Microzooplankton and their role in controlling phytoplankton growth in the marginal ice zone of the Bellingshausen Sea

Abstract

Microzooplankton and their herbivorous activity were quantified on a transect in the Bellingshausen Sea along the 85°W meridian between 70°15'S and 67°30'S. The transect was worked during the Austral spring period of November-December 1992, with stations in pack-ice ( Gertie/Herbie), ice-edge (Isolde), and open-waters ( Jules and Katie) of the marginal ice zone (MIZ). Microzooplankton varied in their abundance in the surface mixed layer within the MIZ transect by more than an order of magnitude. In the waters beneath the pack-ice, microzooplankton concentrations were low (ca 850 organisms I −1); they increased at the ice-edge station (2110 organisms l −1) and reached a maximum ( ca 17,000 organisms l −1) at the open water situated ca 200 km north of the ice-edge. The community was dominated by Protozoa, which contributed >97% of the numerical abundance of microzooplankton and >89% of the standing stocks. Protozoan communities were dominated by naked oligotrich ciliates and heterotrophic dinoflagellates. Microzooplankton biomass in the mixed layer of the MIZ varied between 0.2 and 54 μgC l −1) with the minimum situated at depth under the ice and maximum in open surface waters, respectively. Microzooplankton biomass was strongly correlated to ambient phytoplankton concentrations and averaged 24% of their biomass. Mixed layer standing stocks ranged between 50 and 2021 mgC m −2 at the southern and northern stations, respectively. Microzooplankton herbivorous activity also showed strong latitudinal gradients across the MIZ. Microzooplankton herbivory, investigated using dilution experiments within surface mixed layers, was generally vigorous. Microzooplankton turned over between 3 and 40% of the phytoplankton daily, thereby consuming between 0.04 and 31.21μgC l −1) day −1 in these experiments. The magnitude of this activity was positively related to sea-water temperature, to phytoplankton concentration, and to the ambient microzooplankton concentrations. A multiple regression model of microzooplankton herbivory was developed to produce an integrated analysis of their impact on phytogenic carbon flux. Microzooplankton in the mixed layer were estimated to graze 21 and 3260 mg phytoplankton-C m −2 day − at the southern pack-ice and northern open-water stations, respectively. This grazing flux accounted for 21 and 271% of the concurrent daily phytoplankton production, respectively. Microzooplankton are clearly important in determnining the fate of phytoplankton during the ice-melt period in the Bellingshausen Sea MIZ.