Mesozooplankton production, grazing and respiration in the Bay of Bengal: Implications for net heterotrophy

Abstract

Mesozooplankton samples were collected from the mixed layer depth along a central (along 88ºE) and a western transect in the Bay of Bengal during four seasons covered between 2001and 2006 in order to investigate spatio-temporal variability in their biomass. At these stations, grazing and respiration rates were measured from live zooplankton hauled in from the surface during December 2005. Akin to the mesozooplankton "paradox" in the central and eastern Arabian Sea, biomass in the mixed layer was more or less invariant in the central and western Bay of Bengal, even as the chl a showed marginal temporal variation. By empirical equation, the mesozooplankton production rate calculated to be 70-246 mg C m-2 d-1 is on par with the Arabian Sea. Contrary to the conventional belief, mesozooplankton grazing impact was up to 83% on primary production (PP). Low PP coupled with very high zooplankton production (70% of PP) along with abundant bacterial production (50% of the PP; Ramaiah et al. 2009) is likely to render the Bay of Bengal net heterotrophic, especially during the spring intermonsoon. Greater estimates of fecal pellet-carbon egestion by mesozoopankton compared to the average particulate organic carbon flux in sediment traps, implies that much of the matter is recycled by heterotrophic communities in the mixed layer facilitating nutrient regeneration for phytoplankton growth. We also calculated that over a third of the primary production is channelized for basinwide zooplankton respiration that accounts for ~ 52 M tC annually. In the current scenario of global warming, if low (primary) productive warm pools like the Bay of Bengal continue to be net heterotrophic, negative implications like enhanced emission of CO2 to the atmosphere, increased particulate flux to the deeper waters and greater utilization of dissolved oxygen resulting in expansion of the existing oxygen minimum zone are imminent.

Spatial variability of upper ocean POC export in the Bay of Bengal and the Indian Ocean determined using particle‐reactive 234Th

“…Spatial variability in POC export and the associated biogeochemical processes occurring in this region are discussed below. Fernandes and Ramaiah [2016] reported that the Bay of Bengal exhibited net heterotrophy during spring intermonsoon period and low PP along with high bacterial production and high zooplankton production rendered net heterotrophy in the bay. At times of low autotrophic production, sizeable heterotrophic bacterial biomass and particulates served as source of food for microzooplankton and mesozooplankton.…”