Determining the Condition

Water Quality

  Classifying the Condition of Our Lake (Trophic State)

Trophic state is a method to classify lakes and is an indicator of water quality. Common characteristics used are clarity, chlorophyll (a measure of algae present), and total phosphorus concentration.   There are 4 stages of the Trophic State Method.

  • Oligotrophic: Low biological productivity. Oligotrophic lakes are very low in nutrients and algae, and typically have high water clarity and a nutrient-poor inorganic substrate. Oligotrophic water bodies are capable of producing and supporting relatively small populations of living organisms (plants, fish, and wildlife). If the water body is thermally stratified, hypolimnetic (deep water) oxygen is usually abundant.
  • Mesotrophic: Moderate biological productivity and moderate water clarity. A mesotrophic water body is capable of producing and supporting moderate populations of living organisms (plant, fish, and wildlife). Mesotrophic water bodies may begin to exhibit periodic algae blooms and other symptoms of increased nutrient enrichment and biological productivity.
  • Eutrophic: High biologically productivity due to relatively high rates of nutrient input and nutrient-rich organic sediments. Eutrophic lakes typically exhibit periods of oxygen deficiency and reduced water clarity. Nuisance levels of macrophytes and algae may result in recreational impairments.
  • Hypereutrophic: Dense growth of algae throughout summer. Dense macrophyte beds, but extent of growth may be light-limited due to dense algae and low water clarity. Summer fish kills are possible.: lakes have low nutrient content, and thus are very clear, produce few weeds, and do not support large fish populations.

The introduction of nutrients from the watershed leads to algae blooms, oxygen depletion, and weed growth. Nutrients are introduced by non-point source pollution. Rainfall and subsequent runoff carry nutrients and pollutants from the watershed into the lake. Effluents from shoreline septic systems are an important non-point source of nutrients. Soil erosion from construction sites also introduces nutrients. In addition, erosion contributes to sedimentation and turbidity, and can be harmful to fish and aquatic organisms. Non-point source pollution is difficult to control, because nutrients are introduced at relatively small concentrations over large areas. Land-use management strategies in the watershed must be implemented to control this type of pollution.

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