Crabs will scurry on to dry land, and eels will lay their heads on the bank gasping for air. Anoxia has always been a problem in the deepest parts of our sounds during the summer. During hot, dry summers, when there is little wind and rainfall, the water begins to stratify, or form horizontal layers. The upper layer of water always contains plenty of dissolved oxygen because of the exchange that takes place at the surface and because plants and algae produce oxygen through photosynthesis. However, in the saltier, lower level, very little photosynthesis goes on because sunlight cannot filter through.
Excess nutrients flowing into our coastal waters can compound problems caused by anoxia.
Nutrients such as nitrogen and phosphorus, are food for algae and other microscopic organisms. As algae multiply, or bloom, they remove the oxygen from the water. As they die, the algae sink and decompose. This processes uses up even more oxygen and results in the death of fish, crabs, oysters, clams and other creatures. These nutrients can enter estuaries via any number of sources upstream including livestock operations, agricultural runoff, sewage treatment plants, and industrial discharges.