Selective Grazing by a Tropical Copepod (Notodiaptomus iheringi) Facilitates Microcystis Dominance

Ewaldo Leitão, Kemal A. Ger, Renata Panosso

Top-down grazer control of cyanobacteria is a controversial topic due to conflicting reports of success and failure as well as a bias toward studies in temperate climates with large generalist grazers likeDaphnia. In the tropical lowland lakes of Brazil, calanoid copepods of theNotodiaptomuscomplex dominate zooplankton and co-exist in high abundance with permanent blooms of toxic cyanobacteria, raising questions for grazer effects on bloom dynamics (i.e., top-down control vs. facilitation of cyanobacterial dominance). Accordingly, the effect of copepod grazing on the relative abundance ofMicrocystisco-cultured with a eukaryotic phytoplankton (Cryptomonas) was evaluated in a series of 6-day laboratory experiments. Grazer effects were tested in incubations where the growth of each phytoplankton in the presence or absence of the copepodNotodiaptomus iheringiwas monitored in 1 L co-cultures, starting with a 6-fold initial dominance ofCryptomonasby biomass. Compared to the no grazer controls,N. iheringireduced the growth of both phytoplankton, butCryptomonasgrowth was reduced to negative values whileMicrocystisgrowth continued positively despite grazers. Hence, in a matter of 6 days selective grazing byN. iheringiincreased the biomass ofMicrocystisrelative toCryptomonasby an order of magnitude compared to controls, and thus, facilitated the dominance of this cyanobacterium. To account for the potential effect of allelopathy, we performed a secondary experiment comparing the abundance and growth rate ofMicrocystisandCryptomonasin single and mixed co-cultures in the absence of grazers. The growth rate ofMicrocystiswas unaffected by the presence or relative abundance ofCryptomonas, and vice versa, indicating no allelopathic effects. Our results suggest that selectively grazing zooplankton may facilitate cyanobacteria blooms by grazing on their eukaryotic phytoplankton competitors in nature. Given that selective grazers predominate zooplankton biomass in warmer waters, grazer facilitation of blooms may be a common but poorly understood regulator of plankton dynamics in a warmer and more eutrophic world.