The extent that natural lakes in the United States of America have been changed by cultural eutrophication

We used paleolimnological data for 240 lakes from the U. S. Environmental Protection Agency's (USEPA) 2007 National Lakes Assessment to estimate the extent that natural lakes in the coterminous United States have been changed by anthropogenic activities. In order to detect cultural eutrophication, we analyzed data on diatominferred concentrations of total nitrogen (TN), total phosphorus (TP), specific conductance (SC), and pH as determined from lake sediments from the tops and bottoms of sediment cores from 240 natural lakes where the bottom of the core was judged to represent conditions prior to European settlement. We found no statistically significant increases in the average concentrations of TN, while TP decreased by 14% in this population of lakes since the time of European settlement. We also analyzed data from 48 reference lakes used by the USEPA to determine the relative condition of the sample lakes. Paired t-tests showed the TN concentrations were not significantly different between the two time periods (p > 5%), while the average TP concentrations had significantly decreased by 26% since presettlement times (p < 5%). There were statistically significant increases in SC (17%) and pH (0.05 pH units). There were no statistically significant differences between the changes in TP, TN, SC, and pH in the 240 sample lakes and the changes in the 48 reference lakes. The proportions of lakes categorized as oligotrophic, mesotrophic, eutrophic, and hypereutrophic for the presettlement time period were not significantly different from the proportions found in 2007.