Role of eddies on organic matter production and f-ratios in the Bay of Bengal

Abstract

In order to examine the sources of nutrients and f-ratios in the cyclonic (CE) and anticyclonic eddy (ACE), the content and isotopic composition of carbon and nitrogen in suspended particulate organic matter (POM) were analyzed in one CE and two ACEs in the western Bay of Bengal (BoB). Relatively higher concentrations of Chlorophyll-a (Chl-a) and nutrients were observed in the CE and no-eddy (NE) regions than ACE region. Higher concentrations of particulate organic carbon (POC) and nitrogen (PN) were observed in the CE and NE than ACE in the BoB. Isotope mixing model (SIAR) suggested that POM pool mainly consists of marine plankton biomass (~80%) and contribution of terrestrial organic matter is insignificant (<20%). The depth of nitracline was shallower in the CE due to divergence while it was deeper in the ACEs due to convergence of upper ocean. The isotopic composition of nitrogen in the POM varied between 2.4% and 8.5‰ and relatively lower values were associated with deeper nitracline depth in the ACE and vice versa in the CE. This suggests that upward mixing of subsurface nitrate supported primary production in the CE and regenerated nitrogen in the ACE region. The estimated f-ratios using δ15NPOM and depth of nitracline were almost double in the CE (0.31 ± 0.04) and NE (0.28 ± 0.05) than ACEs (0.19 ± 0.09) in the BoB. Though upward mixing of subsurface low DO water intensified oxygen minimum zone (OMZ) below 75 m in the CE region, enhanced phytoplankton biomass and its export may further intensify. In contrast, convergence of DO rich surface water and low f-ratios in the ACE resulted in weaker OMZ. This study suggests that eddies significantly influence primary and export productions that may lead to modifications in intensity of OMZ in the BoB. >50 eddies form every year in the BoB and its impact on primary and export production leading to intensification/weakening of OMZ should be evaluated using both observations and models.