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 (2016-2020) is 94 square miles compared to an average of 208 square miles from 1987-2000, a 55 percent reduction. The hypoxic area also has declined by 61 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). While achieving a measurable reduction in hypoxia from 2016-2012 is a major achievement, further reductions in the area of hypoxia are needed through 2035 in order to fully attain water quality standards and achieve the ecosystem target goal.
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.
In 2020, the area of hypoxic waters was 63 square miles, a decrease of 25 square miles from 2019. Strong wind events, including from Hurricane Isaias, in late July and early August, likely contributed to the reduction. Strong winds from the east in the summer can help mix the bottom waters with the more oxygen-rich surface waters to improve oxygen levels in the bottom.
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.
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.).
Besides a reduction in the area of hypoxia compared to the pre-TMDL baseline, the duration of hypoxia for 2020 is below the average duration for the time series (see the duration of hypoxia, supporting indicator, 1987-2020).
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.
2019 Long Island Sound CTDEEP and IEC Hypoxia Review Report