Low oxygen content in the waters is called Hypoxia. Hypoxia has plagued Long Island Sound for decades. The condition of Hypoxia is when the seawater has less than 3 parts per million of oxygen in it. At this low and dangerous level many fish, crabs and lobsters can become sick and die. Even greater amounts can be very troublesome. A healthy oxygen level is over 6 parts per million.
Below is a map showing the oxygen levels this august in the Sound. Now as we approach the end of September the waters are recovering because the water is starting to cool. Cool water can hold more oxygen than warm water. The Red colored area on the map in the extreme western end of the Sound and off the Norwalk area indicates less than 3 part per million Oxygen .
The main culprit for hypoxia is nitrogen a product of sewage treatment plants and fertilizers .
Soundkeeper has worked to reduce nitrogen inputs to the Sound on many fronts including a law suit against New York City to reduce its discharges into the Sound.