The effect of zooplankton grazing on estuarine blooms of the toxic dinoflagellate Gonyaulax tamarensis

A series of short-term in situ experiments was conducted in two Cape Cod embayments to estimate mortality rates of the toxic dinoflagellate, Gonyaulax tamarensis, resulting from grazing by zooplankton. Rates of grazing by the whole zooplankton community and by specific zooplankton populations were measured at various points in the G. tamarensis bloom cycle. The planktonic larvae of the spionid polychaete Polydora ligni and the tintinnid ciliate Favella sp. were important grazers in the systems studied. Gonyaulax-specific clearance rates effected by Polydora ranged from 0.02 to 0.5 ml individual−1 h−1; for Favella the range was about an order to magnitude lower. Peak population densities were close to 900 and 400 individuals 1−1 for P. ligni and Favella, respectively. Whether measured directly or predicted as the product of individual clearance rates and numerical abundance, rates of grazing were often higher than estimated algal division rates in years when blooms failed to develop. A simulation model corroborated the results of the field study, demonstrating that grazing can be a significant source of mortality during blooms, and can suppress bloom development when grazers are abundant.