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important part of the biological system within a lake. However, too great an abundance of algae <br />adversely affects both the ecology of the lake as well as the suitability of the lake for use by <br />people. <br />Phosphorus is the plant nutrient that most often controls the amount of algal production in a lake. <br />This is because, compared with other plant nutrients phosphorus in a lake is generally lowest <br />relative to demand by algae. If phosphorus concentrations are low, algal growth will be low. <br />Conversely, high phosphorus concentrations often foster high algal productivity. <br />Under enriched conditions (i.e., high phosphorus concentrations in the water column), <br />populations of certain types of algae can explode during the summer growing season. One of the <br />most common nuisance algal types is blue -green algae or cyanobacteria. They are particularly <br />problematic for several reasons, including: <br />• They decrease water clarity, which affects the perceived suitability of the lake for direct <br />contact recreation such as swimming <br />• They form objectionable surface scum. The decomposition of surface scum often forms a <br />pattern on the water's surface much like spilled paint and generates strong odors that <br />negatively impact the recreational use an aesthetic appeal of the lake <br />• The ecology of bluegreen algae is such that most of the population dies off naturally. <br />Decomposition of this organic mass depletes the lake of oxygen. The resulting lower <br />dissolved oxygen concentration can threaten the survival of desirable game fish, make the <br />lake more suitable for rough fish, and trigger other undesirable conditions, such as release <br />of phosphorus held in the bottom sediments of the lake <br />• Excessive algal growth can cause taste and odor problems in drinking water if the lake is <br />used as a source of raw water. This can substantially increase treatment costs for the <br />water prior to distribution <br />• On occasion, bluegreen algae can secrete toxins poisonous to warm blooded animals. <br />While few human deaths have been reported, there are many documented cases of <br />wildlife and domestic animal deaths as a result of ingestion of these toxins <br />City of Roseville 35 <br />Parks Natural Resource Management <br />