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What is the main cause for the mid summer algae blooms in the lake?
Most of the lakes in southern Minnesota are naturally fertile due to the productive soils of their watersheds. The realization of this natural fertility has been substantially increased as a result of human activities in most watersheds, however. Lake fertility may best be described in terms of nutrients, primarily phosphorus and nitrogen, which enter the water. In many ways, phosphorus is a limiting nutrient in our waters and a small amount of phosphorus can generate a large result. Phosphorus comes from many sources including fertilizers, industrial products and soils. Currently, phosphorus levels in Clear lake are at a record high for the lake and are higher than levels in most area lakes. Midsummer algae blooms on Clear lake are a result of several factors: one is the presence of curled pondweed, an exotic plant which originated in Asia. Another is the decline of natural vegetation in the lake. Curled pondweed has been in Minnesota for more than a century and is widely distributed. It has some unique attributes which allow the plant to dominate a lake’s plant community for periods of time. Curled pondweed starts growing under the ice in December, depending upon light penetration. After ice-out the plant grows rapidly. It suppresses algae and thus the water is usually clear during May and June. By July the plants die back, releasing nutrients into the water which algae quickly utilize, typically resulting in an algae bloom shortly after the pondweed disappears. The poor stands of native emergent and submergent vegetation on Clear lake coupled with the high levels of phosphorus allow the pondweed to flourish. Currently the volume of water entering Clear lake far exceeds the level needed to maintain the lake level at a run out elevation. Annual fluctuation in water levels of several feet are necessary to reestablish many types of natural emergent vegetation. The current water budget for Clear lake will not allow for such changes, which prior to dams and watershed development would occur as natural condition dictated. Last summer Clear Lake maintained a higher water level than many lakes having a much larger watershed. This phenomenon suggests the lake remained high due to ground water support or from other sources in the watershed. Without these fluctuations the likelihood of reestablishing native vegetation is low.