Inhibitory effect of Anabaena sp. on in situ filtering rate of Daphnia

We found that in situ filtering rates of Daphnia spp. measured in a lake containing Anabaena were significantly lower than those measured in a filament-free lake. Even after accounting for the depressing effects of high nannoplankton biomass concentration, filtering rates in the lake with Anabaena were 64% lower than those from the filament-free lake. We also found that filtering rates for Daphnia pulex in laboratory experiments were lower when Anabaena was present in experimental beakers than when Chlorella was present. When Anabaena was removed from Three Mile Lake water, filtering rates compared closely with predicted rates based on nannoplankton concentration and carapace length alone. Our analysis indicates that the presence of Anabaena filaments depresses Daphnia grazing rates in general, and that the filaments themselves are ingested at a lower rate than algae such as Chlorella.