Influence of basin-scale and mesoscale physical processes on biological productivity in the Bay of Bengal during the summer monsoon
Physical forcing plays a major role in determining biological processes in the ocean across the full spectrum of spatial and temporal scales. Variability of biological production in the Bay of Bengal (BoB) based on basin-scale and mesoscale physical processes is presented using hydrographic data col...
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Veröffentlicht in: | Progress in oceanography 2007-03, Vol.72 (4), p.364-383 |
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Zusammenfassung: | Physical forcing plays a major role in determining biological processes in the ocean across the full spectrum of spatial and temporal scales. Variability of biological production in the Bay of Bengal (BoB) based on basin-scale and mesoscale physical processes is presented using hydrographic data collected during the peak summer monsoon in July-August, 2003. Three different and spatially varying physical processes were identified in the upper 300m: (I) anticyclonic warm gyre offshore in the southern Bay; (II) a cyclonic eddy in the northern Bay; and (III) an upwelling region adjacent to the southern coast. In the warm gyre (>28.8 super(o)C), the low salinity (33.5) surface waters contained low concentrations of nutrients. These warm surface waters extended below the euphotic zone, which resulted in an oligotrophic environment with low surface chlorophyll a (0.12mgm super(-) super(3)), low surface primary production (2.55mgCm super(-) super(3)day super(-) super(1)) and low zooplankton biovolume (0.14mlm super(-) super(3)). In the cyclonic eddy, the elevated isopycnals raised the nutricline upto the surface (NO sub(3)-N>8.2 mu M, PO sub(4)-P>0.8 mu M, SiO sub(4)-Si>3.5 mu M). Despite the system being highly eutrophic, response in the biological activity was low. In the upwelling zone, although the nutrient concentrations were lower compared to the cyclonic eddy, the surface phytoplankton biomass and production were high (Chl a - 0.25mgm super(-) super(3), PP - 9.23mgCm super(-) super(3)day super(-) super(1)), and mesozooplankton biovolume (1.12mlm super(-) super(3)) was rich. Normally in oligotrophic, open ocean ecosystems, primary production is based on 'regenerated' nutrients, but during episodic events like eddies the 'production' switches over to 'new production'. The switching over from 'regenerated production' to 'new production' in the open ocean (cyclonic eddy) and establishment of a new phytoplankton community will take longer than in the coastal system (upwelling). Despite the functioning of a cyclonic eddy and upwelling being divergent (transporting of nutrients from deeper waters to surface), the utilization of nutrients leading to enhanced biological production and its transfer to upper trophic levels in the upwelling region imply that the energy transfer from primary production to secondary production (mesozooplankton) is more efficient than in the cyclonic eddy of the open ocean. The results suggest that basin-scale and mesoscale processes infl |
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ISSN: | 0079-6611 |
DOI: | 10.1016/j.pocean.2006.09.012 |