Size-fractionated phytoplankton biomass and primary production in the eastern Indian sector of the Southern Ocean in the austral summer 2018/2019

Abstract

The Southern Ocean significantly contributes to primary production in the global ocean. In this study, size-fractionated primary production and chlorophyll a (Chl a), which are not well elucidated, were determined in the eastern Indian sector in the austral summer of 2018–2019. The Southern Antarctic Circumpolar Current Front, Southern Boundary of the Antarctic Circumpolar Current (SB), and Antarctic Slope Front (ASF) were set as study areas, and sampling stations were divided into the three areas: to the north of the SB, the area between the SB and ASF, and to the south of the ASF. Primary production was 85.1–385.7 mg C m−2 day−1 north of the SB, 131.4–192.5 mg C m−2 day−1 between the SB and ASF, and 95.6–472.7 mg C m−2 day−1 south of the ASF. The Chl a biomass integrated within the euphotic zone in each area was 19.8–46.1 mg m−2, 10.3–19.9 mg m−2, and 17.0–77.9 mg m−2, respectively. Although no significant difference was observed in the primary production and Chl a biomass among the three areas, the biomass tended to be lower between the SB and ASF than in the other two areas. The large phytoplankton (>10 μm) significantly contributed to the primary production and the Chl a biomass at every station north of the SB, accounting for approximately 65–90%. In the other two areas, there were some stations where large phytoplankton accounted for 60–85%, and others where large phytoplankton did not dominate. The flow to the secondary production was one order of magnitude higher to the north of the SB than to the south of the SB. The time since sea-ice melt possibly plays an important role in determining the dominant phytoplankton size and hence size structure of phytoplankton in the survey area. The features in phytoplankton size structure observed north and south of the SB may not be unique to the areas. High phytoplankton biomass and primary production were also observed in the west of 130°E, which was similar to the results of the previous study. A time lag between the west and east surveys, about one month, may have caused the high-west and low-east patterns.