Grazing on Pfiesteria piscicida by microzooplankton

The potential grazing pressure of natural assemblages of microzooplankton ( > 200 pm size fraction) on cultured non-toxic zoospores (NTZ) of Pfiesteria piscicida (Dinamoebiales, Dinophyceae, Pyrrhophyta) was measured approxima tely weekly during the summer of 1999 using surface water samples from a ti dal 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 diac etate, and added to treatments with (> 200 pm) and without (>1.2 pm) the na tural microzooplankton assemblage. The dominant micrograzers on P. piscicid a NTZ were large tintinnids and oligotrichous ciliates. Grazing mortality d epended on species composition as well as abundance of microzooplankton. Th e instantaneous rate of grazing mortality varied from 0 to 10.2 d(-1) and w as >2 d(-1) in 6 out of 10 experiments. In previous studies, the maximum in stantaneous rate of growth of NTZ was >2 d(-1); thus, microzooplankton graz ing 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 comp ared, grazing was inhibited with a recently toxic strain (271A-1) of P. pis cicida. Intervals of low grazing pressure may present windows of opportunit y for growth of P. piscicida NTZ, which, in some cases, can become toxic in the presence of fish.