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Top-down regulations in trophic chain depend on herbivore grazing traits and producers edibility. By actively selecting nutritious eukaryotic phytoplankton and rejecting toxic cyanobacteria, grazing copepods may contribute to cyanobacteria blooms and dynamics. In tropical environments, for instance, copepods commonly cooccur with cyanobacteria blooms, raising the question whether they can facilitate cyanobacteria dominance. We experimentally tested the effects of two groups of copepods with different feeding modes – calanoid Notodiaptomus iheringi (active filter feeding) and cyclopoid Thermocyclops decipiens (ambush feeder) – on the competition of an eukaryotic phytoplankton Cryptomonas and the filamentous cyanobacteria Cylindrospermopsis raciborskii. We assessed grazing in 1L batch cultures for seven days, starting with 10-fold initial dominance of Cryptomonas. Copepods demonstrated initial rejection of Cylindrospermopsis filaments, but while cyclopoids slightly increased grazing on cyanobacteria in extended experimental periods, calanoids reversed to clear more particles of cyanobacteria. Despite differences on grazing, both zooplankton decreased cyanobacteria filaments size similarly, reducing it in ~70%. We also performed experiments testing competition between phytoplankton that showed no interference on each other growth rates, assuring that results from grazing experiment are addressed to zooplankton feeding. Here, we demonstrated that copepod selectively avoidance of filamentous cyanobacteria Cylindrospermopsis may facilitate its dominance, depending on time and alternative food concentration. Cyclopoid, rather than calanoid, grazing may be an appropriate mechanism to explain filamentous cyanobacteria dominance, as they had lower impact on cyanobacteria abundance. This mechanistic approach understanding trophic dynamics using traits of different groups (i.e. copepods and cyanobacteria) is especially relevant in light of the more intense warmer and eutrophic world that will promote cyanobacteria bloom, and increase complexity of such interactions.

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 like Daphnia. In the tropical lowland lakes of Brazil, calanoid copepods of the Notodiaptomus complex 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 of Microcystis co-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 copepod Notodiaptomus iheringi was monitored in 1 L co-cultures, starting with a 6-fold initial dominance of Cryptomonas by biomass. Compared to the no grazer controls, N. iheringi reduced the growth of both phytoplankton, but Cryptomonas growth was reduced to negative values while Microcystis growth continued positively despite grazers. Hence, in a matter of 6 days selective grazing by N. iheringi increased the biomass of Microcystis relative to Cryptomonas by 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 of Microcystis and Cryptomonas in single and mixed co-cultures in the absence of grazers. The growth rate of Microcystis was unaffected by the presence or relative abundance of Cryptomonas, 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.