Daphnia‐Phytoplankton Interactions: Density‐Dependent Shifts in Resource Quality

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Abstract
In mesotrophic Lake Mitchell, density fluctuations of Daphnia cause rapid shifts within algal assemblages, while total phytoplankton cell densities persist at comparable levels before and after grazer fluctuations. The rapid shifting balance between naked flagellates and several digestion—resistant species is evident in both seasonal patterns and in enclosure experiments. The balance between flagellates and resistant species helps explain asymmetrical demographic responses of competing smaller bodied cladocerans to changes in Daphnia density. Shifts between refractory components and accessible, edible algae constitute a prime reason why general measures of phytoplankton (e.g., chlorophyll α, biomass, biovolume, primary productivity) may fail to reflect impacts of Daphnia fluctuations. Rapid changes in resource quality may also provide a simple explanation for why Daphnia's tendency to show unstable oscillatory dynamics in the laboratory is dampened in natural populations.