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 o f one basin with H2SO4 began at ice-out in 1985. Chemical and biologic al responses measured during successive 2-yr periods at pH approximate ly 5.6, 5.1, and 4.7 verified some but not all impacts predicted at th e outset. Changes in major, minor, and trace ions generally agreed wit h predictions. Internal alkalinity generation (IAG) increased at lower pH, and sulfate reduction eliminated approximately 50% of added H2SO4 . Sediment cation exchange was important in IAG and acidified surface sediments, possibly diminishing the lake's ability to counteract furth er H+ inputs. Mass loss of oak leaves was reduced at pH 5.1 (birch lea ves at pH 4.7). Population parameters were more sensitive than communi ty measures for plankton. Species composition changed at each pH, espe cially at pH 4.7. Many changes in zooplankton and benthos were indirec t responses to an algal mat that developed at lower pH or to food web interactions; these were not predicted accurately. Sensitivity of majo r fishes to lower pH was Ambloplites rupestris > Micropterus salmoides > Pomoxis nigromaculatus > Perca flavescens. Fish production was redu ced at pH's above those resulting in population decreases.