Phytoplankton community composition around the Crozet Plateau, with emphasis on diatoms and Phaeocystis

Phytoplankton community composition has important implications for upper-ocean biogeochemistry and export production. The species composition of the phytoplankton community (diatoms, Phaeocystis antarctica) was examined around the Crozet Plateau (Southern Ocean) during the CROZEX project in order to address spatial changes in species composition. Phytoplankton biomass was dominated by P. antarctica to the north of the Crozet Plateau, while south of the plateau a diverse community including several large diatom species (e.g., Corethron pennatum) occurred. The phytoplankton community in the vicinity of the Crozet Plateau and Crozet islands included a mixture of P. antarctica and Thalassionema nitzschioides. High numbers of empty diatom frustules characterised many of the sampling sites, with similar species present in the water column as both full and empty cells. Multivariate analysis was used to analyse both the level of similarity between the different sampling sites and the co-occurrence of species. Cluster analysis of the sampling sites showed distinct clusters that differed in terms of species composition and environmental parameters. Recurrent species analysis showed groupings which relate to phytoplankton cell size; groupings separated (i) small (∼ 15 μ m) diatom species, (ii) medium (∼ 35 μ m) and large (∼ 90 μ m) diatom species as well as P. antarctica colonies, and (iii) giant (∼ 300 μ m) diatoms. The abundance and distribution of the different floral groups identified by multivariate analysis are linked to the different environmental regimes, although the relationships are different for small and large species due to the interaction between factors important for phytoplankton growth (e.g., light, nutrients, iron) and mortality (e.g., grazing).