Seasonal patterns of protoplankton and calanoid copepods in the Firth of Forth : an investigation or Copepod grazing and the effect of diatoms upon reproductive processes

Seasonal variability in the zooplankton of the Firth of Forth, including mesozooplankton and microplankton, were investigated across a full annual cycle. Grazing rates and prey selectivity by the dominant calanoid copepods (Acartia discaudata, Acartia clausi, Centropages hamatus and Temora longicornis) were also examined. Investigations were conducted upon Acartia discaudata and field collected prey under simulated diatom bloom conditions, using the species Thalassiosira rotula, in order to test the hypothesis of diatom inhibition of calanoid copepod reproductive processes. There were fundamental differences between the spring and autumn blooms, typical of temperate estuaries. The spring bloom was composed of highly abundant, small cells amounting to less than half the biomass of larger, less numerous microplankton present in autumn. Copepods dominated the mesozooplankton undergoing seasonal shifts in species’ dominance primarily in response to physical factors. Results of grazing experiments indicate a disproportionate preference for motile prey compared to ubiquitous concentrations of diatoms. Copepods switch to blooming diatom species when present in concentrations > 80 cells ml-1. Ciliates generally contributed < 25% to copepod carbon ingestion. Acartia discaudata, Acartia clausi, and Centropages hamatus selectively consumed dinoflagellate and ciliate taxa whereas Temora longicornis remained an indiscriminate grazer during the entire study. Despite increasing rates of egg production in Acartia discaudata, at high concentrations (> 1 x 103 cells ml-1), Thalassiosira rotula inhibited hatching success such that recruitment to naupliar stage 2 was severely impaired compared with eggs hatched from females fed ~0.3 x 103 cells ml-1 concentrations of T. rotula. This is the first recorded evidence of embryogenic inhibition in A. discaudata