Ecosystem Targets and Supporting Indicators

Extent of Hypoxia

Target: Measurably reduce the area of hypoxia in Long Island Sound from pre-2000 Dissolved Oxygen TMDL averages to increase attainment of water quality standards for dissolved oxygen by 2035, as measured by the five-year running average size of the zone.

Show/Hide Table Data

Progress
Area in Square Miles
YearArea of
Hypoxia (sq. mi.)
Five-year Running
Average (sq. mi.)
Percent to Goal
baseline*208
1987309
1988251
1989328
1990174
199112223763
19928019179
199320218183
199439319477
199530522068
199622024063
19973023065
199816822367
199912116989
2000173142106
2001133125120
2002130145103
200334518083
200420219776
200517719776
200619921171
200716221769
200818018482
200916917785
201010116293
2011130148101
201228817486
20138015497
201487137109
201538125120
2016197138109
20177095158
20185289169
20198989169
20206394160
202114283181
*baseline is avg. of hypoxia from beginning of wq monitoring program to TMDL agreement (1987-2000).

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. 

Challenges

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.

How is This Target Measured?

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.

Importance

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.

Contact

Dr. James Ammerman, Long Island Sound Study [email protected]

Source of Data

CT DEEP (primary data source), also the Interstate Environmental Commission for Western Long Island Sound, and LISICOS.

DATA NOTES

  • The technical explanation on how the target was selected is found in Appendix B of the Comprehensive Conservation and Management Plan.

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