Limnology: Dr. Carlson’s Trophic State Index

Freshwater lakes and reservoirs, precious resources under tremendous stress, go through a natural eutrophication process. This natural process can be accelerated by development and pollution in the watershed.

As more nutrients enter the freshwater system, algae increase, causing the water to become cloudier, warmer, and reducing the dissolved oxygen concentration. As a lake/reservoir’s water quality can be in constant change, monitoring its water quality to maintain its desired state is essential.

The trophic state index (TSI), developed by Dr. Robert Carlson, is used to measure water quality of a lake or reservoir. Water quality has four general states:

  • oligotrophic,
  • mesotrophic,
  • eutrophic, or;
  • hypereutrophic

Students working with Dr. Lundberg determine the TSI by three parameters: water clarity, chlorophyll-a, and total phosphorus. The water clarity is determined by a Secchi disc, the chlorophyll-a by a fluorometer, and the total phosphorus by a UV-Vis spectrophotometer after acid digestion. Dissolved oxygen/temperature profiles are also created.

Dr. Lundberg’s project began in collaboration with Dr. Daniel Downey at James Madison University, providing reports on Shenandoah Valley and George Washington National Forest reservoirs to the Virginia Department of Game and Inland Fisheries, National Forest Service, and local managers.

In the summer of 2013, Dr. Lundberg and his students continued this project in collaboration with AW Research Laboratories on lakes in Brainerd, Minnesota, a popular resort area with approximately 500 lakes within a 30-mile radius.

Trophic State Index

Image: Dr. Daniel Lundberg and Brandon McMillan (Biology major/Chemistry minor) on Stickley Quarry, south of Winchester, VA.

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Limnology: Dr. Carlson's Trophic State Index Research Project

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