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SITilATION ANALYSIS LAKE OWAS50 The Basin Lake Owasso has a surface area of 383 acres. The lake's maximum depth is 36 feet, however much of the lake is shallow. In fact 76% is less than 15 feet deep which means that Lake Owasso is shallow by metro norms. The Watershed Lake Owasso receives surface runoff from a tributary area of 3,491 acres (about 9 times the lake's surface area). Drainage to Lake Owasso carries phosphorus, which has been estimated' to be 1,450 pounds per year. Runoff also carries other kinds of pollution, like sediment, salt or toxic chemicals, but no measurements or estimates have been made to quantify other kinds of pollution in the runoff to Lake Owasso. Except for localized erosion concerns, there is no evidence that other kinds of pollution in runoff are causing problems in Lake Owasso. Water Level Water level fluctuations occur in Lake Owasso. Ramsey County pumped groundwater into the lake for many years, but that operation has been suspended -the last year pumping occurred was 1991. The pumping mitigated low water levels during the summer. Because there has been `wet' weather since the pumps were discontinued, there have been minimal problems with low water levels in Lake Owasso. It is likely that low water levels will occur at times in the future. Water Quality Eutrophication is a process by which lakes become enriched with nutrients -usually phosphorus -that cause an increase in the amount of algae in the lakes. Eutrophication is often a concern because the manifestations of over- fertilization are perceived as nuisances. In the extreme, highly eutrophic lakes are plagued by persistent algae blooms, which diminish the recreational use and environmental value of the lake. There are several indicators that are commonly used to track eutrophication and its manifestations: Indicator SiQniflcance Nutrients Phosphorus is considered the limiting nutrient in most lakes. This means that it is the element in shortest supply relative to the growth needs of algae. Phosphorus is measured from lake water collected in the middle of the lake. Sometimes nitrogen can also limit algae growth. Algae Algae are microscopic plants that float in lakes. Algae become nuisances when they become abundant. A particular kind of algae -blue-green algae - are a particular nuisance because they form scums. All algae become more abundant as the level of nutrients in the water increases. The level of algae is determined by measuring chlorophyll - a green pigment - in lake water. Clarity The transparency of lake water is readily observed by everyone. As the level of algae increases, the water clarity decreases. Clarity is measured using a Secchi disk, which is a 8-inch white or black-and-white disk lowered over the side of a boat until it disappears. Normally, as phosphorus levels increase, so does: ^ The abundance of algae and the frequency of algae blooms ^ The predominance of blue-green (or scum-forming) algae ^ A reduction in water clarity