Plankton diversity and dynamics in the upper surface of the Indian sector of the Southern Ocean ecosystem and biogeochemical processes

In the Southern Ocean (SO) marine ecosystem, the interplay between CO2 sink and climate change is closely linked to the shifting plankton communities, leading to potentially wide-ranging biogeochemical effects. Within the Indian sector of the Southern Ocean (ISSO), the phytoplankton community was predominantly composed of Fragilariopsis spp., followed by Chaetoceros sp., while the zooplankton consisted mainly of Pleuromamma spp. and Oithona similis. Plankton diversity is collectively influenced by variables such as temperature, photosynthetically available radiation (PAR), and macronutrient availability. During the austral summer of 2019, significant latitudinal variations in phytoplankton were observed at major fronts in the region, including the Subtropical Front (STF), Sub Antarctic Front (SAF), Polar Front (PF), and South of Polar Front (SPF). At the STF, flagellates dominated (87%), while their presence decreased to 12% at the SPF, while diatoms progressively increased from the STF (<1%) to the SPF (87%) through the SAF and PF. Chlorophyll a (Chl-a) concentrations at the surface were highest in the PF (0.62 mg m-3), followed by the SPF (0.56 mg m-3), SAF (0.51 mg m-3), and STF (0.28 mg m-3). The integrated Chl-a values across the water column were higher at the SPF (21.49 mg m-2) than at the STF (14.73 mg m-2). The dominant diatom species at the surface along the fronts, except for STF, was Fragilariopsis spp. (-5 x 105 Cells L-1), followed by Chaetoceros sp. (-1 x 105 Cells L-1), and Coscinodiscus sp. (-5 x 104 Cells L-1). Conversely, at the STF, the most abundant flagellate species were Gyrodinium spp. (-2.4 x 104 Cells L-1), followed by Protoperidinium sp. (-2 x 104 Cells L-1), and Amphidinium sp. (-1 x 104 Cells L-1). Zooplankton biovolume in the upper 200 m ranged from 0.71 to 10 ml 100 m-3, with relatively higher biovolume and numerical abundance found in the SAF compared to the PF. The higher Chl-a concentration in the PF and lower zooplankton biovolume could have significant negative consequences for higher trophic levels. PAR showed higher values up to 40 m than observed depths along the fronts. Additionally, variations in salinity and temperature were recorded, with a significant decline in temperature from the STF to the SPF. Nitrate (NO3-), phosphate (PO43-), and silicate (SiO44-) concentrations were higher at the SPF than at lower latitudes. Overall, these findings indicate that the plankton community composition in the study region is influenced by a diverse set of governing variables over time, highlighting the significant implications for the regional biogeochemical cycle.