Oxygen Depletion Investigations of Legend Lake, Menominee County, Wisconsin

Abstract

Severe to moderate dissolved oxygen depletion occurs in four of the eight lake basins of Legend Lake, Menominee County, Wisconsin. All of the basins show less than 1 mg/1 throughout their hypolimnetic stratum in mid-summer. Two of the basins also show less than 1 mg/1 throughout their metalimnetic stratum in mid-summer. Winter oxygen deficits extend to within two meters of the surface in four of the lake basins. Depletion occurs in spite of chlorophyll a values that indicate an oligotrophic lake. Lake morphometry and water exchange rates varied widely between lake basins. Winter depletion of dissolved oxygen was correlated: first, with the increasing amount of sediment organic matter; second, with the amount of aquatic plants and algae; third, with the shallower mean depth; and fourth, with the lowest hydraulic loading rate. Water column areas that are nearer the eastern dam outlet are more dilute in nutrients and do not experience severe winter oxygen depletion. There is therefore an apparent water quality gradient of better conditions in the lake basins nearer the eastern dam. Groundwater is an important component of the overall hydrology; a significant amount of water both enters and leaves Legend Lake as groundwater. Also, there is a large difference in hydraulic residence times among the lake basins. Stratification in the deep hole basins, a continuing supply of iron from the watershed, and marl deposition combine to trap phosphorus in the sediments. Not all Legend Lake basins have the same processes occurring and therefore vary widely in their capacity to remove phosphorus and other nutrients from the water column. There could be some algae bloom problems for some of the shallower, longer retention time lake basins in the future unless watershed phosphorus inputs are reduced at their source.