Picocyanobacteria show warm water preference in the south-central Arabian Sea (North Indian Ocean) during the summer monsoon

The Arabian Sea, a part of the north Indian Ocean shows a trend of increasing sea surface temperature (SST) over a decadal scale. This area is particularly important due to high phytoplankton growth which is mostly governed by atmospheric forcing and also a place for carbon burial. Hence it is imperative to understand the responses of phytoplankton to this warming trend. The summer monsoon (June-August) winds develop a low-level atmospheric jet (Findlater Jet) blowing across the central Arabian Sea. The positive wind stress curl in the north of this jet leads to open ocean upwelling with consequent nutrient enrichment and phytoplankton bloom. The negative curl in the south results in down-welling and deepening of the mixed layer depth. During the winter monsoon, the wind direction reverses and speed weakens, but in the northern part the cold convective mixing occurs due to the cooling and densification of surface waters and also fuels high phytoplankton growth. However, the south remains oligotrophic, low productive, and warmer compared to the north. We present here two data sets of phytoplankton taxonomy done by both marker pigment analyses by HPLC and light microscopy collected in two field campaigns during summer monsoons 2017 (August) and 2018 (August) along the central Arabian Sea (64°E, 11 -21° N in 1 ° interval). The northern part of the Findlater Jet was mostly occupied by the cooler waters and highest nutrient levels that promoted large diatom-dominated phytoplankton biomass. The southern part was oligotrophic with deep mixed layers, warm, and dominated by (~50%) picocynaobacteria and Prochlorococcus (containing zeaxanthin and DV-Chla) followed by smaller chain-forming diatoms, and heterotrophic dinoflagellates. We have observed that the upwelling strength was stronger in 2018 with cooler waters and higher nutrient levels compared to 2017. The occurrences of warmer waters in 2017 supported higher growth of picocynaobacteria. This is also consistent with other global analyses of long-term trends observed from the Indian Ocean. We have considered a box of 64-66°E and divided it into two sectors, south (18-21°N) and north (11-15°N). The satellite-derived SST data from 2000-2024 indicates a warming trend during summer monsoon both in the north and south. However, no such trend was noticed during winter. This observation suggests that warming during summer monsoon may directly influence the phytoplankton community and may affect carbon transfer and cycling in this dynamic basin.