ZOOPLANKTON FEEDING ECOLOGY - GRAZING DURING ENCLOSURE STUDIES OF PHYTOPLANKTON BLOOMS FROM THE WEST-COAST OF SWEDEN

The effects of grazing on natural phytoplankton populations was examined during enclosure studies on the effects of nutrients, zooplankton, and ctenophores on phytoplankton bloom dynamics on the west coast of Sweden in May, 1990. Grazers included the copepods Acartia clausi Giesbrecht, Centropages hamatus Lilljeborg, and Oithona similis Claus, and the marine cladoceran Podon polyphemoides Leuckart. These species ingested various diatoms, dinoflagellates and athecate microflagellates, but grazing was minimal. Clearance rates of the copepods were < 1 ml.animal-1.h-1, while those of cladocerans reached 2.5 ml.animal-1.h-1, at microflagellate concentrations of 1442-3519 cells.ml-1. Despite the low levels of copepod and cladoceran grazing, there was considerable removal of Chl a by grazers < 90-mu-m in longest dimension. These micrograzers were present in experimental and control containers. Athecate ciliates < 20-mu-m in longest dimension were abundant (up to 1730 cells.ml-1) in some samples, and these ciliates were the most probable micrograzers. Rates of clearance of these ciliates by A. clausi (0.13-0.81 ml.animal-1.h-1) and C. hamatus (0.46-0.81 ml.animal-1.h-1) were high, even though ciliates were less abundant than most co-occurring phytoplankton taxa. Copepod and cladoceran grazing appeared to have little impact on phytoplankton dynamics. Rather, copepods were selective consumers of the ciliates that were probably the dominant grazers.