Copepods functional traits determine duration of facilitation on filamentous cyanobacteria
predator-prey dynamics; tropical zooplankton, Notodiaptomus iheringi, Thermocyclops decipiens, Cylindrospermopsis raciborskii, artificial food-webs
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 dynamics. In tropical environments, for instance, copepods commonly co-occur 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 (raptorial 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 shredded cyanobacteria filaments, reducing filament size 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 is not constant. Copepods were able to decrease overall biomass and shorten filaments of prey. Copepods grazing may not be the most appropriate mechanism to explain filamentous cyanobacteria blooms. While environmental correlations due to long term monitoring are important and may reveal interesting trends, a mechanistic approach to understand trophic dynamics of separate different groups (i.e. copepods and filamentous cyanobacteria) is necessary. This is especially relevant in light of the more intense warmer and eutrophic world that will promote cyanobacteria bloom, and increase complexity of such interactions.