Photos of the Long Island Sound

Issues & Actions

Water Quality



Imagine if the air around you had only a scarce amount of oxygen. Now, consider what fish and other wildlife in the Sound face every summer when dissolved oxygen levels drop to low levels. The condition, known as hypoxia, occurs in the Sound every summer when dissolved oxygen levels in bottom waters fall below 3 mg/L. Hypoxia forces some fish and invertebrates to scatter, while making others more susceptible to disease. When concentrations fall below 2 mg/L conditions become suffocating; marine life unable to flee may die. Since 1987, LISS has tracked the area and duration of hypoxia. In 2007, hypoxia lasted 58 days, and at its peak affected 162 square miles—about four times the size of Manhattan.


  • Years of research, monitoring and modeling has helped the Long Island Sound Study to identify nitrogen sources in the Sound as a significant cause of decreased DO levels. In a process called eutrophication, excessive discharges of nutrients such as  nitrogen fuel the growth of planktonic algae. The algae, and planktonic animals that feed on algae die, settle to the bottom of the Sound, and decay, using up oxygen in the process.
  • To reduce nitrogen to the levels necessary to improve DO levels and meet water quality standards, the states of Connecticut and New York with  EPA have adopted a 58.5 percent nitrogen reduction target by 2017 from early 1990s baseline levels. This plan is known as a Total Maximum Daily Load or TMDL.
  • Since the 1990s, LISS’s partners have invested hundreds of millions of dollars to reduce nitrogen by upgrading wastewater treatment plants which discharge nitrogen into waterbodies as a byproduct of sewage effluent. Overall, since 1994, when nitrogen discharges peaked, upgrades at the region’s 106 plants have resulted in a reduction of 94,343 pounds of nitrogen per day (as of 2013).
  • In 2007, Connecticut received EPA’s Blue Ribbon Water Quality Trading Award for its innovative nitrogen credit exchange program.
  • Communities are also looking to reduce nitrogen from fertilizer and pet and animal wastes that can discharge into rivers and streams that flow into the Sound as polluted stormwater runoff.
  • Resource managers are looking at bioextractive technologies to “harvest” algae in the Sound or develop shellfish farms that feed on nutrient-rich plankton.

Toxic Contaminants and Pollution


The discharge of chemicals into the Sound and its tributaries has often been associated with manufacturing processes. During the industrial revolution heavy metals started to accumulate in the sediments of the Sound. Concentrations of mercury off Norwalk Harbor, for example, increased by more than 1,300 percent from 1820 to 1955.


  • With the advent of environmental regulation, product bans, and a decline in manufacturing,concentrations of many contaminants in sediments began to decline in the mid-20th century. Sediment concentrations of mercury, for example, have dropped by more than a third, and copper and zinc have declined as well.
  • Since 1988, toxic chemical discharges directly into the Sound and its tributaries have decreased by 93 percent and airborne discharges throughout the entire watershed have decreased by 88 percent, according to the EPA’s Toxics Release Inventory (TRI) database.
  • Concentrations of mercury and PCBs can be consumed by the Sound’s marine life, which can be eaten by humans. As a result, state health departments issues warnings to the public to limit the number of fish to eat per month.
  • In 2006 and 2007 LISS funded a bi-state study to monitor the concentrations of toxic contaminants in popular sportfish and in lobster. The study showed a decrease in PCBs in fish, which led to advisories for slightly more consumption of bluefish and striped bass for certain people.



For many, the most obvious sign of poor water quality occurs when a “no swimming” or “no shellfishing” sign gets posted because of potential pathogen contamination. Pathogens are disease-causing bacteria and viruses that enter the Sound from inadequately treated human sewage and domestic and wild animal wastes. Sources of pathogens include stormwater runoff carrying animal waste from paved surfaces and lawns, and human waste from improperly maintained septic systems. Some older communities also still use an early generation of sewer system that collects stormwater runoff and sanitary sewage into the same pipe. During dry weather, these combined sewer systems transport wastewater to the sewage treatment plant. During rainfall, if the combined wastewater volume exceeds the capacity of the plant, the system overflows and excess wastewater is discharged directly without adequate treatment. Other sources of pathogens can be leaking sewage pipes, illegal connections that bring sanitary waste to storm sewers, sewage treatment equipment failure, and discharge of sewage from boats.


  • Generally, the Sound’s 203 beaches are safe for swimming.To avoid illnesses caused by pathogens, health departments will close beaches when monitoring data indicate contamination or “preemptively” after a rainstorm at sites known to be susceptible to contamination.
  • Shellfish beds are also regularly monitored by state regulatory agencies to assure that shellfish harvested in commercial and recreationally-approved areas are safe to eat.
  • Since 2007, Connecticut, and since 2011, New York, have prohibited vessels with toilets from discharging sewage into Long Island Sound. The Sound has been designated as a No Discharge Area (NDA) by the Environmental Protection Agency.
  • Health departments work with state agencies and interstate commissions to trace sources of pollution such as illegal sewage connections into rivers or storm drains that are being used to dump pet waste. Interstate Environmental Commission, for example, along with state and local agencies in Connecticut and New York, has been working to eliminate potentially harmful pathogen discharges into the Byram River, an interstate waterway 13 miles long. The efforts include locating illegal connections to storm sewers or cross connections between sanitary and storm sewers that empty into the Byram River.

Floatable Debris


Trash floating in coastal waters and bays or washed up on the beach is called floatable debris. Floatable debris reduces the enjoyment of the Sound, can be a nuisance or hazards for boaters, and can harm wildlife.


  • The MARPOL treaty of 1988 made much of ocean dumping illegal.
  • New York City Department of Environmental Protection deploys a vessel, the Cormorant, that collects tons of debris each year from Long Island Sound and other New York City waters.
  • Volunteers groups in New York and Connecticut collect debris in Long Island Sound Beaches as part of the International Coastal Cleanup in September each year. These clean-ups are partially funded through the Long Island Sound Futures Fund small grants program.

Did You Know?

What is Hypoxia?

What is Hypoxia?

View this chart for a visual explanation of how Hypoxia occurs. Download PDF

Environmental Indicator

Habitat Protection

Habitat protection environmental indicators include River Miles Restored, Coastal Habitat Restoration, Submerged Aquatic Vegetation. Learn more

Nutrient Remediation Workshop

The International Workshop on Bioextractive Technologies for Nutrient Remediation explored the potential benefit of seaweed and shellfish cultivation and harvest for nutrient mitigation in nearshore estuarine environments. Details


No Discharge Areas

In 2006, Connecticut took major strides toward banning sewage discharged from boats.
Learn more

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