partially mix (such as with the passing of a cold front accompanied by strong winds and cold rains), allowing some of these nutrients to “escape” into the epilimnion and potentially stimulate an algal bloom. For similar reasons, algal blooms often are seen at fall turnover as nutrient-rich bottom water is brought to the lake surface where there is ample sunlight to support algae growth. Ammonia-nitrogen also can have an impact on fish. Fish are sensitive to ammonia and are repelled by high levels in the water.

Metals and Other Compounds: Some metals and other elements—notably iron, manganese, and sulfur (as hydrogen sulfide)—also become increasingly soluble and are released from anoxic bottom sediments. These compounds cause taste and odor problems—a potentially serious concern in drinking water supply reservoirs. Additionally, hydrogen sulfide concentrations above 1 mg/L are lethal to many gamefish as well as some zooplankton (microscopic animals that are an important fish food).

Fish:  Low oxygen levels may restrict where fish can go in a lake and limit the types and numbers of fish in the hypolimnion. Warmwater fish (e.g., bass and bluegill) need at least 5 mg/L of dissolved oxygen to survive, while coldwater fish (e.g., trout) require 6-7 mg/L. In eutrophic lakes, as summer progresses and dissolved oxygen levels become too low in the hypolimnion, fish are confined to the epilimnion and a portion of the metalimnion.

As ice covers a lake in early winter, there usually is adequate oxygen in the water to sustain fish and other aquatic organisms. You may be surprised to learn that certain algae and rooted aquatic plants grow right through the winter and photosynthesize, producing oxygen. However, bacterial decomposition of organic matter on the lake bottom can consume more oxygen than photosynthesis can replace, causing a decline in dissolved oxygen levels as the winter season progresses. If enough snow covers the ice or if the ice is opaque, sunlight may be inadequate or unable to penetrate and photosynthesis will stop. If the lake’s supply of oxygen falls too low before ice- out, a partial or total fishkill can occur.

Temperature

In summer-stratified lakes, water temperatures decrease from the surface to the bottom. As discussed above, a warm surface layer (the epilimnion) “floats” on a colder layer (the hypolimnion).

Different fish species prefer different water temperatures. Hence, a lake’s temperature variations are important in influencing what types and how many fish will live and reproduce in that lake. If the colder, deeper waters of the hypolimnion have enough oxygen, then that area will provide a refuge for fish species that prefer, or require, cold water temperatures. However, if dissolved oxygen levels become too low in the hypolimnion and fish are forced into the warmer surface waters, coldwater fish species may not be able to survive.