US Long-Term Ecological Research Network
The aquatic chemistry of aluminum, iron, manganese, cadmium, copper, lead, and zinc in an experimentally-acidified seepage lake
Little Rock Lake, a small (18 ha), low-alkalinity (25 \mueq/L, pH 6.1) seepage lake in northern Wisconsin, was divided into two basins by a flexible, inert barrier, and beginning in May 1985, the north basin was acidified in three two-year steps to pH 5.6, 5.1 and 4.7. The annual-average pH of the reference basin remained near 6.1. At pH 5.6, dissolved (0.4 \mum pore-size filtered samples) Mn and Fe increased in the epilimnion of the acidified basin. At pH 5.1 and 4.7, dissolved Al, Fe, Mn, Cd, and Zn in the epilimnion of the acidified basin were elevated over reference-basin levels. At pH 4.7, dissolved Pb in the acidified basin became elevated over reference-basin levels. Dissolved Cu remained similar in both basins down to pH 4.7. Enclosure experiments were conducted in the treatment basin to make predictions about its response to acidification. The results from these experiments were in reasonable qualitative agreement with whole-lake results for dissolved Al, Mn, Cd, Cu, and Zn, but some differences were noted for Pb and Fe. Sedimentation of metals, as measured by sediment traps, was also affected by acidification. Sediment-trap samples from the epilimnion of the acidified basin at pH 5.6 contained less Mn (\mug/g dry wt basis) than samples from the reference basin, and at pH 5.1, contained less Al, Mn, and Pb than reference-basin samples. The Cu content of sediment-trap samples from both basins was similar during these two periods of acidification. Redox cycling of Fe and Mn occurred in the anoxic bottom waters of the acidified basin during stratification. Elevated concentrations of dissolved and particulate Al, Cd, Cu, Pb, Zn, and dissolved organic carbon (DOC) were also observed in the bottom waters at this time, and apparently were released from surface sediments along with Fe and Mn. Except for Fe, whole-basin concentrations of dissolved and particulate metals and DOC did not increase measurably after autumn turnover because of the relatively small volume of the hypolimnion compared with whole-basin volume.
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