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Physical processes influence the biological productivity: An observation in Bay of Bengal during summer monsoon
Although large rivers bring nutrients to Bay of Bengal, this oceanic area is considered to have a lower biological productivity than its western counterpart, the Arabian Sea. Seasonally reversing monsoon winds and associated circulation patterns plays a major role in determining biological processes across the full spectrum of spatial and temporal scales. NIO's scientists studied variability of biological production in the Bay of Bengal based on basin-scale and mesoscale physical processes using hydrographic data collected during the peak summer monsoon in July?August, 2003. They identified three different and spatially varying physical processes in the upper 300 m: (i) anticyclonic warm gyre off-shore 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 the low salinity surface waters were found to contain low concentrations of nutrients. These warm surface waters extended below the euphotic zone, resulting in an oligotrophic environment with low surface chlorophyll-a, low surface primary production and low zooplankton biovolume. In the cyclonic eddy, though the elevated isopycnals raised the nutricline upto the surface making the system highly eutrophic, the response in the biological activity was found to be low. In the upwelling zone, although the nutrient concentrations were lower compared to the cyclonic eddy, the surface phytoplankton biomass and production were high, and mesozooplankton biovolume 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 influence the abundance and spatial heterogeneity of plankton populations across a wide spatial scale in the Bay of Bengal. The multifaceted effects of these physical processes on primary productivity thus play a prominent role in structuring of zooplankton communities and could consecutively affect the recruitment of pelagic fisheries. For more information, please read: Muraleedharan, K.R.; Jasmine, P.; Achuthankutty, C.T.; Revichandran, C.; DineshKumar, P.K.; Anand b, P.; Rejomon, G. Influence of basin-scale and mesoscale physical processes on biological productivity in the Bay of Bengal during the summer monsoon. Prog. Oceanogr.: 72(4); 2007; 364-383 Ramaswamy, V.; Nath, B.N.; Vethamony, P.; Ilangovan, D. Source and dispersal of suspended sediment in the macro-tidal Gulf of KachchhMar. Pollut. Bull.: 54(6); 2007; 708-719.
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