopted The Comprehensive Conservation and Management Plan in 1994, and built upon that plan with an expanded agreement in 2002. 

Connecticut’s share of the total nitrogen pollution in Long Island Sound is about one-third, and New York’s is two-thirds. In 2001, the Federal Environmental Protection Agency approved the New York and Connecticut joint plan for implementing a Total Maximum Daily Load (TMDL). The TMDL is the maximum amount of pollutants that can be discharged while still allowing water quality standards to be attained.
Despite the greater nitrogen discharges from 2013 through 2015, DEEP reports that Connecticut met the goal for the "trade-equalized load," which takes into account the distance of inland treatment plants from Long Island Sound. A portion of the reduction in nitrogen discharges in 2016 was due to the drought, when less stormwater flowed into and through the sewer systems, and another portion was the result of capital improvements in New Haven. Also, more nitrogen is discharged when the weather is cold, so the warmth of 2016 probably was a factor.
The effectiveness of Connecticut's approach to reducing nitrogen in the Sound is confirmed in two ways. First, the bottom chart shows the average level of dissolved nitrogen in the water of Long Island Sound. Levels have improved as Connecticut has reduced its nitrogen discharges.
Second, the United States Geological Survey published a report in 2016 that analyzed the nutrients being carried to the Sound by Connecticut's rivers and streams; since 2001, the total amount of nitrogen was reduced by more than ten percent. 
Large uncontrolled quantities of nitrogen enter Long Island Sound when rainfall carries fertilizer from residents' lawns along with the pollutants that have accumulated on impervious surfaces

 Technical Notes:

1. The top chart shows the area of Long Island Sound (both states combined) that had adequate oxygen levels throughout each year. The sampling area (2700 square kilometers) does not include the whole Sound (3400 square kilometers). The areas not sampled are shallow waters (less than two meters deep) near shore, which generally do not experience hypoxia; bays; the eastern end of the Sound, which is not expected to experience hypoxia; and an area in the far western end, which probably becomes hypoxic in most years. 

2. More about the new hypoxia goal: Progress toward the goal should be assessed using a five-year rolling average. One or two years of promising data could be natural variability at work. The five-year rolling average is not shown here, but can be calculated or inferred fairly easily.

3. Hypoxia was redefined by DEEP in 2011. Areas of the Sound are now considered hypoxic where a liter of water contains less than 3.0 mg of dissolved oxygen. This is the criterion that was used prior to 2004. From 2004 through 2010, DEEP used 3.5 mg/l as the determining level. The threshold was returned to the 3.0 level in 2011 to be consistent with the definitions used by New York and the Long Island Sound Study. Data for all previous years were recalculated to show the area of the Sound having adequate oxygen under the current definition (that is, at least 3.0 mg/l).

4. The nitrogen in the bottom chart is total dissolved nitrogen in the bottom waters of Long Island Sound.