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.

View Implementation Actions for Extent of Hypoxia

Show/Hide Table Data

Area in Square Miles
Year	Area of Hypoxia (sq. mi.)	Five-year Running Average (sq. mi.)	Percent to Goal
baseline*	208
1987	309
1988	251
1989	328
1990	174
1991	122	237	63
1992	80	191	79
1993	202	181	83
1994	393	194	77
1995	305	220	68
1996	220	240	63
1997	30	230	65
1998	168	223	67
1999	121	169	89
2000	173	142	106
2001	133	125	120
2002	130	145	103
2003	345	180	83
2004	202	197	76
2005	177	197	76
2006	199	211	71
2007	162	217	69
2008	180	184	82
2009	169	177	85
2010	101	162	93
2011	130	148	101
2012	288	174	86
2013	80	154	97
2014	87	137	109
2015	38	125	120
2016	197	138	109
2017	70	95	158
2018	52	89	169
2019	89	89	169
2020	63	94	160
2021	142	83	181
2022	87	87	170
2023	127	102	145
2024	43	92	163
*baseline is avg. of hypoxia from beginning of wq monitoring program to TMDL agreement (1987-2000).

Status and Trends

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 2019-2023 was 102 square miles of hypoxia compared to an average of 208 square miles from the 1987-1999 baseline, a  51 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 2019-2023 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 increased slightly by 15 square miles from last year’s five-year average of 87 square miles (for 2019-2023) the maximum area of hypoxia increased – from 87 square miles in 2022 to 127 square miles in 2023. Dry summer conditions during summer 2022 likely reduced nutrient loading to the Sound from the watershed, likely contributing to the observed reduction in hypoxic area. Excess nutrient loading contributes to conditions that result in hypoxic, or oxygen-depleted, waters.

In assessing trends, LISS uses the five-year rolling average, instead of from year to year, 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.

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 james.ammerman@longislandsoundstudy.net

Source of Data

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