Water Quality Reconstruction from Fossil Diatoms: Applications for Trend Assessment, Model Verification, and Development of Nutrient Criteria for Lakes in Minnesota, USA

Diatom reconstructions of historical phosphorus and chloride concentrations and sediment accumulation rate, based on sediment cores from 55 lakes in Minnesota, provide a unique opportunity for examining temporal and spatial trends in eutrophication, validating eutrophication models, and providing historical perspective for developing nutrient criteria. Sediment cores, obtained by a piston corer between 1995 and 1998, were sectioned and dated. Sections corresponding to 1750, 1800, 1970, and 1993 time-periods were used in this analysis. Distinct regional patterns in historic (mean of 1750 and 1800) phosphorus concentrations were evident: Northern Lakes and Forests lakes averaged 15 ?g/L (± 1), North Central Hardwoods Forests lakes averaged 24 ?g/L (± 2), and Western Corn Belt Plains lakes averaged 47 ?g/L (± 6). Comparing these values to the recent 1990s values suggests no change in the NLF lakes (15 ?g/L ± 1), but significant increases in the CHF (38 ?g/L ± 5) and WCP lakes (67?g/L ±16). Comparisons were made between the historic values and two empirical models routinely used to help set in-lake phosphorus goals in Minnesota. The first model (Vighi and Chiaudani 1985), based on the morphoedaphic index, is used to predict the background phosphorus concentration of a lake. The second (MINLEAP) is an ecoregion-based model that predicts in-lake phosphorus based on morphometry, watershed area, and ecoregion characteristics (Wilson and Walker 1989). Comparisons between historic phosphorus concentrations and model predictions indicate good correspondence at model-predicted values less than about 30 ?g/L. Lastly, the use of diatom-inferred values is explored as a part of Minnesota's efforts to develop water quality standards for nutrients.