Functional characteristics of plankton in relation to biophysical processes in the Indian Sector of Southern Ocean: current status of ecosystem & future perspectives

Abstract The plankton (phytoplankton and zooplankton) plays a major role in southern Ocean ecosystem and global climate by sinking of atmospheric carbon dioxide (CO 2 ) and regulating biogeochemial cycle. In the study we are comparing the phytoplankton and zooplankton contributions and biomass, distinct patterns that emerge across Southern Ocean frontal zones during the austral summers of 2018 and 2020. In 2018, flagellates, particularly Gyrodinium sp., dominated carbon biomass in the Subtropical Front (STF), with contributions of 0.25 μg C/L, followed by Tripos sp., Dinophysis sp., and Protoperidinium sp. In the Subantarctic Front (SAF), Fragilariopsis sp. was prominent at 0.4 μg C/L, followed by Tripos sp. and Dinophysis sp. The Polar Front (PF), Fragilariopsis sp. significantly dominated at 1.7 μg C/L, followed by other species ranges 0.22-0.24 μg C/L. While the Southern Polar Front (SPF), Fragilariopsis sp. contributed 0.5 μg C/L. During 2020, Gyrodinium sp. became the key contributor in the STF with 1 μg C/L, followed by SAF (0.45 μg C/L), and ciliates at 0.11 μg C/L. In the PF, Fragilariopsis sp. and Thalassiosira sp. contributed at 0.6 μg C/L and 0.8 μg C/L respectively followed by Gyrodinium sp. (0.3 μg C/L). In the SPF, Fragilariopsis sp. dominated with 0.87 μg C/L followed by Gyrodinium sp. and Thalassiosira sp. The class abundance of cryptophytes dominated in the STF during 2018; however the dominance of diatom and cyanobacteria was prominent in 2020, while in SAF cryptophytes and green flagellates abundance showed a consistent. The diatoms were prevalent in the PF and SPF throughout study period, except in PF. Chlorophyll- a and other diagnostic pigments peaked contrarily across fronts and years, reflecting varying phytoplankton compositions and biomass. Zooplankton both calanoid and cyclopoid were increasing trend in PF and SPF region than STF and SAF in study period indicated their preferential food abundance of diatom biomass. On the other hand the chateognaths sp. was increased trend towards PF and SPF during 2018; whereas it decreased its trend in 2020 inferred the species with food preferential and environmental settings. Collectively, these findings underscore the dynamic relationship between phytoplankton and zooplankton communities, their shifts in distribution and abundance bases a significant role in biogeochemical cycle and climate change.