Indicators:  Sound + Shore

Oxygen in Long Island Sound

A large area of western Long Island Sound was affected by hypoxia (oxygen levels too low to support aquatic life) in August 2006.

Hypoxia is the condition in the water when oxygen levels are too low to support desirable forms of life, including fish and lobsters. (For this indicator, hypoxia is defined as less than or equal to 3 mg/l of dissolved oxygen.) Hypoxia occurs when the nitrogen in pollution stimulates excessive growth of aquatic plants, which die and are consumed by oxygenusing bacteria. Connecticut's goal is to “eliminate the adverse impacts of hypoxia resulting from human activities.” All of the hypoxia has occurred in the western two-thirds of the Sound. Weather greatly influences hypoxia, making year-to-year changes less important than long-term trends. Mild winters followed by relatively cool summers result in fairly uniform water temperatures and less hypoxia in the depths.

The second largest area of hypoxia was observed in 2003, and scientists believe it is attributable to an expansive brown algae bloom in the western end of the sound, which was most likely fueled by a large amount of rain (and nitrogen-bearing runoff) early in the summer. A cool beginning to the summer of 2004 led to less hypoxia, and favorable weather led to another small improvement in 2005 (but see technical notes below). The first week of August 2006 saw a return of hypoxia to a larger area.

To reduce the nitrogen inputs that cause hypoxia, Connecticut and New York adopted a comprehensive management plan in 1994 and built upon that Plan with an expanded agreement in 2002. Connecticut’s progress in reducing nitrogen pollution is illustrated in the Nitrogen indicator.

Technical notes: The graph shows the area of Long Island Sound that had adequate oxygen levels throughout the year. The sampling area (2700 square kilometers) does not include the whole sound (3400 square kilometers). The areas not sampled are shallow waters (less than two meters deep) near shore, which generally do not experience hypoxia; embayments; the eastern end of the sound, which is not expected to experience hypoxia; and an area in the far western end, which probably becomes hypoxic in most years.

In 2004, the DEP redefined hypoxia to include waters with less than or equal to 3.5 mg/l of dissolved oxygen (replacing the older 3.0 mg/l standard). There has not been time to re-calculate past years’ data to reflect the new standard, so this indicator still uses the older 3.0 mg/l standard.