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MEASURING EUTROPHICATION IMPACTS ON DISSOLVED OXYGEN CONCENTRATIONS IN LAKE ST. CROIX
Kent Johnson, Metropolitan Council Environmental Services
In 2008, the Minnesota Pollution Control Agency (MPCA) and Wisconsin Department of Natural Resources (WDNR) listed Lake St. Croix as impaired, based on "nutrients, eutrophication, and biological indicators." In 2009, the St. Croix Implementation Team will complete a TMDL plan to address the 20% reduction in phosphorus loading necessary to restore lake water quality.
Metropolitan Council Environmental Services (MCES), local citizen volunteers, and the National Park Service have been monitoring eutrophication indicators (total phosphorus, chlorophyll-a, and Secchi depth) in Lake St. Croix since 1976, 1999, and 2007, respectively. These monitoring data have been valuable for identifying long-term water quality trends, as well as the current eutrophication impairment.
However, eutrophication impacts on dissolved oxygen (DO) concentrations in Lake St. Croix have not been well measured or understood. To better determine the prevalence of hypolimnetic anoxia in the lake, local citizen volunteers, with MCES support, obtained monthly DO profiles at seven locations throughout Lake St. Croix during the summers of 2008 and 2009. To better understand diurnal and seasonal DO fluctuations in the lake, the National Park Service (NPS) deployed continuous monitoring sondes at the inlet (Stillwater) and outlet (Prescott) during the summer of 2009.
The results of the 2008-2009 DO monitoring work indicate that hypolimnetic anoxia was commonly observed throughout Lake St. Croix during the summer months, especially in July and August. DO concentrations appear to be controlled largely by water temperature in the early summer, and become more influenced by primary productivity later in the summer. DO concentrations and diurnal fluctuations in these concentrations were typically higher at Stillwater than Prescott, perhaps due to upstream river processes or higher concentrations of phosphorus. Down-lake hypolimnetic anoxia may be contributing to lower DO concentrations at the lake outlet near Prescott.