An analysis of selective herbivory in an acid lake and its importance in controlling phytoplankton community structure

Abstract

Two types of experiments were performed at Cheat Lake (pH ∼4.5), West Virginia, to test the hypothesis that selective herbivory in acidified lakes controls phytoplankton community structure, resulting in Peridinium inconspicuum dominance. Six replicate in situ cell-count feeding experiments were performed to determine whether Cheat lake herbivores feed selectively, rejecting P. inconspicuum as a food item. Two replicate in situ enclosure experiments were also performed, where treatment was physical exclosure of zooplankton from large bags suspended in the lake, in order to examine phytoplankton community response to grazing pressure removal. In all six feeding experiments, P. inconspicuum was extremely dominant. However, its population was never significantly reduced by grazers, despite the fact that zooplankton density inside the grazing chambers was ∼170 times the natural lake level. The herbivores fed preferentially on rare unicellular green algae. In both bag experiments, however, zooplankton removal had no significant effect on total phytoplankton volume or community structure. P. inconspicuum consistently acounted for 76–99% of phytoplankton volume in bags where herbivory was essentially reduced to zero. Despite very selective feeding, at their low densities in this lake, the herbivores appear to have little influence in regulating species composition in the phytoplankton community.