Microzooplankton grazing on Prorocentrum minimum and Karlodinium micrum in Chesapeake Bay

Potential grazing rates on the bloom forming dinoflagellates Prorocentrum minimum and Karlodinium micrum were measured in Chesapeake Bay during summer (2000). Cultured P. minimum and K. micrum cells were fluorescently labeled with 5-chloromethylfluoroscein diacetate and introduced to <200 mum filtered water. Microzooplankton grazing was assessed by measuring the disappearance of labeled prey over time. Grazing on P. minimum and K. micrum was highest between lower oligohaline to midmesohaline regions of the open bay, where microzooplankton biomass was greatest. In June, grazing rates on P. minimum were high at all stations, apparently because of naked (NHD) and thecate heterotrophic dinoflagellates. In July, grazing pressure on P. minimum was related to aloricate oligotrich and choreotrich biomass (r(2) = 0.5107, p = 0.030), whereas g for K. micrum was correlated with Oxyrrhis marina (r(2) = 0.7217, p = 0.004) abundance. In August, grazing on P. minimum was correlated with abundance of the NHD Gyrodinium spp. (r(2) = 0.6621, p = 0.006) and Polykrikos kofoidii (r(2) = 0.6617, p = 0.010) abundance. Microzooplankton biomass peaked within the mesohaline regions of Chesapeake Bay during all months, and these assemblages were dominated by heterotrophic dinoflagellates. On the basis of these results, microzooplankton grazing is an important loss to P. minimum and K. micrum populations in Chesapeake Bay.