Ecosystem Targets and Supporting Indicators
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Meeting the hypoxia reduction ecosystem target by increasing oxygen concentrations in the bottom waters is ahead of schedule. The target is measured as the five-year rolling average of the bottom water area with < 3 mg/L dissolved oxygen. The five-year rolling average for 2017-2021 was 83 square miles of hypoxia compared to an average of 208 square miles from the 1987-1999 baseline, a 61 percent reduction. Based on the 2015 CCMP, a 28 percent reduction from the baseline (to about 150 square miles) is necessary to achieve a measurable reduction (see data note). While achieving a measurable reduction in hypoxia from 2017-2021 is a major achievement, further reductions in the hypoxic area are needed through 2035 in order to fully attain water quality standards and achieve the ecosystem target goal.
The five-year average hypoxic area decreased even though the maximum area of hypoxia increased from 2020 to 2021 – 63 square miles in 2020 to 142 square miles in 2021. The continued declining trend is due to the removal of the 2016 measurement, a particularly large hypoxic area, from the current period, 2017-2021. 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 average out over time.
For summer 2021, record or near-record August average air temperatures throughout the Long Island Sound watershed probably contributed to warming the Sound’s surface waters and causing the large hypoxic area in the bottom waters. A fact sheet from the IUCN explains that warmer ocean water holds less oxygen and is more buoyant than cooler water. This leads to reduced mixing of oxygenated water near the surface with deeper waters, which naturally contain less oxygen. Warmer water also raises oxygen demand from living organisms. As a result, less oxygen is available for marine life.
Hurricane Henri, which struck the region on Aug. 22, may have contributed to the decline in hypoxic area, which occurred after mid-August. The strong, southeasterly winds from the storm probably enhanced mixing of the heavier, bottom waters with the lighter oxygenated surface waters to increase oxygen concentrations in the bottom waters.
The years 1987-1999 are used as a benchmark (or the baseline) because they represent the beginning of Long Island Sound Study’s water quality monitoring program, prior to the Total Maximum Daily Load (TMDL) agreement in 2000 to reduce nitrogen loads into the Sound.
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 (CT DEEP).
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 CT DEEP 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]
CT DEEP (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:
Hypoxia is more frequent in the western Sound.
2019 Long Island Sound CT DEEP and IEC Hypoxia Review Report