Prorocentrum minimum(clone Exuv) is nutritionally insufficient, but not toxic to the copepod Acartia tonsa

This study tested whether the dinoflagellate Prorocentrum minimum is nutritionally insufficient or toxic to the copepod Acartia tonsa. Experiments were carried out with adult female A. tonsa and the P. minimum clone Exuv, both isolated from Long Island Sound. Initially, the functional and numerical responses of A. tonsa feeding on exponentially growing P. minimum cells were characterized. These experiments revealed that A. tonsa readily ingested P. minimum cells, up to the equivalent of 200% of body carbon day⁻¹, but egg production was relatively low, with a maximum egg production rate of 22% of body carbon day⁻¹. Hence, the egg production efficiency (egg carbon produced versus cell carbon ingested) was low (10%). In a separate experiment, ingestion and egg production rates were measured as a function of food concentration with cells in different growth stages (early-exponential, late-exponential/early-stationary, and late-stationary growth phase) to simulate conditions during a bloom. There was no indication that cells in the stationary phase resulted in lower ingestion or egg production rates relative to actively growing cells. Egg hatching success remained high (>80%) and independent of the cell growth phase. In a third experiment specifically designed to test the hypothesis that P. minimum is toxic, ingestion, egg production and egg hatching success were measured when females were fed mixtures of P. minimum and the diatom Thalassiosira weissflogii, but in which total food concentration was held constant and the proportion of P. minimum in the mixed diet varied. A. tonsa readily ingested P. minimum when it was offered in the mixed diet, with no detrimental effects on egg production or egg hatching observed. Supplementing P. minimum with T. weissflogii increased both the egg production rate and the egg production efficiency. It is concluded that P. minimum is nutritionally insufficient, but not toxic to A. tonsa. Finally, it is estimated that in the field grazing by A. tonsa is approximately equivalent to 30% of the maximum daily growth rate of P. minimum. Hence, copepod grazing cannot be ignored in field and modeling studies of the population dynamics of P. minimum.