MEPS 233:31-38 (2002)  -  doi:10.3354/meps233031

ABSTRACT: Grazing can prevent or substantially decrease net growth of phytoplankton populations if grazing coefficients are similar to or greater than growth coefficients of the phytoplankter. Potential grazing by microzooplankton on small dinoflagellates (<20 µm), such as Pfiesteria piscicida, can be high, but extremely variable in estuaries. Blooms should only be possible during Œwindows of low grazing pressure¹. To better understand this variability, microzooplankton assemblages were collected at stations with low (~5 psu), medium (~10 psu) and high (~15 psu) salinity on the Pocomoke River, MD, and their grazing on cultured, stained non-toxic P. piscicida zoospores was measured. Grazing coefficients varied from 0 to 7.6 d^-1. On most dates average grazing coefficients were >2 d^-1, indicating that microzooplankton grazing had the potential to regulate densities of non-toxic zoospores. Potential grazing was positively correlated with salinity, abundance of photosynthetic and heterotrophic dinoflagellates and with abundance of <20 µm planktonic ciliates. Grazing on zoospores was negatively associated with abundance of cryptophytes. Approximately 26% of the variation in grazing coefficient could be predicted from salinity, whereas 67% of the variation could be predicted from abundance of cryptophytes, dinoflagellates and small ciliates. Net growth of non-toxic zoospore populations is most likely in low-salinity waters with abundant cryptophytes but with low concentrations of microzooplankton.

KEY WORDS: Harmful algal blooms · Biological control · Dinoflagellates · Microzooplankton grazing · Pfiesteria piscicida · CMFDA · Chesapeake Bay · Pocomoke River