New to total primary production ratio (f-ratio) in the Bay of Bengal using isotopic composition of suspended particulate organic carbon and nitrogen
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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 -13Csus, 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 -15Nsus 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 -15Nsus 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.
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