Spatial and temporal distribution of Keratella hiemalis in association with temperature, oxygen, chlorophyll A, and pH in Little Rock Lake, Wisconsin
Keratella hiemalis is unique among rotifers in Little Rock Lake because its presence is restricted to late fall, winter and early spring. K. hiemalis also displays changes in depth distribution over this period. This study examines the seasonal abundance patterns of K. hiemalis and its depth distribution from 1984-1991, in relation to several environmental factors: temperature, oxygen, chlorophyll-a and pH. In addition, a comparison of total abundance of K. hiemalis was made between the north basin of the lake in which pH was experimentally lowered, and the south basin which was unaltered. Temperature was an important influence on the seasonal occurrence of K. hiemalis and on its depth distribution. The depths at which water temperatures were coldest coincided with the highest chlorophyll-a levels, and the highest numbers of individuals/L. Egg production was low throughout the study, indicating that population growth was strongly influenced by the emergence of resting eggs from fall through spring. Individuals were present at a wide range of chlorophyll-a concentrations and oxygen levels including near anoxic conditions, and over a pH range of 5.28 to 6.8. In the acidified basin, K. hiemalis was present in pH 4.68. A cyclical pattern of population reduction and recovery was observed in the years following acidification.