PurposeMany lakes in Wisconsin have elevated concentrations of nutrients, which result in unsightly algal blooms, as well as an over abundance of aquatic plants. The high phosphorus levels often result in runoff from watershed activities. In the case of Little Green Lake there is considerable agriculture in the watershed as well as homes around most of the shoreline. A study conducted by Ramaker and Associates in 1998-99 as well data collected by the U.S. Geological Survey since 1991 indicate that considerable internal loading is occurring in the lake. Modeling by Ramaker and Associates estimated that 70 percent of the total P load comes from internal loading.
ObjectiveThere are several sources from within the lake that can contribute P. These include benthivorous fish, e.g. carp, bullheads; nutrient release following die off of macrophytes, e.g. curly-leaf pondweed; sediment release from anoxic conditions during stratification; and elevated pH during intense algal blooms. Because of the morphometry of Little Green Lake, it is susceptible to periods of stratification and mixing throughout the summer. Its maximum depth (28 ft) is deep enough for stratification to occur during periods of warm weather with little wind, but not deep enough to prevent mixing during moderate wind events. During stratification the bottom waters may become devoid of oxygen allowing phosphorus to be released from the sediments. Subsequent mixing transports the P to the upper waters thus enhancing algal growth which result in noxious algal blooms. While the stratification/mixing is not well documented in lakes it is known to occur in a few lakes.
OutcomeDuring 2003, a destratification system was installed and began operation with the intent of significantly reducing the amount of stratification that occurs during the summer months. By preventing this stratification and the subsequent depletion of oxygen, it was expected that release of phosphorus from the sediments would be greatly reduced and thus reducing the severity of algal blooms. In 2003, because of some technical problems the system did not operate the entire summer. There were no problems in 2004 and the system was in operation from May through September.
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Final report of destratification project as of 2004.
Progress of destratification project as of 2003.
Progress of destratification project as of 2000.