Long Island Sound Monitoring Program
2023 Hypoxia Season Summary

Background

Hypoxia is a condition in which of low or depleted dissolved oxygen concentrations are observed in the waters of Long Island Sound (LIS). Hypoxia impacts up to half of the LIS waters each summer. The primary cause of hypoxia is excess nitrogen from human sources. These sources include sewage treatment plant discharges, storm water runoff and atmospheric deposition.

Dissolved oxygen (DO) levels below 3.0 mg/L are considered hypoxic in LIS. Hypoxic conditions cause impairment and, in some cases, death to aquatic life. Some studies have found DO can become limiting below 4.8 mg/L for sensitive fish species, while more tolerant species are not affected until DO falls below 2.0 mg/L (Simpson et. al., 1995, 1996).

Since 1991, the Connecticut Department of Energy and Environmental Protection (CT DEEP) and the Interstate Environmental Commission (IEC) have conducted an intensive water quality monitoring program on LIS. The program is funded through a grant from the U.S. Environmental Protection Agency’s (EPA) Long Island Sound Study. Data from the surveys are used to quantify and identify annual trends and differences in water quality parameters relevant to hypoxia, in particular nutrients, temperature, and chlorophyll. These data are also used to evaluate the effectiveness of the LIS management programs’ efforts to reduce anthropogenic nitrogen inputs, since nitrogen is a primary contributor to the excessive algae growth that leads to hypoxia in LIS.

During the summer (June - September), surveys across LIS (Figure 1) are conducted at bi-weekly intervals to better define the areal extent and duration of hypoxia. During these surveys stations are sampled for in-situ parameters including dissolved oxygen, temperature, pH, and salinity.

Figure 1: Interactive map of LIS survey stations. Click on each station to find information on sampling index period for that station.


During the summer of 2023, CT DEEP conducted seven surveys across LIS between May 31 and September 7. IEC conducted twelve surveys in western LIS between June 28 and September 12. Hypoxic conditions were documented during three CT DEEP surveys and nine IEC surveys.

Table 1: 2023 Long Island Sound Hypoxia Season Cruise Information.
NC= Not Calculated, NA= Not Applicable
Cruise Start Date End Date Number of Stations Sampled Number of Hypoxic Stations Hypoxic Area (sq mi) Minimum DO Station Where Minimum DO Occurred
WQJUN23 05-31-2023 06-02-2023 17 0 0 6.24 B3
HYJUN23 06-23-2023 06-23-2023 18 0 0 6.38 A4
IEC RUN #1 06-28-2023 06-28-2023 22 0 NC 4.22 H-C1
WQJUL23 07-05-2023 07-10-2023 41 0 0 3.41 A4
IEC RUN #2 07-05-2023 07-05-2023 22 4 NC 1.87 9-413
IEC RUN #3 07-12-2023 07-12-2023 22 4 NC 2.25 H-C1
HYJUL23 07-17-2023 07-19-2023 40 3 42.01 1.85 A4
IEC RUN #4 07-18-2023 07-18-2023 22 12 NC 1.30 9-413
IEC RUN #5 07-25-2023 07-25-2023 22 6 NC 0.63 A5
WQAUG23 07-31-2023 08-03-2023 42 13 126.77 1.69 A4
IEC RUN #6 08-03-2023 08-03-2023 21 8 NC 0.97 H-D
IEC RUN #7 08-09-2023 08-09-2023 22 4 NC 2.57 H-B
IEC RUN #8 08-15-2023 08-15-2023 22 12 NC 1.37 B4
HYAUG23 08-14-2023 08-16-2023 38 8 121.51 1.94 A4
IEC RUN #9 08-22-2023 08-22-2023 22 0 NC 3.17 A3
IEC RUN #10 08-31-2023 08-31-2023 21 4 NC 2.23 H-B
WQSEP23 08-29-2023 09-07-2023 42 0 0 3.56 A4
IEC RUN #11 09-06-2023 09-06-2023 21 0 NC 4.14 H-C
HYSEP23 No Survey Conducted- DO > 3.0 mg/L NA NA NA NA NA NA
IEC RUN #12 09-12-2023 09-12-2023 22 14 NC 1.17 9-409

Weather Summary

Leading up to 2023’s hypoxia survey season, Connecticut experienced its warmest winter on record, with temperatures 2ºF to 8ºF above normal in February. Much of the Northeast ranked within the warmest 20 for maximum temperature averages during the Spring season (Mar-May). Above average precipitation in June caused major flooding across the Northeast and increased the flow of debris and runoff into the region’s rivers. June also had a drop in temperatures and air quality, with winds funneling smoke from Quebec wildfires into the region.

In July the Northeast experienced both record heat and rainfall. Precipitation fell at 200% to 300% of normal, making it the fourth wettest July on record for the Northeast. In August, temperatures dipped with 29 out of the 35 major climate sites recorded as cooler than normal. The Northeast Regional Climate Center (NRCC) also observed 130 percent of normal precipitation within the region.

During August, there were 30 confirmed tornadoes in the Northeast, reaching seven times the region’s August average of four tornadoes (based on 1998-2022 data). September ranked among the 20 warmest Septembers on record for 10 states and ranked among the 20 wettest on record for five states. September’s precipitation was 215% of normal in Connecticut. Two tropical systems affected the Northeast in September. Hurricane Lee produced rough surf and rip currents along the East Coast from around September 13 to 16 and Tropical Storm Ophelia resulted in multiple days of higher-than-usual water levels and contributed to major flash flooding in New York City on the 29th.

During 2022, the Northeast saw 45.40 inches of precipitation, which was 100 percent of normal. So far, 2023 shows increased departures from normal precipitation in Connecticut (Figure 2) and across the Northeastern states.

<i>Monthly rankings for precipitation in Connecticut in 2022 and 2023. Rankings are by NRCC for the 129 years between 1895 and 2023. 1 = driest; 129 = wettest. Data and summary adapted from https://www.nrcc.cornell.edu/ </i>

Figure 2: Monthly rankings for precipitation in Connecticut in 2022 and 2023. Rankings are by NRCC for the 129 years between 1895 and 2023. 1 = driest; 129 = wettest. Data and summary adapted from https://www.nrcc.cornell.edu/

Maximum Hypoxic Area

CT DEEP utilizes Geographic Information System (GIS) software to create interpolations of bottom water dissolved oxygen concentrations across LIS and estimate the area effected by hypoxia on a cruise by cruise basis. Changes in the maximum hypoxic area over the LISS sampling period of record can be seen in this video. 2023 CT DEEP dissolved oxygen maps are available on the DEEP Monitoring Maps webpage. Maps created by the IEC for the far Western Sound 2023 season can be seen in this video.

In 2023, the maximum area of Long Island Sound with bottom water dissolved oxygen (DO) concentrations below 3.0 mg/L was 126.8 sq. mi (328.4 sq. km) and occurred during the WQAUG23 survey conducted 31 July - 3 August 2023 (Figure 3). During this survey four stations had DO concentrations below 2 mg/L, eight stations had concentrations below 3.0 mg/L, and 10 stations had concentrations between 3 and 3.5 mg/L. The lowest dissolved oxygen concentration recorded during the survey was 1.69 mg/L at Station A4.

<i> LIS Dissolved Oxygen concentrations recorded during August 2023 survey</i>

Figure 3: LIS Dissolved Oxygen concentrations recorded during August 2023 survey


The LISS utilizes a five-year rolling average (Figure 4) to determine progress towards meeting the management target of reducing the area of LIS bottom water exhibiting hypoxic conditions. The five-year rolling average is used to assess trends because conditions in any given year could be impacted by variable factors, such as extreme weather conditions. The years 1987-1999 are used as a baseline because they represent the beginning of LISS’s water quality monitoring program prior to the LIS Total Maximum Daily Load (TMDL) developed in 2000 targeting a reduction in nitrogen loads to LIS. Read more about the hypoxia extent ecosystem target established by the LISS.

Figure 4: This interactive graph shows the maximum area of hypoxia in square miles (blue bars), duration of hypoxic conditions in days (white circles), and the five-year rolling average area in square miles (green line). Area in square miles, Duration in days

Duration

The duration of hypoxia refers to the estimated length of time that hypoxic conditions persist in the bottom waters of Long Island Sound. Duration is estimated based on dissolved oxygen concentration readings taken at three water quality monitoring stations in the Western Sound - A3, A4, and B3.

The 2023 hypoxic event lasted an estimated 42 days, beginning on July 12, 2023 and ending on August 22, 2023 (Figure 5).

The earliest onset of hypoxia (based on CT DEEP data only) occurred on June 25, 2002 and the latest end date was September 20, 2005. The average duration over the 33-year time series is 52 days. The longest hypoxic event was 79 days during 2008. The shortest hypoxic event was 26 days in 2017.

Figure 5: Timing and Duration of Hypoxia in Long Island Sound 1991 - 2023 based on DEEP survey results.

Table 2 displays the onset, duration, and end of the hypoxic events from 1991 through 2023 based on CT DEEP data only. Using the LISS dissolved oxygen standard of 3.0 mg/L, the average date of onset was July 12 (+/-9 days), the average end date was September 3 (+/-11 days), and the average duration was 52 days (+/-13 days).

The earliest onset of hypoxia occurred on June 25, 2002, and the latest end date occurred on September 20, 2005.

The maximum area of hypoxia was 393 square miles and occurred in 1994. The longest hypoxic event occurred in 2008 and lasted 79 days. The shortest hypoxic event occurred in 2017 and lasted 26 days.

In 2014, 2016, 2017, 2018, and 2020 there were clear periods where the DO concentration rose above the 3.0 mg/L threshold in the early/middle part of August before dipping again during late August and early September.

Table 2: 2023 Long Island Sound Onset and Duration of Hypoxia Information
Year Estimated Start Date Estimated End Date Duration (days) Maximum Area (sq mi)
1991 07-19 08-28 41 122
1992 07-07 08-30 55 80
1993 07-09 09-10 64 202
1994 07-01 09-06 68 393
1995 07-12 08-16 35 305
1996 08-10 09-12 34 220
1997 07-27 09-12 48 30
1998 07-05 09-16 73 168
1999 07-02 08-21 51 121
2000 07-02 08-06 35 173
2001 07-10 09-14 66 133
2002 06-25 08-28 65 130
2003 07-05 09-03 61 345
2004 07-20 09-12 55 202
2005 07-14 09-20 69 177
2006 07-06 08-27 53 199
2007 07-16 09-11 58 162
2008 07-03 09-19 79 180
2009 07-19 09-01 45 169.1
2010 07-05 08-13 40 101.1
2011 07-06 08-28 54 130
2012 07-10 09-10 63 288.5
2013 07-08 09-07 62 80.7
2014* 07-24 09-09 35 87.1
2015 07-16 09-10 57 38
2016* 07-08 09-03 51 197.5
2017* 07-18 08-29 26 70
2018* 07-30 09-08 35 51.6
2019 07-12 08-28 48 89.4
2020* 07-07 09-10 43 132.5
2021 07-23 09-07 47 142
2022 07-10 09-05 58 86.6
2023 07-12 08-22 42 126.8
Average 07-12 09-03 52 155.6
Deviation ±9 days ±11 days ±13 days ± 84.3 mi2

The University of Connecticut Long Island Sound Integrated Coastal Observing System (LISICOS) utilized continuously collected dissolved oxygen data from the Execution Rocks buoy (Figure 6) to estimate there were 42.30 days with concentrations below or equal to 3.0 mg/L. There were 28.69 days with DO concentrations less than or equal to 2.0 mg/L and 11.01 days with concentrations below or equal to 1.0 mg/L. The lowest DO concentration recorded by the buoy was 0.08 mg/L on July 8.

<i>Execution Rocks LISICOS Buoy Continuous DO Data June 2023 - October 2023</i>

Figure 6: Execution Rocks LISICOS Buoy Continuous DO Data June 2023 - October 2023

Hypoxic Volume

Hypoxic Volume is a measure of the vertical extent or thickness of hypoxia. Hypoxic volume is an important measure for aquatic life uses because fish and crustaceans move throughout the water column.

In 2019, CT DEEP and the O’Donnell lab at the University of Connecticut Marine Sciences Department undertook a project to develop a tool to calculate the hypoxic volume of Long Island Sound. The tool is available to the public and allows users to obtain area and volume estimates on the fly from any survey from 1991-present. The tool utilizes CT DEEP and IEC data.

The maximum volume of water with concentrations below 3.0 mg/L occurred during the HYAUG23 survey and was 1.2 km3 (0.29 mi3). Table 3 compares hypoxic area and volume by survey over the 2023 season. Figure 7 shows a time series of the maximum hypoxic volume from 1991 to 2023.

Table 3: 2023 Volume Estimates of Hypoxic Long Island Sound Bottom Waters Obtained from the UCONN HypVolApp
Area (sq km)
Volume (cubic km)
CRUISE 2 mg/L 3 mg/L 4.8 mg/L 2 mg/L 3 mg/L 4.8 mg/L
WQJUN23 0 0 0 0 0.0 0.0
HYJUN23 0 0 0 0 0.0 0.0
WQJUL23 0 0 205 0 0.0 0.9
HYJUL23 48 114 615 0 0.3 2.9
WQAUG23 124 310 1495 0 0.8 15.7
HYAUG23 36 315 1240 0 1.2 9.5
WQSEP23 NA NA NA NA NA NA
HYSEP23 NA NA NA NA NA NA

Figure 7: The interactive graph plots the maximum hypoxic volume over time. Initial analyses shows strong evidence that the hypoxic volume in LIS is substantially smaller between 2015 and 2019 than 20 years earlier (O’Donnell, et al., 2020).

Frequency of Hypoxia

Figure 8 illustrates how frequently the areas of Long Island Sound experience hypoxia. The colors represent the percentage of years in which hypoxic conditions have occurred in the bottom waters of Long Island Sound. The westernmost areas have experienced hypoxia almost every year since monitoring began.

The bar graph (Figure 9) provides a deeper dive into the data (percentages) used to create the frequency map. The colors of the bars correspond to the map legend.

<i>The frequency of hypoxia in Long Island Sound Bottom Waters from 1994 - 2023</i>

Figure 8: The frequency of hypoxia in Long Island Sound Bottom Waters from 1994 - 2023

Figure 9: The percentage of time a station has been hypoxic over the period of survey record

Data Downloads

Data from all CT DEEP cruises can be downloaded from the UCONN ERDDAP and select data is available for download from the Water Quality Portal . All samples are collected and analyzed under EPA-approved Quality Assurance Project Plans.

Data from IEC cruises can be downloaded through the Water Quality Portal. All IEC samples are collected and analyzed under EPA-approved Quality Assurance Plans.

In addition click the buttons below to download data utilized in the visualizations produced for the 2023 Hypoxia Season Summary.

Contact Information

For more information on the Long Island Sound Water Quality Monitoring Program visit:

CT DEEP Logo

IEC Logo

LISS Logo

LISICOS

Report Prepared by:
Katie O’Brien-Clayton, CT DEEP
katie.obrien-clayton@ct.gov

Acknowledgements:
Cover photo: Rachel Ranelli, 2023 LIS Seasonal Resources Assistant
This Project was funded by the United States Environmental Protection Agency, Long Island Sound Study through grant funds administered by CT DEEP and IEC.

References

O’Donnell, J., Fake, T., and J. O’Donnell. 2020. Computing the Hypoxic Volume of Long Island Sound- Final Report, September 12, 2020. University of Connecticut, Department of Marine Sciences, Groton, CT. Prepared for the Connecticut Department of Energy and Environmental Protection and the Long Island Sound Study

Simpson, D. G., Gottschall, K., and Johnson, M. 1995. Cooperative interagency resource assessment (Job 5). In: A study of marine recreational fisheries in Connecticut, CT DEP Marine Fisheries Office, PO Box 719, Old Lyme, CT 06371, p. 87-135.

Simpson, D.G., Gottschall, K., and Johnson, M. 1996. Cooperative interagency resource assessment (Job 5). In: A study of marine recreational fisheries in Connecticut, CT DEP Marine Fisheries Office, PO Box 719, Old Lyme, CT 06371, p. 99-122.