Zooplankton-phytoplankton interactions in a eutrophic lake

Abstract

Enclosure experiments were made in a cyanobacteria dominated lake (Lake Rotongaio) to assess the impact of zooplankton (>150 μm) grazing on algal growth rates and determine the effect of diel and vertical changes in zooplankton grazing intensity and nutrient (NH 4 -N) regeneration upon abundance of phytoplankton. The filamentous cyanobacterium Anabaena minutissima var. attenuata and diatom Cyclotella meneghiniana showed a negative linear change in abundance with a gradient in zooplankton grazing intensity. Phytoflagellates were not grazed and showed a positive linear change in abundance with increasing zooplankton biomass. These effects, as well as shortening of filament length of Anabaena , were caused by raptorial feeding by the alanoid copepod Boeckella propinqua which dominated the zooplankton. Phytoplankton growth was not stimulated by addition of nutrients, suggesting nutrient regeneration was not important. Diel and vertical changes in feeding and NH 4 -N regeneration rates were measured in March and June 1988. Diel differences were more pronounced in March when the water column was stratified. Specific feeding rates were more important than vertical changes in zooplankton biomass in determining community grazing rates in March, but in June when the water column was mixed, vertical distribution of zooplankton biomass was important. Zooplankton grazing was an important loss process for phytoplankton in the lower part of the epilimnion in Lake Rotongaio.