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i itin utrints <br />The quantity or biomass of algae in a lake is usually limited by the water's concentration of an <br />essential element or nutrient-the "limiting nutrient." (For rooted aquatic plants, the nutrients are <br />derived from the sediments.) The limiting nutrient concept is a widely applied principle in ecology <br />and in the study of eutrophication. It is based on the idea that plants require many nutrients to grow, <br />but the nutrient with the lowest availability, relative to the amount needed by the plant, will limit <br />plant growth. It follows then, that identifying the limiting nutrient will point the way to controlling <br />algal growth. <br />Nitrogen (N) and phosphorus (P) are generally the two growth-limiting nutrients for algae in most <br />natural waters. Analysis of the nutrient content of lake water and algae provides ratios of N:P. By <br />comparing the ratio in water to the ratio in the algae, one can estimate whether a particular nutrient <br />may be limiting. Algal growth is generally phosphorus-limited in waters with N:P ratios greater <br />than 12. Laboratory experiments (bioassays) can demonstrate which nutrient is limiting by growing <br />the algae in lake water with various concentrations of nutrients added. Bioassays, as well as <br />fertilization of in-situ enclosures and whole-lake experiments, have repeatedly demonstrated that <br />phosphorus is usually the nutrient that limits algal growth in temperate lakes. Reducing phosphorus <br />in a lake, therefore, is required to reduce algal abundance and improve water transparency. Failure to <br />reduce phosphorus concentrations will allow the process of eutrophication to continue at an <br />accelerated rate. <br />grass_lake_intro.doc Page 3 09/27/99 <br />