Zooplankton of the West Florida Shelf: Relationships with \u3cem\u3eKarenia brevis\u3c/em\u3e 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. Prior to undertaking the first portion of the study, an examination of the perturbations in the normal zooplankton assemblage within K. brevis blooms, it was first necessary to define the normal zooplankton assemblage on the WFS. To this end, a seasonal analysis of abundance, biomass and community composition of zooplankton was undertaken at 6 stations on the WFS. Monthly sampling was conducted for one year at the 5, 25 and 50- m isobaths. Two major groups in community composition were observed at the near shore (5-m and 25-m) and offshore (50-m) stations. Considerable overlap was seen in community composition between the 5-m to 25-m and 25-m to 50-m isobaths, but little overlap in community composition was observed between the 5-m and 50-m isobaths. Of the 95 species identified, only 25 proved to be important (\u3e90%) contributors to community composition. Near shore, important contributors were Parvocalanus crassirostris, Penilia avirostris, Paracalanus quasimodo, Oithona colcarva, Oikopleura dioica, Centropages velificatus and Pelecypod larvae. As distance offshore increased, important contributors to community composition were Euchonchoichiea chierchiae, Clausocalanus furcatus, Oithona plumifera, Oithona frigida, Oncaea mediteranea, Calaocalanus pavoninius, Oithona similis, and Gastropod larvae. 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. One potential source of nutrients to support K. brevis blooms may be zooplankton regeneration of nutrients. To test this hypothesis, ammonium and phosphate excretion rates of several West Florida Shelf copepods (Labidocera aestiva, Acartia tonsa, Temora turbinata, and Paracalanus quasimodo) 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 l-1 of K. brevis, though phosphate excretion rates were adequate to support even 106 cells l-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