Mahanta, Chandan, and Rajib. K. Goswami

Department of Civil Engineering, Indian Institute of Technology-Guwahati, Guwahati 781039, India, Tel: 91-361-2691000, Fax: 91-361-2690762, E-mail: mahanta_iit@yahoo.com

Contrasting sediment flux to the Northern Indian Ocean from the Indus and the Brahmaputra-Ganges system: A future C-N-P Biogeochemistry perspective with reference to JGOFS and BOBPS

The Northern Indian Ocean is characterised by its peculiar dynamics and trophic conditions deserving studies that integrate physical, geochemical and biological aspects. The discharge by the Indus to the Arabian Sea being less than 10% of what it used to be five decades back makes a comparative impact study with the Bay of Bengal of great interest. Extending the JGOFS to the Bay of Bengal in the form of BOBPS has opened possibilities of contrasting processes and upwelling taking place in the unexplored more dynamic part of the Indian Ocean with respect to the critical roles of the Ganges-Brahmaputra and the Indus. The JGOFS data on the Arabian Sea and the processes integrated with preliminary findings of BOBPS indicate interesting interpretations of the likely impact of Ganges-Brahmaputra flux on the C-N-P biogeochemistry. 95% of the two billion tons annual sediment load (highest in the world) of the Brahmaputra-Ganges system is injected to the Bay of Bengal during the monsoon. This translates into an episodic pulse of more than 5% of the global dissolved and particulate riverine C-N-P load.

The open sea connection to this sudden nutrient flux is of particular importance as time-series studies revealed fluctuations of an order of magnitude over two decades. Distribution of C-N-P and DO in the deep Indian Ocean suggests that the sediment flux serves as a major nutrient source and oxygen sink. The nutrient element excess and oxygen deficiencies observed in the benthic mixed layer is consistent with diffusion along isopycnal surface with circumpolar deep water acting as a sink for C-N-P and source for dissolved oxygen. This unique aspect is likely to be the result of very high rates of respiration and particle dissolution coupled with a high stability of the water column overlying the benthic mixed layer.