Cyanobacteria tend to dominate nutrient-rich, freshwater lakes during late summer. Ambient nitrogen-to-phosphorus (N:P) ratios can strongly influence phytoplankton community structure (e.g., nitrogen fixers favored under low N:P conditions). Although typically regarded as poor food for herbivores, the effect of cyanobacteria on grazers, as well as the reciprocal effect of herbivores on cyanobacteria, varies and depends on the unique suite of traits specific to the dominant cyanobacterium (e.g., intracellular toxin production, morphology) and herbivore (e.g., size, selectivity, tolerance to cyanobacteria). Using six genotypes of the common, large-bodied cladoceran, Daphnia pulicaria, that vary in tolerance to cyanobacteria in the diet, we tested the hypothesis that cyanobacterial species composition, ambient N:P ratios, and Daphnia tolerance to cyanobacteria would mediate the effect of D. pulicaria on cyanobacteria and phytoplankton, in general. Using a full factorial design enclosure experiment, we inoculated half (n = 12) of the enclosures with a high biomass of either Microcystis or Anabaena species, cross-factored with two N:P ratios (28:1 and 4:1, by atoms). We subsequently stocked D. pulicaria into half (n = 12) of the enclosures at a density of 0.1 L-1.
Results/Conclusions
A large positive effect of N:P ratio was observed for phytoplankton (96% increase in chlorophyll in the high N:P enclosures) and cyanobacteria (120% increase in phycocyanin in the high N:P enclosures) by day 8 of the experiment. While D. pulicaria had no effect on total phytoplankton biomass between days 8 and 26, D. pulicaria did suppress cyanobacterial biomass within each N:P treatment, relative to no Daphnia controls (P < 0.05). Furthermore, the effect of Daphnia on cyanobacterial biomass was larger under the higher N:P ratio. By day 63, Daphnia had significantly reduced phytoplankton biomass, but not cyanobacterial biomass, relative to no Daphnia controls within each N:P ratio treatment. Interestingly, none of the effects of N:P ratio or Daphnia presence/absence were contingent on the species of cyanobacteria inoculated into the enclosures at the start of the experiment. Our results indicate that D. pulicaria can have subtle, but contrasting, effects on phytoplankton versus cyanobacterial abundance, and that grazers may actually reduce rather than favor resistant phytoplankton taxa. Despite substantial evidence demonstrating negative effects of cyanobacteria on zooplankton grazers, our results indicate that D. pulicaria can suppress blooms of these seemingly resistant species, in the absence of zooplanktivorous fish.
