Spatial variations and controlling factors in phytoplankton composition at deep chlorophyll maxima in the Bay of Bengal
Abstract
The occurrence of deep chlorophyll maxima (DCM) in the stratified oceanic regions is reported either due to photoacclimation or the availability of nutrients. The influence of mesoscale processes, such as eddies, on the depth of DCM, phytoplankton composition, and the fate of phytoplankton pigments in the Bay of Bengal is unclear. Based on the�in-situ�observations in the water column, the role of eddies on spatial variability in the depth of DCM and phytoplankton composition was studied. The depth of DCM varied between 20 and 80�m. It was shallowed and deepened in the cyclonic (CE) and anticyclonic (ACE) eddy regions by 20�40�m due to convergence and divergence, respectively. The depth of DCM displayed a significant linear relationship with nutricline and an inverse relationship with the percentage of surface photosynthetic active radiation (PAR) reaching the depth of DCM. The chlorophyll-a (Chl-a) concentration in the DCM varied between 0.2 and 1.1�mg�m?3�and was inversely related to the PAR reached and nutrient concentrations at this depth. Prymnesiophytes, chlorophytes, cyanobacteria and prochlorophytes significantly contributed to the phytoplankton in the DCM, but their contribution varied with the amount of PAR received and nutrient concentrations at the DCM. The biomass of prymnesiophytes and chlorophytes increased with an increase in PAR and nutrients at DCM and contrasted with that observed for cyanobacteria and prochlorophytes. The dominance of prymnesiophytes and chlorophytes was observed in regions where shallow (<40m) DCM was present. Cyanobacteria and prochlorophytes were abundant in areas with deep DCM, due to variable light adaptations. Some physical processes bring phytoplankton in the DCM to the surface, and to examine its fate, a microcosm experiment was conducted by incubating DCM waters under surface light. The microcosm experiments suggested that premnesiophytes and chlorophytes decreased by 85�90�%, whereas a 65�% decrease was noticed in the case of cyanobacteria and prochlorophytes from that of the initial. This suggests an increase in Chl-a, after intense physical mixing, may be due to new production arising from nutrients upwelling. The numerical models simulate the depth of DCM, but the composition of phytoplankton groups can be simulated using the governing factors derived in this study to evaluate their role in primary and export production and carbon sequestration in the stratified ocean.
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