The effects of harmful algal species and food concentration on zooplankton grazer production of dissolved organic matter and inorganic nutrients

Harmful algal blooms (HABs), including toxic species, have been increasing in frequency, range, and duration over the past several decades. The effect of a harmful or toxic algal diet on zooplankton nutrient regeneration, however, has not been previously examined. In this study, we determined the effects of non-bloom and bloom concentrations of non-toxic and toxic cultures of HAB species Prorocentrum minimum and Karlodinium veneficum on grazing and production of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP), and inorganic nutrients, ammonium (NH₄⁺) and phosphate (PO₄^3-), by the copepod Acartia tonsa and the heterotrophic dinoflagellate Oxyrrhis marina. Ingestion rates of grazers were significantly higher while feeding on bloom algal concentrations compared to non-bloom algal concentrations, but were always below 1% body Cd^-1 for A. tonsa (ingestion rate range of 0.5-31ngCindividual^-1d^-1) and below 2% body Cd^-1 for O. marina (range of 0.1-8.8pgCindividual^-1d^-1). However, rates of inorganic nutrient and dissolved organic matter (DOM) release, when detected, were always >100% of carbon (C), nitrogen (N), and phosphorus (P) ingested. Additionally, the quantity and forms (organic vs. inorganic) of nutrients released by zooplankton were significantly different between non-toxic and toxic algal treatments, and typically higher grazer DOM release occurred while feeding on toxic algal cultures. DOM was the only detected form of nutrient released from O. marina, and DON and DOP were significant portions of total dissolved N and P released for A. tonsa feeding on toxic K. veneficum (69-84% and 73%, respectively). Low grazing rates on all algal diets used in our study, regardless of cell concentration or toxicity, likely resulted in starvation and subsequent catabolism of grazer body tissue. The potential for additional factors affecting variable grazer nutrient release between toxic and non-toxic algal treatments, including algal nutrient quality and direct toxic effects, are discussed. Our results suggest these grazers may not be capable of controlling bloom formation of these HAB species, and that nutrient cycling dynamics in the coastal ocean may change with increases in the presence of harmful and toxic algal blooms.