Significant population growth over the last three decades, as well as efforts to improve surface-water quality mandated by the Clean Water Act, potentially have had opposing influences on aquatic ecosystems in the U.S. Because historical data on water-quality trends are limited over this time period, we developed a diatom-based transfer function to reconstruct chloride, color, acid neutralizing capacity (ANC), total phosphorus (TP), and pH in 55 Minnesota lakes. The lakes span three different ecoregions, as well as the Twin Cities metropolitan area, and differ in their history of settlement and land use, and in surficial geology, climate, and vegetation. Lakes in the Northern Lakes and Forest ecoregion are nearly pristine, whereas those in the other regions likely are strongly affected by urban or agricultural pollutants. Reconstructions of water-chemistry trends since 1970 suggest that recent human activities have had substantial impacts in both urban and rural areas. Chloride concentrations have increased in many Metro lakes, which may be due to road salts, and phosphorus levels have been steady or rising in agricultural regions. The majority of Metro lakes show some decline in TP, although many of the changes are not statistically significant based on our reconstruction techniques. There is no evidence that atmospheric deposition of sulfate or nitrate has caused acidification or changes in trophic state for remote lakes in the northeastern part of the state.
Bibliographical noteFunding Information:
This study was part of larger project on historical records of nonpoint source pollution that was codirected by Edward Swain (Minnesota Pollution Control Agency). James Almendinger and Ed Swain helped with sediment coring, and Carol Hubbard and Dan Helwig assisted in coordinating the water sampling. Lead-210 dating and sediment handling were the responsibility of Kelly Thommes. This work was funded by the Legislative Commission on Minnesota Resources through a grant to the MPCA and the Science Museum of Minnesota.
- Minnesota lakes
- Transfer function