Microzooplankton grazing in Phaeocystis and diatom-dominated waters in the southern North Sea in spring

The impact of microzooplankton grazing upon phytoplankton production was quantified in surface waters of the Southern Bight of the North Sea, during April 1998. Two sites were studied in order to examine the impact of microzooplankton grazing on phytoplankton communities dominated by either Phaeocystis globosa and large phytoplankton or small phytoplankton taxa. The nearshore site was characterised by a phytoplankton community comprised mainly of P. globosa and chains of diatoms with high productivity (av. 346 ± 185 μgC l−1 d−1) and biomass (280 ± 171 μgC l−1 d−1). In contrast, in the offshore waters relatively small diatoms dominated the phytoplankton where productivity and biomass were more than ca. five times lower than in nearshore waters. Contrary to expectations, the nearshore site supported a high biomass of microzooplankton (av. 22.4 ± 10.6 μgC l−1 d−1) which was dominated by large heterotrophic dinoflagellates, mostly Gyrodinium cf. spirale. Offshore the microzooplankton community contained one-third the biomass of the nearshore community and was dominated by smaller individuals, in particular oligotrich ciliates, Strombidium spp. Dilution experiments were conducted in order to quantify phytoplankton growth and losses due to microzooplankton grazing in the <200 μm size fraction. Phytoplankton specific growth rates (<200 μm) ranged between 0.13 and 0.67 d−1 with highest values associated with offshore waters. In contrast, phytoplankton mortality due to microzooplankton grazing (0.27 to 1.14 d−1) was highest at the nearshore site and exceeded the growth rates of the <200 μm phytoplankton. Biomass specific grazing rates were three-fold higher in nearshore (av. 0.33 ± 0.23 d−1) waters compared to those offshore (av. 0.11 ± 0.09 d−1). These results show that microzooplankton were grazing more vigorously nearshore than offshore and were consistently cropping the production of the <200 μm phytoplankton. This high grazing pressure is likely to drive a shift in community composition from smaller to larger cells, in particular colonies of P. globosa. The high grazing rates on smaller phytoplankton demonstrated in this study illustrate that microzooplankton grazing may be one of the driving forces behind the evolution of the Phaeocystis life-history strategy that involves a transition between solitary and colonial cells.