PREY DISCRIMINATION BY CALANOID COPEPODS ON THE UPWELLING COAST OF WASHINGTON STATE: IMPLICATIONS FOR POPULATION DYNAMICS OF THE TOXIC DIATOM, PSEUDO-NITZSCHIA

M. Brady Olson, Evelyn J Lessard, Chung Huen J. Wong and Megan J. Bernhardt

University of Washington, School of Oceanography, Seattle, WA 98195

As part of the Pacific Northwest ECOHAB project, we measured grazing rates of calanoid copepods during fall of 2003 off the coast of Washington State, USA. We tested the hypothesis that copepods will discriminate amongst particles, particularly against the toxic diatom, Pseudo-nitzschia, in natural assemblages from this upwelling environment. Seven copepod grazing experiments were conducted across and along the shelf using the copepods Calanus pacificus, Metridia pacifica, Acartia longiremis and a small community assemblage dominated by Acartia spp., with minor contributions from Pseudocalanus spp., Paracalanus spp. and Oithona spp. The high productivity and patchiness of this dynamic region allowed us to test our hypothesis across wide-ranging prey biomass concentrations (34 to 1260 µg C l^-1) and species assemblages. In two experiments the small copepod, A. longiremis, cleared Pseudo-nitzschia equal to, or slightly higher, than other available prey. This was surprising given that Pseudo-nitzschia represented only 1 to 2 % of total available C. The larger, offshore copepod, M. pacifica, cleared Pseudo-nitzschia at low rates relative to other prey. M. pacifica showed a preference for microzooplankton, clearing large (> 40 µm) gymnodinoid dinoflagellates and ciliates at the highest rates. The largest copepod, C. pacificus, cleared Pseudo-nitzschia at very low rates, with high clearance rates exclusively on microzooplankton. This finding was especially interesting given that in this experiment autotrophic biomass was very high (1212 µg C l^-1), representing 96% of available prey C. The small copepod community cleared Pseudo-nitzschia at very low rates. Highest clearance rates for the small assemblage was on gymnodinoid and ciliate microzooplankton 20 to 40 µm in length, and on the diatom Thalassiosira spp. Two general patterns emerged from our experiments. First, the dominant prey biomass contributors in each experiment were cleared at low rates relative to other prey types. In most cases the dominants were the autotrophic dinoflagellates Ceratium spp. and Prorocentrum spp., and the diatom Thalassiosira spp. The second pattern was high preference for microzooplankton. High clearance on microzooplankton can result in trophic cascades, which were evident in our size-fractionated chlorophyll data. These patterns indicate that copepods have direct and indirect effects on the plankton community composition on the Washington coast. The fact that Acartia longiremis was the only copepod to have cleared Pseudo-nitzschia at high rates suggests that populations of Pseudo-nitzschia are impacted little by direct copepod predation.