CONTRIBUTION OF ZOOPLANKTON GRAZING TO THE DECLINE OF A CERATIUM BLOOM

Growth rates and vertical distributions of dominant Ceratium spp. were investigated during their August-September bloom in Danish waters. Vertical distributions of potential predators were also studied. Grazing pressure due to ciliates was low. Laboratory experiments show that only the largest copepod species (Centropages hamatus and Centropages typicus) and cladocerans are able to graze Ceratium. Copepod biomass remained almost constant during the investigation period, but cladoceran biomass increased and eventually dominated the mesozooplankton. The vertical distribution of these species was correlated with that of Ceratium spp. The crustaceans grazed from 30 to 70% of the Ceratium production per day, so that a large fraction of the Ceratium cells was grazed in the water rather than sedimenting to the bottom.