WHAT DOES IT TELL US ABOUT A LAKE?

Lakes that are productive (have lots of plant growth, especially algae) tend to have decreased oxygen levels in the lower waters because as the plant material dies, it sinks to the bottom of the lake and the decomposition process uses up oxygen. Because of thermal stratification, well oxygenated water from the surface does not mix with the lower waters. The bottom waters of a productive lake can become anoxic, meaning there is less than 1 part per million of oxygen (ppm) in the water.

Many scientists and lake monitors measure dissolved oxygen levels by depth to tell if the lower levels are becoming depleted of oxygen. Loss of oxygen in the bottom waters of a lake can affect biological and chemical processes in a lake.

Biology
Changes in dissolved oxygen effect the way that organisms are distributed throughout the lake, their behavior, and the way that they interact with other organisms. Loss of oxygen in the bottom waters can have drastic effects on organisms, especially those species which require both cold temperatures and high levels of dissolved oxygen, such as cold-water fish species like lake trout. Although there may be plenty of dissolved oxygen in the surface waters because of photosynthesis and diffusion from the atmosphere, the warmer temperatures here cause stress to the organism and negatively effect their health, reducing their ability to compete and reproduce.

When the lower waters of a lake become anoxic, the amount of habitat available to life in the lake is greatly decreased, especially for those cold water species which cannot thrive in warmer temperatures. This can change or disrupt food web relationships in the lake.

Chemistry
Dissolved oxygen levels can play a significant role in chemical processes in a lake. When the bottom waters of a productive lake become anoxic, this promotes the release of phosphorus from the sediments on the bottom of the lake. For lakes that already have problems with large amounts of phosphorus that can cause excessive algal growth, the release of this additional phosphorus can make the situation even worse.

Resources:
Bronmark, Christer and Lars-Anders Hansson. 1998. The Biology of Lakes and Ponds. New York. Oxford University Press. Kalff, Jacob. 2002. Limnology. Upper Saddle River, N.J. Prentice Hall.

Wetzel, Robert G. 2001. Limnology, Lake and River Ecosystems. New York. Academic Press.

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