Grazer control of nitrogen fixation: synergisms in the feeding ecology of two freshwater crustaceans

Using in situ mesocosm experiments, we compared grazing rates, food-selection patterns and nutrient mineralization rates of a daphniid cladoceran and a diaptomid copepod to determine their influence on phytoplankton composition and the prevalence of N-fixing cyanobacteria. We show that grazing and nutrient mineralization may act synergistically within a grazer taxon. Cladocerans exhibited non-selective grazing and high N:P mineralization; copepods exhibited selective grazing and low N:P mineralization. Through such synergistic effects, freshwater systems dominated by copepods would be expected to be more vulnerable to blooms of N-fixing cyanobacteria than those dominated by cladocerans.