Effects of a storm event on the structure of the pelagic food web with special emphasis on planktonic ciliates

Abstract

Storm-induced changes in the water column structure and the pelagic food web were investigated by daily sampling during a 3 week period (October 24 to November 10, 1988) at a permanent station in the southern Kattegat (Denmark) Subsequent to a storm the primary production increased and the size-distribution of the phytoplankton changed towards larger cells. Some components of the zooplankton community responded to the increase in potential food, either in terms of a functional response (copepods) or in terms of a numerical response (cladocera), whereas no changes in the populations of mixo- and heterotrophic aliates were evident. The dilates were not food limited since growth rates determined in situ were comparable to maximum growth rates measured in the laboratory. Calculations suggest that the total clearance capacity of planktonic copepods (as fraction of water column cleared per unit time) was of the same magnitude as the measured instantaneous growth rates of the ciliates (0.55–0 85 day −1 ). Ciliate populations were therefore probably limited by copepod predation. Ciliates, however, contributed only insignificantly to the diets of copepods. Although the total zooplankton production increased by at least 30% subsequent to the storm (and that of the copepods by 50%), the zooplankton community was unable to assimilate the entire increase in primary production. The relative zooplankton (copepods, cladoceran and ciliates) grazing rate on the primary production decreased ∼30%, and the flow of carbon to biomass accumulation, other grazers and in particular sedimentation is predicted to have increased by 100% following the storm.