Hydrography of the Southern Ocean fronts – The zooplankton link: A detailed study in the Indian Ocean sector

Abstract

Despite its importance, zooplankton diversity, biomass and abundance are still poorly known in the remote Indian sector of the Southern Ocean (SO). Therefore, systematic observations of zooplankton and hydrography have been made in the Indian sector of the SO, with a focus on Copepoda. We collected the data along 57° E transect during austral summer and examined various oceanographic locations in the central Indian Ocean sector of the SO lying in the east of the Kerguelen Plateau. The contribution of copepod abundance was substantial (85 %) to the total zooplankton biomass across the entire study region. We observed surface stratification in the Subtropical Front (STF) and Subantarctic Front (SAF); on the other hand, subsurface stratification was encountered in the Polar Front (PF) and South of PF (S-PF). As a result, nutrients were lower in warmer waters than in colder waters. On the other hand, zooplankton abundance was higher in colder waters than in warmer waters. The highest zooplankton grazing impact was recorded in the PF zone, where zooplankton removed ∼ 81 % of the daily phytoplankton production, while north of the PF zooplankton grazing was generally low. Four copepod species were shown to be highly connected to oceanic fronts. The vertical distribution of zooplankton abundance and species composition was chiefly driven by physical and biological conditions in the Indian sector of the SO. As a result, our findings indicate that physical processes play an important role in organizing plankton diversity, which ultimately determines the fate of the foodweb structure and the biological carbon pump in the studied area.