EFFECTS OF ACIDIFICATION ON MINOR AND TRACE-METAL CHEMISTRY IN LITTLE ROCK LAKE, WISCONSIN

Little Rock Lake is a low‐alkalinity (25 μeq/L, pH 6.1) seepage lake in north‐central Wisconsin. The two main basins of the lake were separated by a polyvinyl barrier in 1984, and acidification of the north basin with H2SO4 began in spring 1985; the south basin is being maintained as a reference. The effects of acidification on dissolved concentrations of Al, Fe, Mn, Cd, Cu, Ni, Pb and Zn have been studied at three scales at Little Rock Lake: (A) laboratory‐scale experiments with well‐mixed sediments; (b) large (4‐ to 5‐m dia) in situ enclosures maintained at pH 4.5 to 6.0; and (c) the lake itself. In general, qualitative agreement is good between the three approaches, but some differences have been observed, especially for Al and Fe. The most significant trace metal response observed in acidifying the north basin from pH 6.1 to 5.1 has been a large increase in dissolved Mn, apparently as a result of hindered oxidation of Mn2+ at lower pH. Dissolved Al and Fe also increased at pH 5.1 but not at 5.6. Water column levels of Cd, Cu, Pb and Zn have not increased at pH 5.1 in the lake, but results of lab and enclosure studies suggest that increased concentrations of Cd and Zn will occur when the lake is acidified to pH 4.6. Sediment cores and surface sediments from both basins of the lake have been collected and analyzed for Al, Fe, Mn, Cd, Cu, Hg, Ni, Pb and Zn. Similar concentrations of most metals were found in surface sediments from both basins of the lake. Near‐surface sediments were enriched in Hg and Pb relative to deeper sediment layers; the enrichment is presumed to be caused by increased atmospheric inputs during the past century, not diagenetic activity. No consistent differences in the Hg content of zooplankton, periphyton and benthic invertebrates were found at pH 5.6 and 5.1 in the acidified basin compared with the pH 6.1 reference basin. Copyright © 1990 SETAC