MICROZOOPLANKTON AND THEIR ROLE IN CONTROLLING PHYTOPLANKTON GROWTH IN THE MARGINAL ICE-ZONE OF THE BELLINGSHAUSEN SEA

Microzooplankton and their herbivorous activity were quantified on a t ransect in the Bellingshausen Sea along the 85 degrees W meridian betw een 70 degrees 15'S and 67 degrees 30'S. The transect was worked durin g the Austral spring period of November-December 1992, with stations i n 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, m icrozooplankton concentrations were low (ca 850 organisms I-1); they i ncreased at the ice-edge station (2110 organisms I-1) and reached a ma ximum (ca 17,000 organisms I-1) at the open water situated ca 200 km n orth of the ice-edge. The community was dominated by Protozoa, which c ontributed > 97% of the numerical abundance of microzooplankton and > 89% of the standing stocks. Protozoan communities were dominated by na ked oligotrich ciliates and heterotrophic dinoflagellates. Microzoopla nkton biomass in the mixed layer of the MIZ varied between 0.2 and 54 mu g Cl-1, with the minimum situated at depth under the ice and maximu m in open surface waters, respectively. Microzooplankton biomass was s trongly correlated to ambient phytoplankton concentrations and average d 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 us ing dilution experiments within surface mixed layers, was generally vi gorous. Microzooplankton turned over between 3 and 40% of the phytopla nkton daily, thereby consuming between 0.04 and 31.21 mu gC I-1 day(-1 ) in these experiments. The magnitude of this activity was positively related to sea-water temperature,lo phytoplankton concentration, and t o the ambient microzooplankton concentrations. A multiple regression m odel of microzooplankton herbivory was developed to produce an integra ted analysis of their impact on phytogenic carbon flux. Microzooplankt on in the mixed layer were estimated to graze 21 and 3260 mg phytoplan kton-C m(-2) day(-1) 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. Microzoo plankton are clearly important in determining the fate of phytoplankto n during the ice-melt period in the Bellingshausen Sea MIZ.