The Arabian Sea (AS), (0-25°N; 45 -80°E), a land locked tropical basin in the North Indian Ocean, is one of the highest upwelling induced productive regions in the global ocean, and supports huge fisheries resources to its surrounding countries. This basin is also characterized by the existence of the persistent mid-depth oxygen minimum zone (120-1000m). The reversal of Indian monsoon winds strongly impacts its surface currents, hydrographical characteristics and biogeochemical processes, and creates high seasonal variability. The ongoing climate change with multiple stressors (e.g. increasing CO2, warming, and trace metal input) can have significant impact on the phytoplankton community from this productive basin and monitoring their responses are imperative to know the ecosystem service potential in future.

The present study was conducted as a pilot study on board [R.V. Sindhu Sadhana, SSD40] during a monsoon [Aug' 2017] cruise. Our preliminary observations showed high spatial variability in terms of physicochemical and biological parameters along the East coast of AS. During the peak monsoon, the upwelling signature (high biomass coupled with nutrient concentrations) was noticed in the south west coast of India. Surface water pCO2 variability was correlated to both physical and biological parameters.

Phytoplankton responses under CO2 and trace metal [copper (Cu) and zinc (Zn)] enrichment were monitored on board. A nutrient enriched diatom bloom [dominated Leptocylindrus sp; Nitzschia sp.] was initiated on board and was grown under ambient and high CO2 levels (»800matm) in combination with different Cu concentrations for three days. Phytoplankton community composition, cell count, biochemical composition, particulate organic carbon (POC), biogenic silica (BSi) production, stable isotopes of carbon (d13CPOM) were analyzed. There was a significant change noticed in the phytoplankton community composition both in response to CO2 and Cu. But no considerable change in per cell quota of carbon or BSi was seen between the treatments. In the second experiment (dominated by Chaetoceros sp), trace amount of Zn was added under both ambient and high CO2 levels. Zn did not show any growth stimulating impact, rather was seen to be deleterious under high CO2 +Zn treatments. Cell count was significantly decreased under high CO2 and Zn treated samples. However, total POC production did not show any statistically significant difference between Zn added or CO2 enriched samples. d13CPOM showed depleted values under high CO2+Zn treated samples and could be due to slow growth which was indicated in low cell count.  Some dominant diatom species from this upwelling system might have some strong adaptability to such variability and could be less susceptible to the ongoing/future changes. Inclusion of temperature impact in combination with the presently studied stress factors may show some different responses and needs to be considered in future studies.