2008 Lake Water Quality Assessment Report for Six McLeod County Lakes

Six lakes in McLeod County, (Silver, Cedar Marion, Hook, Stahl's and Otter) were monitored in 2006 as a part of a study to assess the levels of the blue-green algal toxin, microcystin, in eutrophic Minnesota lakes. Details from that study may be found in Lindon and Heiskary (2007). Cedar, Otter and Marion were also sampled in 2008 to allow for 303(d) assessments of these lakes. All six lakes lie near the transition of the North Central Hardwood Forest and Western Corn Belt Plains Ecoregions. Watershed-to-lake ratios vary from 2:1 (Silver) to >1,000:1 (Otter). Watershed land use is characterized by a mix of land uses with agriculture being prominent in each watershed. All six lakes are considered shallow, with most having 100 percent littoral. As a result, the lakes are generally well-mixed throughout most of the summer, though periodic temporary stratification may occur during warm, calm periods. Dissolved oxygen (DO) frequently falls below 2 milligram per liter (mg/L) near the bottom of the lakes during calm periods and this may help promote internal recycling of phosphorus from the lake sediments. Based on trophic status measures (total phosphorus (TP), chlorophyll-a, and Secchi) all six lakes are considered eutrophic or hypereutrophic. Based on chlorophyll-a concentrations and algal assessments, nuisance blue-green algal blooms were common on these lakes. For most of the lakes, there was minimal long-term data available and no distinct trends over time were evident. Model estimates indicate all lakes have modern-day TP concentrations in excess of background (e.g. pre-European); however, Minnesota Lake Eutrophication Analysis Procedure (MINLEAP) model estimates for Marion, Stahl's and Otter indicate observed TP is equal to or lower than that expected based on lakes with similar morphometric and watershed characteristics in these ecoregions. All six lakes exceed or are near their respective nutrient impairment thresholds, and it is likely Cedar, Marion and Otter will join Hook (listed in 2008) on the 2010 303(d) list for nutrient impairment. Silver and Stahl's may lack sufficient data for the 2010 assessment.