Nutrient recycling by Daphnia reduces N 2 fixation by cyanobacteria

A field experiment in an experimentally eutrophied lake (Lake 227) tested the hypothesis that differential recycling of nitrogen and phosphorus by Daphnia (a zooplankter with low body N:P ratio) affects the physiological status of cyanobacteria, including rates of N 2 fixation (stoichiometric recycling hypothesis). After a 5‐d incubation in 2.4‐liter bottles, Daphnia treatments had lower standing stocks of algae (based on particulate carbon or Chl a ) and higher concentrations of dissolved nutrients than did Epischura (a high N:P ratio consumer) or control treatments lacking macrozooplankton. The N:P ratio of dissolved nutrients was higher in the Daphnia treatments than in control or Epischura treatments, consistent with the stoichiometric recycling hypothesis, and was associated with greater algal P deficiency. Measurements of absolute and C‐specific N 2 fixation rates following the incubation showed that Daphnia treatments experienced a 50% reduction in N 2 fixation relative to Epischura and control treatments. This reduction is consistent with the higher N:P ratio of the dissolved pool in the Daphnia treatment, as N 4 ‐N inhibits N 2 fixation. Thus, by differentially recycling N 4 ‐N relative to P, Daphnia reduce the advantage (N 2 fixation) cyanobacteria have~over other phytoplankton. We suggest that this mechanism may be an important factor in the ability of Daphnia to reduce the incidence of cyanobactetial blooms in lakes with low N:P loading ratios.