Zooplankton grazing can facilitate and control the proliferation of harmful algal blooms caused by Dinophysis acuminata in NY, USA, estuaries

While harmful algal blooms (HABs) caused by the obligate-mixotroph, Dinophysis acuminata, have been associated with the ciliate, Mesodinium rubrum, the role of zooplankton grazing in the occurrence of these HABs has been under studied. Here, the dynamics of D. acuminata blooms were tracked within two NY, USA, harbors, over three years (2019–2021) during which grazing by native protozooplankton and introduced copepods (Acartia tonsa) was evaluated experimentally using an Imaging FlowCytobot (IFCB) to quantify plankton between 20 µm and 150µm. During each year, protoozooplankton grazing on Dinophysis was low during the bloom initiation period but exceeded cellular growth rates of Dinophysis during the peak bloom period, suggesting that a lack of grazing permitted bloom initiation but grazing onset facilitated bloom decline. The addition of juvenile Acartia increased Dinophysis growth rates in 10 of 14 experiments with differences being significant during three, bloom peak experiments. This finding suggests juvenile copepods can trigger a trophic cascade, potentially consuming Dinophysis-predators and releasing this HAB from grazer control. In contrast, adult Acartia and other mesozooplankton had minimal effects on Dinophysis densities suggesting that mesozooplankton did not directly impact blooms. Collectively, this study demonstrates that reduced grazing pressure at the onset of blooms facilitates bloom development, enhanced grazing during bloom peak can facilitate bloom decline, and that juvenile copepods can promote blooms via the induction of a trophic cascade.