ABSTRACT: The potential grazing pressure of natural assemblages of microzooplankton (<200 µm size fraction) on cultured non-toxic zoospores (NTZ) of Pfiesteria piscicida (Dinamoebiales, Dinophyceae, Pyrrhophyta) was measured approximately weekly during the summer of 1999 using surface water samples from a tidal tributary of the Chesapeake Bay (Chicamacomico River, MD, USA) in which fish kills associated with Pfiesteria-like dinoflagellates have occurred. NTZ of P. piscicida (strain FDEPMDR23, an apparently non-toxic strain) were stained with a vital green fluorescent dye, 5-chloromethylfluorescein diacetate, and added to treatments with (<200 µm) and without (<1.2 µm) the natural microzooplankton assemblage. The dominant micrograzers on P. piscicida NTZ were large tintinnids and oligotrichous ciliates. Grazing mortality depended on species composition as well as abundance of microzooplankton. The instantaneous rate of grazing mortality varied from 0 to 10.2 d^-1 and was >2 d^-1 in 6 out of 10 experiments. In previous studies, the maximum instantaneous rate of growth of NTZ was <2 d^-1; thus, microzooplankton grazing has the potential to prevent net growth of NTZ. However, in 1 out of 2 experiments in which grazing on different strains of P. piscicida were compared, grazing was inhibited with a recently toxic strain (271A-1) of P. piscicida. Intervals of low grazing pressure may present windows of opportunity for growth of P. piscicida NTZ, which, in some cases, can become toxic in the presence of fish.

KEY WORDS: Harmful algal blooms · Biological control · Dinoflagellates · Microzooplankton · Microzooplankton grazing · Pfiesteria piscicida · Tintinnids · Oligotrichs · Ciliates · CMFDA

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Published in AME Vol. 22, No. 3 (2000) on October 26
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 2000