1. The diatom stratigraphy of ^^"Pb-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 modem 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 modem 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.

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Widespread drought is among the most likely and devastating consequences of future global change. Assessment of drought impacts forecast by atmospheric models requires an understanding of natural drought variability, especially under conditions more arid than today. Using high-resolution lake-sediment records from the northern Great Plains, we show pronounced 100-to 130-yr drought cycles during the arid middle Holocene (8000 calendar yr BP). During drought phases, grass productivity declined, erosion and forbs increased, and fuel limitation reduced fire importance. Intervening humid decades saw grass production rise, with stabilization of soils and renewed fire as fuels became abundant. Although both C 3 and C 4 grasses declined during droughts, a lasting shift to C 3 dominance occurred during a single drought ϳ8200 calendar yr BP. During the more humid Late Holocene (2800 calendar yr BP), climate was less variable and without evident drought cyclicity. Consequently, drought severity during past, and possibly future, arid phases cannot be anticipated from the attenuated climate variability evident during contemporary humid phases. Our study demonstrates that agriculturally important grassland ecosystems respond sensitively to drought variability, uncertainty in which has profound implications for the future of these ecosystems.

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• We studied mercury deposition in Lake Chungará (18°S) over the last 2700 years. • Parinacota volcano produced 20 tephra layers recorded in lake sediments. • Lake primary production was the main, not limiting, carrier of Hg to the sediment. • Volcanoes contributed to~30% of Hg inputs to the lake over the study period. • Last 400 years anthropogenic Hg emissions overwhelmed the volcanic activities.