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

Abstract

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 waterconcentrations 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 organicN. Annual maximum concentrations of N and P forms at 9 m depth in the hypolimnion increased with decreasing pH, but most of theincrease resulted from a drought-induced decline in lake levelduring 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 hydrologicfactors were more important in controlling silica levels than waspH. Most of the predictive hypotheses made about the effects ofacidification on nutrient forms and processes or their recoveryin the lake were qualitatively correct.