Role of eddies on controlling total and size-fractionated primary production in the Bay of Bengal

Abstract

In order to examine the role of eddies on total and size-fractionated primary production, two cyclonic (CE), one anticyclonic (ACE) and no-eddy (NE) regions were sampled in the BoB during pre-summer monsoon (June 2019). The upper ocean is strongly stratified due to freshwater discharge from major rivers. Low (high) nutrients concentrations were noticed in the ACE (CE) regions due to convergence (divergence). Prominent existence of deep Chlorophyll-a maximum (DCM) was observed and its depth is governed by mixing associated with eddies. High concentration of zeaxanthin and fucoxanthin was observed in the upper 50 m and DCM respectively. The dominant contribution of picoplankton biomass (40-80%) to total phytoplankton biomass was observed in the photic zone whereas micro and nanoplankton contributed between 10 and 30%. The photic zone integrated total primary production was higher in the CE and NE than ACE regions associating with higher nutrients in the former than latter region. The primary production by microphytoplankton was higher in the CE and NE than ACE regions. Higher picophytoplankton production was observed at depth below 10 m from surface (10-80%) than nano and microphytoplankton (1-30%). The microphytoplankton production was higher in the CE (164 ±16 mgC m-2 d-1) than ACE (60 ±26 mgC m-2 d-1) due to availability of nutrients in the former region resulted from upwelling of subsurface waters. The photic zone integrated total and microphytoplankton primary production displayed linear relation with nutrients (nitrate and phosphate) and inverse relation was observed with picophytoplankton suggesting that the availability of nutrients due to eddy-driven mixing determined the contribution of primary production by different size classes. This study suggests that eddy-driven nutrients increased contribution of primary production by microphytoplankton leading to enhanced export production under CE in the BoB and therefore, these regions can be considered as an efficient regions of carbon sequestration for the atmospheric CO2. More than 30 eddies form every year with life time of 3-6 months and its impact on sinking carbon fluxes and atmospheric CO2 sequestration in the BoB needs evaluation using numerical modelling