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Monsoons increase growth of tiny plants in the Bay of Bengal

  • Satellite (MODIS) image of chlorophyll patch in the southern Bay of Bengal during the Summer Monsoon. Credit: Authors.

Satellite (MODIS) image of chlorophyll patch in the southern Bay of Bengal during the Summer Monsoon. Credit: Authors.

A current rising as a result of the Indian Summer Monsoon during June to September in the Bay of Bengal increases the growth of phytoplankton, minute plants that carry out photosynthesis in the sea. This results in the movement of organic carbon, or carbon flux, in the region.

Some ocean currents, like the one observed here, result in an upwelling of nutrients, which can enhance the “productivity” of a particular region, ie. the amount of photosynthesis and resultant nutrient fixing. Researchers from CSIR – National Institute of Oceanography,  Indian Institute of Science, Bangalore and Integrated Coastal and Marine Area Management, Chennai have quantified the extent to which the current associated with the Indian Summer Monsoon, the  Summer Monsoon Current (SMC), increase the phytoplankton growth.

Phytoplankton are microscopic plants that float in large numbers near the surface of sea water. Phytoplankton are extremely important in maintaining earth's ecosystem. They not only provide food for the marine world but, by photosynthesis, they produce one half of the oxygen produced on earth. They also act as a sink for carbon dioxide.

The study uses a combination of field sampling and satellite data tp show that the size of the area over which the phytoplankton grows, also called the chlorophyll patch, is increased due to the SMC in the Bay of Bengal.

"The open ocean current that flows south of Sri Lanka during the Summer Monsoon (June – September) is known as the Summer Monsoon Current. It flows from the Arabian Sea to the southern Bay of Bengal with a strong linkage to the Indian summer monsoon wind stress. The present paper showed the reasons behind the Summer Monsoon Current – associated large chlorophyll patch in the southern Bay of Bengal", said R. Jyothibabu, the lead author of the paper.

Though multiple rivers empty into the Bay of Bengal, carrying nutrients that should encourage productivity, the Bay is not as productive as the Arabian Sea. However, the shallow waters of the continental shelf lining the coast seem to use up these nutrients, without leaving much for the open ocean areas. The freshwater from the rivers, however, create a low salinity “cap” that has some interesting effects: very few phyloplankton can grow and storms and cyclones form easily.

This study has shown that the constant nutrient flow from southern Sri Lanka to southern Bay of Bengal, by the Summer Monsoon Current, is largely responsible for the increase in size of the chlorophyll patch. While smaller phytoplankton generally predominated in southern Bay of Bengal, the regions affected by the SMC had a greater proportion of larger phytoplankton. This was due to the presence of higher amounts of nitrates and silicates, which favour growht of larger phytoplankton. The larger phytoplankton in turn contribute to the chlorophyll biomass and higher organic carbon flux in the area, during the southwest monsoon.

The study was conducted under the Continental Tropical Convergence Zone (CTCZ) Programme, under the Indian Climate Research Programme (ICRP).

About the authors:

P. N. Vinayachandran is a senior scientist and Professor of Oceanography at the Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India. http://caos.iisc.ernet.in/faculty/vinay/vinay.htm

Email: vinay@caos.iisc.ernet.in; Phone number (Office): 080-2293 3065

R. Jyothibabu, N.V. Madhu, C. Karnan, L. Jagadeesan and A. Anjusha are from the CSIR – National Institute of Oceanography, Regional Centre, Kochi, India.

R.S. Robin is from the Integrated Coastal and Marine Area Management, Chennai, India.

 

The paper has been accepted for publication in the Journal of Marine Systems, and appeared online on 7th November.  http://www.sciencedirect.com/science/article/pii/S0924796314002565