QUANTITATIVE TROPHIC RECONSTRUCTION FROM SEDIMENTARY DIATOM ASSEMBLAGES - A CAUTIONARY TALE

1. The diatom stratigraphy of Pb-210-dated sediment cores is used to reconstruct the recent trophic histories of four oligo/mesotrophic lakes in the lower peninsula of Michigan, U.S.A. Quantitative trends in total phosphorus concentrations are reconstructed from the cores with a calibration data set based on modern diatom assemblages and water-chemistry variables in forty-two Michigan lakes that span a range of nutrient concentrations and biological productivities. 2. The total phosphorus reconstructions suggest that phosphorus concentrations have changed very little over the last 200 years, with modern values approximately equal to those at the time of settlement. Three of the four lakes, however, do show a trend to higher phosphorus concentrations at the time of logging and settlement and a subsequent decline. 3. The quantitative inferences of trophic change are compared with interpretations based on diatom species composition and the accumulation of diatoms, biogenic silica and carbonate in the sediments, and some discrepancies occur among the various lines of evidence. These comparisons are used to discuss the sensitivity of various proxies for reconstructing the nature and magnitude of trophic change.