By measuring Nitrogen in a lake over long periods of time, we can get an idea if the lake system is changing or staying stable. Several things can lead to increases in Nitrogen levels in a lake. Nitrogen may be added to a lake by way of waste water from urban or developed areas or from high concentrations of animals such as bird colonies. Nitrogen can also be deposited from the atmosphere.

Nitrogen in the atmosphere comes mostly from the burning of fossil fuels, and can be deposited in lakes and have a significant effect on nitrogen levels. It is estimated that nitrogen inputs to the land from the atmosphere have doubled globally as a result of human activities. Air masses are able to carry pollution long distances, resulting in pollution being deposited far from where it was produced. In the Northeast, we are especially venerable to this type of pollution as it is produced in the mid-west from coal-burning power plants, and is carried east by our weather patterns and deposited on our lakes, streams and land masses. Even places in the Midwest and western U.S. are being heavily affected by nitrogen pollution from the atmosphere. According to scientists, Lake Superior has been enriched by as much as 300 µg/L of Nitrogen during this century, presumably due to air pollution. Some lakes like Lake Tahoe in California , which were nitrogen limited in the past, are now phosphorus limited because of increases in nitrogen in the lake from atmospheric deposition.

Normally the growth in lakes is limited by the amount of phosphorus in the system, but in lakes where phosphorus levels are very high, growth is limited by nitrogen. In this type of situation certain blue-green algae species have developed special adaptations that allow them to fix nitrogen from the air at the surface of the water. Nitrogen fixation by blue-green algae may in some cases contribute up to 50% of the annual nitrogen input to a lake.

A lot of the nitrogen found in lakes is in the form of living organisms, but it also exists in molecular form as nitrate, nitrate and in the reduced form, ammonium.

Nitrogen levels in lakes not affected by human activities tend to range from 100µg/L to1,500 µg/L and lakes which receive nutrient input from human activities and wastes from animals tend to range from 1,500 µg/L to over 6,000µg/L

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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