Acartia tonsa grazing on the harmful dinoflagellate Dinophysis acuminata reduces copepod survival and increases extracellular toxin concentrations

ABSTRACT: Dinophysis spp. synthesize lipophilic toxins and form harmful algal blooms (HABs) across the globe. Zooplankton can play a role in controlling HABs and be a vector for HAB toxins; however, no study has explored the grazing and survival of copepods fed cultured Dinophysis. Here, the copepod Acartia tonsa isolated from New York, USA, was fed 3 strains of D. acuminata from the eastern USA (Massachutsetts, New York, Virginia), as well as 2 non-toxic prey (Rhodomonas salina and Gymnodinium aureolum). Grazing and survival rates of A. tonsa were quantified along with toxins. A. tonsa fed on D. acuminata at rates similar to R. salina and G. aureolum. Mixed-prey experiments suggested that D. acuminata was not acutely toxic to A. tonsa. Extracellular levels of okadaic acid (OA) significantly increased (p ≤ 0.05) and extracellular pectenotoxin (PTX2) increased by 50% upon exposure to copepods, suggesting that grazers stimulated extracellular toxin release. During 3 wk survival experiments, copepods consuming D. acuminata as a sole food source displayed significantly lower survival rates compared to those fed a control diet of R. salina (p < 0.05). This depressed survivorship was ameliorated by feeding the copepods a diet of D. acuminata mixed with G. aureolum, suggesting that nutritional deficiencies drove mortality. Since grazing on Dinophysis may be low when prey abundance is low, reduced grazing may contribute to bloom development; however, as blooms intensify, grazing may increase, potentially causing a reduction in copepod survival and continued bloom progression. Finally, grazing-induced increases in OA and PTX2 could enhance the introduction of Dinophysis-derived toxins into food webs.