New to total primary production ratio (f-ratio) in the Bay of Bengal using isotopic composition of suspended particulate organic carbon and nitrogen

Abstract

Variations in the content and isotopic compositions of carbon and nitrogen in the suspended particulate matter were analysed to identify sources of nutrients and compute f-ratios (ratio of new production to primary production) in the Bay of Bengal (BoB) during the post-monsoon period (November-December 2013). Remote sensing data during our observation period indicated that the study region encompassed meso-scale cyclonic (CE) and anticyclonic eddies (ACE) and was influenced by tropical cyclone Madi between 6 and 12 December 2013. Relatively higher concentrations of nitrate were observed in the CE and in the cyclone-influenced region compared to that in the ACE. Variable concentrations of nutrients controlled the size distributions of phytoplankton. Overall, the picoplankton population dominated, but the contributions of micro- and nano-plankton populations were significantly higher in the CE region because of higher concentration of nutrients. The δ13Csus, C:N molar ratios and C: Chl-a ratios suggested that suspended matter is mainly contributed by in situ processes during the study period. The δ15Nsus of suspended matter suggested that vertical supply of nutrients brought from subsurface water supported new production in the CE and cyclone-influenced regions, whereas regenerated nutrients supported phytoplankton production in the ACE. The f-ratios computed using δ15Nsus and depth of nitracline were high in the CE (0.41 ± 0.14) and cyclone-influenced region (0.54 ± 0.12) and lower in the ACE (0.27 ± 0.06). This study revealed that the meso-scale CEs are more important in enhancing exportable production in the BoB. As more than 30 CEs form annually in the BoB, their impact on basin-wide new production and export flux to the deeper ocean must be evaluated using both observations and high-resolution models.