Zooplankton of the West Florida Shelf: Relationships with Karenia brevis blooms

Blooms of the toxic dinoflagellate Karenia brevis are common on the West Florida Shelf (WFS), yet little is known of the relationships between zooplankton and K. brevis. A comprehensive analysis was undertaken to examine (1) perturbations in zooplankton community composition within K. brevis blooms (2) the contribution of zooplankton ammonium and phosphate excretion to K. brevis bloom nutrient requirements, and (3) the role of zooplankton grazing in K. brevis bloom termination. Monthly sampling was conducted for one year at the 5, 25 and 50-m isobaths and within K. brevis blooms. Two groups in community composition were observed at the near shore (5-m and 25-m) and offshore (50-m) stations. Of the 95 species identified, only 25 proved to be important (>90%) contributors to community composition. Variations in abundance and biomass between non-bloom and bloom assemblages were evident, including the reduction in abundance of 3 key species within K. brevis blooms. To test the hypothesis that zooplankton regeneration may be a source of nutrients for K. brevis blooms, ammonium and phosphate excretion rates of several West Florida Shelf copepods were measured and prorated to a 24-hour day. These excretion rates were then extrapolated to other West Florida Shelf zooplankton, combined with available literature excretion rates for some taxa, and applied to zooplankton abundances found for K. brevis blooms on the West Florida Shelf in 1999 and 2001. Ammonium excretion rates were found to be inadequate to support all but 104 cells 1-1 of K. brevis, though phosphate excretion rates were adequate to support even 106 cells 1-1 of K. brevis. Grazing assessment was conducted for three common zooplankton species that were found within two K. brevis blooms, A. tonsa, P. quasimodo, and L. aestiva, using 14C labeled K. brevis cells. Grazing rates were then applied to the zooplankton community and grazing assessed. Grazing pressure was occasionally heavy, and was capable of reducing K. brevis to background concentrations at stations in the 1999 bloom and at 1 station in the 2001 bloom. Generally, however, grazing pressure proved to be insufficient to reduce K. brevis to background concentrations during the 1999 and 2001 blooms.