Abstract. Sediment cores were collected to investigate multiple stresses on Clear Lake, California, USA, through the period of European occupation to the present day. Earlier workers suggested the hypothesis that the use of mechanized earthmoving equipment, starting in the 1920s and 1930s, was responsible for erosion, mercury (Hg) contamination, and habitat loss stresses. Cores (;2.5 m in depth) were collected in 1996 and 2000 from each of the three arms of the lake. Carbon-14 dating suggests that these cores represent as much as 3000 years of the lake's history, beginning long before European settlement. Total mercury (TotHg) and methylmercury (MeHg), dry matter, water, carbon, nitrogen, phosphorus, sulfur, and the stable isotopes 13 C and 15 N were measured at 5-cm intervals. Nearly all parameters show major changes at depths of 58-135 cm, beginning at ca. 1927 (dated with 210 Pb). Accepting this date for concomitant major changes in seven cores yields an estimated 8.6 mm/yr average sedimentation rate after 1927. Pre-1927 sedimentation rates were ;1 mm/yr. Total mercury and MeHg, dry matter, phosphorus, and 15 N increase significantly, whereas nitrogen, sulfur, carbon, and water content decrease significantly above the 1927 horizon. Both TotHg and MeHg show extremely large increases (roughly 10-fold) above the 1927 horizon. A peak in inorganic deposition rate and minimum values for percentage of water is present at depths corresponding to ca. 1970. Interestingly, the first 75 years of European settlement in the Clear Lake basin (including the most productive years of the Sulphur Bank Mercury Mine) appeared to have had undetectable effects on lake cores. Changes since 1927 were dramatic. The large increase in Hg beginning about 1927 corresponds to the use of heavy equipment to exploit the ore deposit at the mine using open-pit methods. Increases in sediment deposition from increased earthmoving in the basin and sulfate loading from the mine are the most likely explanations for the dramatic changes seen in the post-1927 sections of the cores.

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We investigated the effects of cultural eutrophication on the coupling between pelagic primary producers and benthic consumers in Lake Tahoe. Spatial and temporal changes in zoobenthos energetics were documented by measuring Ͼ40 yr of change in pelagic primary production through 14 C incubations, reduction in clarity by Secchi and light measurements, and sedimentation rates. Effects on zoobenthic primary consumers (oligochaete and chironomid) and an obligate benthic secondary consumer (Catostomus tahoensis) were determined by comparing ␦ 13 C values of historical and contemporary samples. A model that considers primary production (benthic or pelagic) contributions and their respective ␦ 13 C signals was used to examine the factors contributing to zoobenthic energy shifts. Spatially, zoobenthos exhibited a strong positive relationship between lake depth and pelagic isotopic signals. For depths at which ambient 1% light levels have shifted with time (50-85 m), pelagic primary producer and zoobenthic consumer coupling was positive. Historically, zoobenthos from this depth zone obtained 27% of their energy from phytoplankton sources. After 43 yr of eutrophication, they obtained 62% from pelagic sources. A simple model indicated that increased pelagic production and resultant export of matter combined with the loss of benthic primary production contributed to the change in zoobenthos energetics. This change was passed on to higher consumers, with the benthic fish Tahoe sucker (Catostomus tahoensis) now deriving ϳ21% of its energy from pelagic primary production sources. This study demonstrates how lake eutrophication increases the coupling between pelagic and benthic habitats.

Evidence from this study suggests the existence of a significant modern source for atmospheric Hg deposition in the Sierra Nevada, on the continental west coast of the United States. Concentrations of both lead (Pb) and mercury (Hg) in the sediments of Lake Tahoe deposited prior to 1850 are similar to concentrations in the catchment bedrock, but their concentrations in modern sediments have increased 6-fold for Pb (average 83 ppm) and 5-fold for Hg (average 0.191 ppm). The lake occupies a relatively pristine, nonindustrialized subalpine basin, with a watershed to lake surface ratio of only 1.6. Excess accumulation of trace metals in these sediments should closely reflect direct atmospheric deposition. On average, since 1980 there have been approximately 17 mg of Pb and 38 μg of Hg deposited annually/m2 in excess of the baseline flux. While Pb emissions occurred locally in the Tahoe Basin, from combustion of leaded gasoline until about 1985, the deposition of atmospheric Hg must represent a predominately regional to global source of contamination. Ratios of total modern flux to preindustrial flux are 29 for Pb and 24 for Hg. The flux ratio for Pb is somewhat higher than reported from the eastern United States and Canada but is not atypical. The flux ratio for Hg is much higher than that observed in most other natural aquatic systems without point-source contamination.