Effects of zooplankton grazing on phytoplankton communities in Mt Bold Reservoir, South Australia, using enclosures

Abstract

Enclosure experiments demonstrated that zooplankton grazing changed the composition of the phytoplankton community in Mt Bold Reservoir. Phytoplankton biomass as measured by chlorophyll a did not change within the enclosures but changed across the experiments in response to zooplankton grazing. The chlorophyll a : phaeophytin a ratio did not reflect zooplankton grazing activity. Phytoplankton species richness and diversity did not change but the frequencies of many individual phytoplankton taxa differed in response to zooplankton grazing. Neither taxonomic identity nor phytoplankton size as measured by greatest axial linear dimension and volume determined the susceptibility of a taxon to grazing. This suggests that other criteria are important in food selection, criteria which vary between experiments. Multivariate statistical techniques successfully differentiated the grazed and the ungrazed phytoplankton communities based on the different frequencies of the component taxa. There was an indication that, within the enclosures, zooplankton grazing advanced the phytoplankton community along a temporal path. Microzooplankton grazing was not examined in these experiments but there was evidence that it was significant.