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
Abstract
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.