Nutrient recycling by Daphnia reduces N2 fixation by cyanobacteria

MacKay, Neil A., James J. Elser

Limnol. Oceanogr., 43(2), 1998, 347-354 | DOI: 10.4319/lo.1998.43.2.0347

ABSTRACT: 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 N2 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 N2 fixation rates following the incubation showed that Daphnia treatments experienced a 50% reduction in N2 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 N4-N inhibits N2 fixation. Thus, by differentially recycling N4-N relative to P, Daphnia reduce the advantage (N2 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.

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