Copepod feeding selectivity on microplankton, including the toxigenic diatoms

As part of the Pacific Northwest ECOHAB project, we measured clearance rates and feeding selectivity of calanoid copepods off the coast of Washington State, USA, during fall of 2003. We tested the hypothesis that copepods discriminate amongst prey, particularly against the toxic diatoms Pseudonitzschia spp. in natural assemblages from this highly productive, upwelling environment. Seven 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. Three general patterns emerged from our experiments. First, all copepods, except A. longiremis in 1 experiment, showed neutral preference or discriminated against Pseudo-nitzschia, but preference did not appear related to cellular domoic acid concentrations. Second, 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 diatom Thalassiosira spp. or the autotrophic dinoflagellates Ceratium spp. and Prorocentrum spp. The third 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 could have both direct and indirect effects on the plankton community composition on the Washington coast. However, our estimates of total potential grazing suggest that copepod grazing impact on Pseudo-nitzschia populations is negligible.