Experimental Acidification of Little Rock Lake, Wisconsin: Chemical and Biological Changes over the pH Range 6.1 to 4.7

The two basins of this seepage lake were separated by a vinyl curtain in August 1984 after a year of background studies, and acidification of one basin with H 2 SO 4 began at ice-out in 1985. Chemical and biological responses measured during successive 2-yr periods at pH ~5.6, 5.1, and 4.7 verified some but not all impacts predicted at the outset. Changes in major, minor, and trace ions generally agreed with predictions. Internal alkalinity generation (IAG) increased at lower pH, and sulfate reduction eliminated ~50% of added H 2 SO 4 . Sediment cation exchange was important in IAG and acidified surface sediments, possibly diminishing the lake's ability to counteract further H + inputs. Mass loss of oak leaves was reduced at pH 5.1 (birch leaves at pH 4.7). Population parameters were more sensitive than community measures for plankton. Species composition changed at each pH, especially at pH 4.7. Many changes in zoopiankton and benthos were indirect responses to an algal mat that developed at lower pH or to food web interactions; these were not predicted accurately. Sensitivity of major fishes to lower pH was Ambloplites rupestris > Micropterus salmoides > Pomoxis nigromaculatus > Perca flavescens. Fish production was reduced at pH's above those resulting in population decreases.