| Summary: | Ice melting in the Antarctic summer and upwelling of the Circumpolar Deep Water (CDW) near the Antarctic continent have been suggested to regulate the distributions of microbial communities. However, an understanding of these relationships in the Indian Ocean sector of East Antarctica is lacking. We report on the distribution and relationships of virio- and picoplankton at 77 stations (including 29 stations throughout the entire water column and 48 stations from the epipelagic zone) from 35° to 78°E and 62° to 70°S, covering the Cosmonaut Sea, Cooperation Sea and Prydz Bay. The epipelagic stations were grouped into four clusters based on the virio- and picoplankton composition and abundance. Clusters three and four, which corresponded with areas of upwelling and ice retreat, had high abundances of autotrophic picoeukaryotes; this resulted in subsequent high abundances of heterotrophic prokaryotes and virioplankton. Cluster one and two stations correspond with a high nutrient, low chlorophyll (HNLC) area, where the abundances of autotrophic and heterotrophic picoplankton are low. Viruses were significantly correlated with heterotrophic prokaryotes in the ice-covered areas but more strongly correlated with autotrophic picoeukaryotes in areas associated with ice-edge blooms. Throughout the water column in the Cooperation Sea, nutrient supply from the upwelling of the CDW induced an increased abundance of heterotrophic prokaryotes, particularly in the meso- and bathypelagic zones of stations in the northern part of the area. High viral abundances were detected in the meso- and bathypelagic zones of stations with high abundances of virio- and picoplankton in the epipelagic zone. This study shows that the summer distribution of virio- and picoplankton in the Indian Ocean sector of East Antarctica was mainly controlled by ice-melting and upwelling of the CDW.
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