Upper-ocean dynamics and nutrient coupling control the contribution of deep chlorophyll maxima to phytoplankton productivity in the Indian Sector of the Southern ocean

Abstract

The deep chlorophyll maximum (DCM) significantly influences water-column integrated phytoplankton production (IPP) and carbon export in the Indian Sector of the Southern Ocean (ISSO). The characteristics of DCMs exhibit strong regional variability, influenced by the interplay among physical dynamics, nutrient inputs, and biological interactions. Using combined in situ and satellite observations, this study investigated the characteristics, driving factors, and significance of DCMs to phytoplankton productivity in the ISSO. The DCM dynamics in the ISSO are strongly influenced by upper ocean stratification, nutrient-light tradeoff, watermass structure, and sea-ice variability. The study indicated that biomass-driven DCMs occurred under moderate stratification and nutrient availability, whereas photoacclimated DCMs were reported under strong stratification and reduced light conditions. In the Seasonal sea-ice Zone (SIZ), the DCM formation was facilitated by the metwater-driven buoyancy formed by the sea-ice dynamics in association with the westerly anomalies. While episodic restratification and mixing led by zonal wind modulated DCM formation at the Polar front (PF-1), particularly during 2016. Across the ISSO, DCMs contributed ∼ 3.8% to 19.4% to the IPP. The biomass-driven DCMs enhanced IPP significantly compared to those formed by photoacclimation, contributing less to IPP. These findings highlight the significant influence of DCM layer production on water column productivity and on the efficacy of the biological carbon pump (BCP) in the ISSO.