Temporal and Spatial Patterns of Abundance of the Chydoridae (Cladocera) in Lake Itasca, Minnesota

Abstract

The chydorid populations of Lake Itasca, Minnesota, were monitored over a 2.5—yr period by pattern sampling. Of the 24 species collected, 15 were sufficiently abundant to allow analysis of population trends. Chydorid species have two basic patterns of abundance. One group of species exhibits only a summer density peak. The second group shows one abundance peak in summer and a second peak in fall or winter. All species experience a precipitous decline in density following the summer peak. Population growth in chydorids is limited by different factors during different times of the year. During periods of ice cover most species are apparently limited by a reduction in primary productivity due to reduced light penetration rather than by water temperature. Spring chydorid populations are limited by temperature. Summer populations, at least for abundant species, are limited by predation. Chydorid species are distributed patchily within Chara beds. Patchiness is moderate in spring, declines to low levels during summer, and peaks in the fall for most species. Patchiness remained about constant during the summer decline in density. Experimentation showed that heterogeneity of the physical habitat is not a necessary cause of patchiness. A Bray—Curtis—type ordination was performed on data from a complete year of monitoring. The primary component of variation extracted from the data matrix is due to temporal differences in abundance among species. The second major source of variation is due to spatial differences in abundance (habitat preference). The most abundant chydorid species tent to have similar habitats and temporal patterns of abundance.