RESPONSES OF NUTRIENTS TO EXPERIMENTAL ACIDIFICATION AND RECOVERY IN LITTLE ROCK LAKE, U.S.A.

摘要

Nutrient concentrations and cycling processes were studied in Little Rock Lake, Wisconsin, U.S.A., during experimental acidification from pH 6.1 to target pH values of 5.6, 5.1 and 4.7 and during the first four years of recovery. Surface water concentrations of ammonium, nitrate, soluble reactive phosphate, and total phosphate were not affected, but total nitrogen was lower from year 2 at pH 5.6 until the end of recovery year 3 (pH 5.3). The decrease was attributed to lower dissolved organic N. Annual maximum concentrations of N and P forms at 9 m depth in the hypolimnion increased with decreasing pH, but most of the increase resulted from a drought-induced decline in lake level during acidification that brought the 9 m sampling location closer to the sediment. In contrast to findings elsewhere, acidification of Little Rock Lake had no effect on rates of nitrification during winter ice-cover. Dissolved silica concentrations increased slightly at pH 4.7, probably because of a pH-induced decrease in the diatom population, but hydrologic factors were more important in controlling silica levels than was pH. Most of the predictive hypotheses made about the effects of acidification on nutrient forms and processes or their recovery in the lake were qualitatively correct.