Mesoscale distribution of protozooplankton communities and their herbivory in the western Scotia Sea of the Southern Ocean during the austral spring

Abstract

The distribution and structure of protozooplankton communities and the impact of their grazing on phytoplankton during spring were studied as part of the 15th Korean Antarctic Research Program in the western Scotia Sea. Water mass identities were determined based on physicochemical properties, with four regions identified: north of the Polar Front (NPF), Polar Front (PF), South of the Polar Front (SPF), and the South Antarctic Circumpolar Current region (SACCr). Physicochemical characteristics of the water column significantly influenced the distribution and structure of phytoplankton and protozooplankton communities. The NPF was characterized by relatively warmer water, low nutrient, low chlorophyll <em>a </em>(chl<em>a</em>), and pico-sized phytoplankton predominance (i.e., cyanobacteria and eukaryotic picoflagellates). Nano-sized phytoplankton such as<em> Phaeocystis antarctica</em> and <em>Cryptomonas</em> sp. dominated in the SACCr with its colder water, higher nutrient, and higher chla concentrations. Despite the relatively slightly high chla concentration in the PF, microsized phytoplankton, especially diatoms, were abundant. Large changes in protozooplankton biomass and community were observed between water masses. Heterotrophic nanoflagellates including choanoflagellates and nanociliates declined in abundance from the NPF to SACCr, whereas heterotrophic dinoflagellates (HDF) and microciliates increased in abundance from the NPF to SACCr. Ciliates declined in importance from the NPF to SACCr, accounting for over 50% of the total protozooplankton biomass in the NPF. In contrast, HDF comprised over 50% of the total in the SPF, PF, and SACCr. The depth-integrated protozooplankton biomass ranged from 443.2 to 934.0 mg C m？2, and was highest in the PF and lowest in the NPF. These relationships suggest that the spatial variation in the community and biomass of protozooplankton appears to be primarily governed by the community and size structure of phytoplankton. Protozooplankton consumed an average of 76.9% of daily phytoplankton production. Therefore, protozooplankton were the major consumers of the diverse phytoplankton community, and protozooplankton grazing is one of the most important loss processes affecting phytoplankton biomass and composition during spring in the western Scotia Sea.