Some species of fish can no longer survive in Onondaga Lake because of the lack of dissolved oxygen in the lower lake waters.

Fish and other aquatic life in Onondaga Lake need oxygen in the lake water to survive. If the level of oxygen dissolved in the lake water is too low, the fish and other animals must move to water with higher oxygen levels, or they will die.

Decomposition of Algae

Most of the algae produced in the upper layers of Onondaga Lake ultimately settle to the bottom where they are decomposed by bacteria and fungi. Oxygen is used up as part of the decomposition process. The greater the algae production in the upper layers, the greater the loss of oxygen from the lower layers of the lake. Presently, oxygen is lost from the bottom of Onondaga Lake very rapidly compared to other lakes. The oxygen in the lake's lower waters is gone by July. For more information about algae in Onondaga Lake, see Pollutants and Sources: Phosphorus.

Oxygen and Cold Water Fish

Fish like trout, salmon, and whitefish are referred to as cold water fish because they prefer the colder, deeper lake waters. Before the turn of the century, cold water fish flourished in Onondaga Lake. Cold water fish, which require at least 5 milligrams of oxygen per liter of water, can no longer survive in Onondaga Lake because of the lack of oxygen in the lower lake waters. For more information about fish in Onondaga Lake see The Lake's Fishery.

By-products of Decomposition

The by-products of algae decomposition such as hydrogen sulfide, methane and ammonia accumulate in the lower lake waters following the loss of oxygen. These substances are distributed throughout the lake during fall turnover. As a result, the oxygen concentration throughout the lake in some years drops below 4 milligrams per liter. These oxygen crises not only violate environmental standards, they also drive fish out of the lake outlet and into the Seneca River. Such severe annual losses of oxygen have not been reported in the upper waters of any other lakes in the United States.