Trace metal research at Little Rock Lake, Wisconsin: background data, enclosure experiments, and the first three years of acidification
Water-column and sediment-trap monitoring programs were used to investigate the important mechanisms regulating the biogeochemical cycling of Al, Fe, Mn, Cd, Cu, Pb and Zn in the acidified and reference basins of Little Rock Lake, an oligotrophic seepage lake in northern Wisconsin. Background lake water concentrations of dissolved (0.4 μm pore-size filtered samples) metals were (μg l-1): Al, 7; Fe, 17; Mn, 12; Cd, 0.05; Cu, 0.3; Pb, 0.1; and Zn, 3. The two main basins of the lake were divided by a polyvinyl barrier, and stepwise acidification (2 years each at pH 5.6, 5.1 and 4.6) of the north basin with H2SO4 began shortly after ice-out in April 1985. The south basin (annual average pH 6.1) is being maintained as a reference. Of the metals measured in this study, only dissolved Mn increased (from 12 to 30 μg l-1) when the north basin was acidified from pH 6.1 to 5.6; this was accompanied by a 70\% decrease in the sedimentation rate of particulate Mn in this basin. Further acidification of the north basin to pH 5.1 resulted in a further increase in dissolved Mn (from 30 to 40 μg l-1, a two-fold increase in dissolved Fe (from 30 to 60 μg l-1), and a 45\% elevation in dissolved Al (to 15.5 μg l-1) over reference basin levels (10.7 μg l-1). No increases in dissolved Cd, Cu, Pb, or Zn were observed in the acidified basin during the first 3 years of acidification. Analysis of suspended particulate matter collected from both basins revealed lower levels of particulate Al, Fe, Mn, and Pb in the acidified basin, demonstrating that metals have reduced affinity for particulate matter at the lower pH of the acidified basin. Enclosure (i.e. limnocorral) experiments in the littoral and pelagic zones of the north basin in 1984 and in 1985 accurately predicted the pH levels at which dissolved Al, Fe, and Mn first increased in this basin.