Protist community structure and diversity at the West Antarctic Peninsula in austral fall – reconciling metabarcoding, microscopy and pigment analyses

Although the West Antarctic Peninsula (WAP) is the most studied region in Antarctica, there are still many gaps in our knowledge on planktonic food web dynamics, especially outside the summer season. Most studies focused on members of the classic Antarctic paradigm of a short diatom – krill – top predator food chain, although microbial processes have been shown to be a major pathway in the pelagic food web at the WAP. Microzooplankton, for instance, are dominant grazers in summer and potentially serve as important food source for macrozooplankton, such as krill, when large phytoplankton are less abundant. This link might gain importance as recent warming of surface waters at the WAP is associated with changes in phytoplankton community composition from large diatoms to small cryptophytes, while the dominance of key macrozooplankton grazers is shifting from krill to salps. To better understand and predict the consequences of such changes for the lower trophic food web, a comprehensive understanding of the protist community is crucial. We collected plankton samples at ten different stations and four different depths around the South Shetland Islands and the Antarctic Peninsula during a cruise in austral fall (March – May 2018). The samples were analyzed using a broad range of tools, such as microscopy, pigment analysis, flow cytometry and amplicon sequencing. We then related protistan community structure to macrozooplankton abundances, dissolved and particulate nutrients and a range of abiotic parameters. Our results indicate significant differences in protist diversity and community composition among depths and sampling sites. While diversity increased with depth, temperature and dissolved nutrient concentrations were important determinants for community composition. By providing a comprehensive picture of the protist distribution over depth and in space, this study will serve as groundwork for investigating potential consequences of a shift from the predominant krill population to an increasing occurrence of salps.