MEPS 286:133-144 (2005)  -  doi:10.3354/meps286133

ABSTRACT: Grazing of mesozooplankton on phytoplankton, ciliates and other microplankton in the Mississippi River plume was studied by on-deck zooplankton addition incubations during March 2002. Diatoms, numerically predominated by the pennate diatom Pseudonitzschia pseudodelicatissima, were the most abundant microplankton in the plume. We observed that large cells of all types dominated the mesozooplankton diet and that phytoplankton generally comprised the largest dietary component. Microzooplankton contributed between 2 and 60% to the mesozooplankton diet. At the near-field station (nearest the discharge point of the river), P. pseudodelicatissimi concentration was low and consumption of diatoms, ciliates and dinoflagellates by mesozooplankton reflected available concentrations. In the mid-field stations, P. pseudodelicatissimi attained very high concentrations (17000 cells ml^-1) but comprised only a small portion of the mesozooplankton diet, which was instead dominated by ciliates and dinoflagellates. At the far-field station (approximately 60 km distance from the discharge point), P. pseudodelicatissimi concentration was intermediate but mesozooplankton clearance rates were still higher on ciliates and dinoflagellates at these stations. This pattern may have been established by changes in the composition of the mesozooplankton grazer community, by the inability of some mesozoopllankton to efficiently ingest the long (>100 µm) and large-sized diatoms, or by the production of toxins by P. pseudodelicatissimi that prevent it from been grazed by mesozooplankton. Our findings are consistent with an earlier published conceptual model in that (1) the abundance of microzooplankton (ciliates) was high in the near- to mid-field and then decreased toward the far-field, in parallel with phytoplankton stock; (2) mesozooplankton consumed large rather than small prey, thereby affecting the structure of the phytoplankton and microzooplankton community; (3) phytoplankton, dominated by diatoms, were the major food source for mesozooplankton in the plume.

KEY WORDS: Mesozooplankton grazing · Diet composition · Microzooplankton · Ciliates · Mississippi River plume