Ecosystem Targets and Supporting Indicators
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Meeting the ecosystem target (measured as the maximum area of bottom waters with dissolved oxygen ≤ 3 mg/L) based on a five-year rolling average of hypoxia is ahead of schedule. The five-year rolling average (2015-2019) is 89 square miles compared to an average of 208 square miles from 1987-2000, a 57 percent reduction. The hypoxic area also has declined by 63 percent from the peak five-year period (1992-1996), which was 240 square miles. Based on the last 20 years of interannual variability, a 28 percent reduction is necessary to achieve a measurable reduction (see data note). Further reductions in the area of hypoxia are needed in order to fully attain water quality standards for dissolved oxygen.
The years 1987-2000 are used as a benchmark (or the baseline) because they represent the beginning of Long Island Sound Study’s water quality monitoring program up to the December 2000 Total Maximum Daily Load (TMDL) agreement to reduce nitrogen loads into the Sound.
Compared to 2018, the area of hypoxia increased from 52 square miles to 89 square miles. However, the 2019 five-year rolling average of the maximum area (2015-2019) of hypoxic waters remained the same as the 2018 five-year rolling average (2014-2018), which also happens to be 89 square miles.
In assessing trends, LISS uses the five-year rolling average because conditions in any given year could be impacted by variable factors, such as extreme changes in heat or precipitation, which would be hard to compare to the normal conditions over a long period of time. Hypoxia in the western Sound also appeared more intense (lower oxygen concentrations) in 2019, perhaps due to the hot and wet summer weather.
As shown in the chart above, there is considerable annual variability in the maximum area of hypoxic waters, in part due to annual variations in weather (temperature, wind, rainfall, etc.).
While the area of hypoxia has been reduced compared to the pre-TMDL baseline, the duration of hypoxia has remained unchanged at 56 days compared to the pre-TMDL baseline (see the duration of hypoxia, supporting indicator, 1987-2017).
Warming water temperatures will reduce the amount of oxygen that the water can contain, making it more difficult to meet the target long term. In addition to weather variables affecting the area of hypoxia year to year, longer-term climate influences will affect the vulnerability of the Sound to hypoxia. Improvements in monitoring, including increased monitoring in embayments, will better define areas affected by hypoxia, and the factors contributing to it.
Routine monitoring of bottom-water hypoxia is done monthly throughout the year and biweekly in the summer by the Connecticut Department of Energy and Environmental Protection (CTDEEP).
Additional year-round monitoring is conducted by the Interstate Environmental Commission in Western Long Island Sound and the Narrows. The Long Island Sound Integrated Coastal Observing System (LISICOS) also deploys real-time monitoring instruments on buoys across the Sound, including three with bottom water oxygen sensors in the Western Sound. The three monitoring programs help provide a comprehensive long-term data set on both the area and duration of hypoxia, with the monitoring data going back to 1987 (initially conducted by the University of Connecticut from 1987-1990, and beginning with CTDEEP since 1991).
Bottom hypoxia is measured by lowering instruments with multiple sensors (including dissolved oxygen) through the water column from a research vessel or smaller boat.
Hypoxia, a deficiency in the amount of oxygen in the water, can be harmful or lethal to fish, invertebrates, and other animals and therefore decrease or eliminate them from Long Island Sound.
Hypoxia may also limit the growth of animals that are exposed but not killed.
Dr. James Ammerman, Long Island Sound Study [email protected]
CTDEEP (primary data source), also the Interstate Environmental Commission for Western Long Island Sound, and LISICOS.
Here are links to find data on dissolved oxygen levels and the extent of hypoxia in Long Island Sound:
Frequency of Hypoxia, 1994-2019
Hypoxia is more frequent in the western Sound.
2018 Combined Report Final