Microplastics suppress copepod grazing of the harmful algal bloom-forming dinoflagellate, Alexandrium catenella

While harmful algal blooms (HABs) are prominent occurrences in coastal zones where microplastic concentrations are maximal, the manner in which microplastics influence food webs and HABs are poorly understood. This study explored how polystyrene microplastics effected copepod grazing of the saxitoxin-synthesizing harmful dinoflagellate, Alexandrium catenella, as well as a non-toxic dinoflagellate, Gymnodinium aureolum. Experiments were performed using multiple densities of microplastics (250– 10,000 mL-1) of differing sizes (4 and 10 µm), as well as multiple densities of microalgae. The addition of 10 µm microplastics at densities of 1,000 – 10,000 beads mL-1 consistently and significantly reduced grazing on A. catenella while 10,000 µm beads mL-1 (10 µm diameter) consistently and significantly increased grazing on G. aureolum (p<0.05 for all). The addition of 4 µm beads (10,000 beads mL-1) did not alter grazing on A. catenella but did mitigate grazing disruption caused by 10 µm beads when offered as a mixture of bead sizes. When A. catenella and G. aureolum were combined, the addition of 10 µm beads increased grazing on A. catanella compared to a mono-algal treatment, while grazing rates on G. aureolum were unchanged. In contrast, the algal mixture without bead additions yielded enhanced grazing on G. aureolum while grazing on A. catenella was unchanged compared to mono-algal treatments. Microplastics 10 µm and smaller are poorly studied but likely have disproportionate interactions with lower levels of marine food webs. This study highlights potential co-stressor effects of HABs and microplastics on zooplankton grazers which could promote HABs.