Direct and Indirect Effects of Zooplankton Grazing on Phytoplankton in a Hypereutrophic Lake

Abstract

In situ column enclosures were successfully colonized by introduced monocultures of Daphnia galeata mendotae or Bosmina longirostris after extermination of native zooplankton and fish with CO2. Bosmina enclosures proceeded from green (Scenedesmus, Ankistrodesmus) to blue-green (Microcystis) dominated phytoplankton communities accompanied by increasing algal biomass, gross primary productivity and pH. Concurrently, Daphnia enclosures featured periods of low biomass, primary productivity and pH with a reduced proportion of blue-greens. Field observations, laboratory experiments and previous studies suggest that Daphnia may reduce the relative abundance of blue-green algae and total phytoplankton biomass by (1) grazing directly on the large, colonial algae such as Microcystis and (2) through heavy grazing pressure and subsequent low standing crops of phytoplankton producing a set of abiotic factors (low pH, high transparency, reduced total phosphorus) favoring edible, non-blue-green forms which are more efficiently consumed. This dual interaction provides a solution to the paradox of managing 'inedible' blue-green algae by manipulations to favor large zooplankton.