Short-term toxicity effects of Prymnesium parvum on zooplankton community composition

Harmful algal blooms (HABs) can disrupt aquatic communities through a variety of mechanisms, especially through toxin production. Herbivorous and omnivorous zooplankton may be particularly susceptible to HAB toxins, due to their close trophic relationship to algae as grazers. In this study, the acute toxigenic effects of the haptophyte Prymnesium parvum on a zooplankton community were investigated under laboratory conditions. Total zooplankton abundances decreased during 48-h exposure, although species responses to P. parvum densities varied. Changes in community composition were driven by declines in Daphnia mendotae and Keratella spp. abundances, which resulted in an average shift in copepod abundance from 47.1 to 72.4%, and rotifer abundance from 35.0 to 7.1%. Total cladocerans were relatively unchanged in relative abundance (11.1-10.4%), though the dominant cladoceran shifted from Daphnia mendotae (61.3% of cladocerans) to Bosmina longirostris (81.5% of cladocerans). Daphnia mendotae and Keratella spp. are known to be non-selective or generalist feeders and were likely harmed through a combination of ingestion of and contact by P. parvum. Proportional increases in copepod and Bosmina abundances in the presence of P. parvum likely reflect selective or discriminate feeding abilities in these taxa. This study corroborates previous field studies showing that P. parvum can negatively affect zooplankton, reflecting species-specific differences in zooplankton-P. parvum interactions. Such changes can alter zooplankton community composition, leading to substantial food-web consequences and potential long-term ecosystem-level impacts in lakes that experience blooms.