Abundance, biomass and distribution of microzooplankton in the Irish Sea

The microzooplankton of the Irish Sea (˜54°N 5°W) were investigated by microscope-based image analysis for the late spring-early summer period in 1987, 1988, 1989 and 1992. The microzooplankton community was dominated by protozoa, with a total of 56 types recorded. The community comprised mostly aloricate oligotrich ciliates of the genus Strombidium, the endosymbiont-containing ciliate Mesodinium rubrum and heterotrophic dinoflagellates of the class Gymnodiniales. Ciliates ranged in length from 8 to 163 μm, with the majority 50 μm. Microzooplankton abundance in surface waters ranged between 1000 and 60 000 cells 1−1. Abundance was higher in surface waters than at depth. The cell concentration varied over the April-June period, and highest numbers were found during May in 1987 and 1988. Microzooplankton concentrations varied between years, with the lowest numbers recorded in 1989. The biomass of microzooplankton ranged from 2 to 22 μg C 1−1 in 1992. Concentrations reached 34 and 41 μg C l−1 in May 1987 and April 1988, respectively. Distinct patterns of distribution of different microzooplankton were found. Mixed waters in the eastern sector of the Irish Sea were dominated by small-sized microzooplankton, whereas larger species dominated the stratified westerly waters. These results are compared to estimates of microzooplankton in other regions and it is concluded that microzooplankton are an important constituent of the Irish Sea plankton.