Lake Assessment Program - Lake Mary, Douglas County

Lake Mary is located in Douglas County, near Alexandria, MN. This lake has a surface area of 2,371 acres, mean depth of 13.5 feet and a maximum depth of 40 feet. Because of its large surface area and fetch the it tends to be well mixed through most of the summer. The total watershed is about 18,580 acres (including the lake). Land use in the watershed is characterized by cultivated (61 percent), open water/marsh (22 percent), grassland/pasture (9 percent), forested (5 percent), and residential/urban areas (the remaining 3 percent). These land use percentages are typical of what would be expected for lake watersheds in the North Central Hardwood Forests (NCHF) ecoregion of the state. Lake Andrew and Pocket Lake are within the Mary watershed and will serve to trap nutrients and sediments from that portion of the watershed, which in turn reduces the loading to Lake Mary. Lake Mary was sampled during the summer of 2005 by Minnesota Pollution Control Agency (MPCA) staff and volunteer Gary Bahn as part of the advanced Citizen Lake-Monitoring Program Plus. Water quality data collected during the study revealed a summer-mean total phosphorus concentration of 28 ?g/l, chlorophyll-a concentration of 8 ?g/l, and a Secchi transparency of 6.6 feet. All three of these values are well within the typical range exhibited by reference lakes in the NCHF ecoregion. Total phosphorus, chlorophyll-a, and Secchi transparency all help to characterize the trophic status of a lake. For Mary Lake, these measures indicate mesotrophic-eutrophic conditions.