Chemical Limnology of Soft Water Lakes in the Upper Midwest

Water samples from 36 lakes in northern Minnesota, Wisconsin, and Michigan were collected and analyzed during 1983-84. All study lakes were dilute and had total alkalinities of less than 150 μ eq· L-1. Minnesota lakes have hydrologic inputs from the watershed and inputs of base cations derived from the watershed. Study lakes in Minnesota had higher total alkalinities, dissolved organic carbon, and noncarbonate alkalinity as a result of watershed inputs. Lakes in Michigan and Wisconsin were precipitation-dominated seepage lakes that have lower concentrations of base cations than lakes in Minnesota. All of the study lakes have lower sulfate concentrations than expected, based on atmospheric wet deposition and evapotranspiration. Pore water samples collected from one of the study lakes--Little Rock Lake--in Wisconsin were used to calculate diffusive fluxes between the sediment and water column. According to these calculations, the sediments were a source of total alkalinity and Ca2+ and a sink for SO42-. The sediment-water exchange of total alkalinity, Ca2+, and SO42- appears to be important in the whole-lake budgets of these ions for Little Rock Lake.