Oceanography The Official Magazine of
The Oceanography Society
Volume 29 Issue 02

View Issue TOC
Volume 29, No. 2
Pages 222 - 231


Effects of Freshwater Stratification on Nutrients, Dissolved Oxygen, and Phytoplankton in the Bay of Bengal

By V.V.S.S. Sarma , G.D. Rao, R. Viswanadham , C.K. Sherin , Joseph Salisbury , Melissa M. Omand, Amala Mahadevan, V.S.N. Murty , Emily L. Shroyer, Mark Baumgartner, and Kathleen M. Stafford 
Jump to
Article Abstract Citation References Copyright & Usage
Article Abstract

The Bay of Bengal (BoB) is strongly density stratified due to large freshwater input from various rivers and heavy precipitation. This strong vertical stratification, along with physical processes, regulates the transport and vertical exchange of surface and subsurface water, concentrating nutrients and intensifying the oxygen minimum zone (OMZ). Here, we use basinwide measurements to describe the spatial distributions of nutrients, oxygen, and phytoplankton within the BoB during the 2013 northeast monsoon (November–December). By the time riverine water reaches the interior bay, it is depleted in the nutrients nitrate and phosphate, but not silicate. Layering of freshwater in the northern BoB depresses isopycnals, leading to a deepening of the nutricline and oxycline. Oxygen concentrations in the OMZ are lowest in the north (<5 µM). Weak along-isopycnal nutrient gradients reflect along-isopycnal stirring between ventilated surface water and deep nutrient-replenished water. Picoplankton dominate the phytoplankton population in the north, presumably outcompeting larger phytoplankton species due to their low nutrient requirements. Micro- and nanoplankton numbers are enhanced in regions with deeper mixed layers and weaker stratification, where nutrient replenishment from subsurface waters is more feasible. These are also the regions where marine mammals were sighted. Physical processes and the temperature-salinity structure in the BoB directly influence the OMZ and the depth of the oxycline and nutricline, thereby affecting the phytoplankton and marine mammal communities.


Sarma, V.V.S.S., G.D. Rao, R. Viswanadham, C.K. Sherin, J. Salisbury, M.M. Omand, A. Mahadevan, V.S.N. Murty, E.L. Shroyer, M. Baumgartner, and K.M. Stafford. 2016. Effects of freshwater stratification on nutrients, dissolved oxygen, and phytoplankton in the Bay of Bengal. Oceanography 29(2):222–231, https://doi.org/10.5670/oceanog.2016.54.


Ballance, L.T. and R.L. Pitman. 1998. Cetaceans of the western tropical Indian Ocean: Distribution, relative abundance, and comparisons with cetacean communities of two other tropical ecosystems. Marine Mammal Science 14:429–459, https://doi.org/10.1111/j.1748-7692.1998.tb00736.x.

Baumgartner, M.F., T.V.N. Cole, P.J. Clapham, and B.R. Mate. 2003. North Atlantic right whale habitat in the lower Bay of Fundy and on the SW Scotian Shelf during 1999–2001. Marine Ecology Progress Series 264:137–154, https://doi.org/10.3354/meps264137.

Baumgartner, M.F., K.D. Mullin, L.N. May, and T.D. Leming. 2001. Cetacean habitats in the northern Gulf of Mexico. Fishery Bulletin 99:219–239.

Casey, K.S., T.B. Brandon, P. Cornillon, and R. Evans. 2010. The Past, Present and Future of the AVHRR Pathfinder SST Program. Pp. 273–287 in Oceanography from Space: Revisited. V. Barale, J.F.R. Gower, and L. Alberotanza, eds, Springer, https://doi.org/10.1007/978-90-481-8681-5_16.

de Boer, M.N., R. Baldwin, C.L.K. Burton, E.L. Eyre, K.C.S. Jenner, M.-N.M. Jenner, S.G. Keith, K.A. McCabe, E.C.M. Parsons, V.M. Peddemors, and others. 2002. Cetaceans in the Indian Ocean Sanctuary: A Review. Whale and Dolphin Conservation Society Science Report. Chippenham, UK, 52 pp.

Gauns, M., M. Madhupratap, N. Ramaiah, R. Jyothibabu, V. Fernandes, J.T. Paul, and S. Prasanna Kumar. 2005. Comparative accounts of biological productivity characteristics and estimates of carbon fluxes in the Arabian Sea and Bay of Bengal. Deep Sea Research Part II 52:2,003–2,017, https://doi.org/10.1016/j.dsr2.2005.05.009

Grasshoff, K., M. Ehrhardt, and K. Kremling, eds. 1992. Methods of Seawater Analysis. Verlag Chemie, Weinheim, 634 pp., https://doi.org/​10.1002/9783527613984.

Krishna, M.S., M.H.K. Prasad, D.B. Rao, S. Viswanadham, V.V.S.S. Sarma, and N.P.C. Reddy. 2015. Export of dissolved inorganic nutrients to the northern Indian Ocean from the Indian monsoonal rivers during discharge period. Geochemica et Cosmochemica Acta 172:430–443, https://doi.org/10.1016/j.gca.2015.10.013.

Latha, T.P., K.H. Rao, P.V. Nagamani, E. Amminedu, S.B. Choudhury, C.B.S. Dutt, and V.K. Dadhwal. 2015. Impact of Cyclone PHAILIN on chlorophyll-a concentration and productivity in the Bay of Bengal. International Journal of Geosciences 6:473–480, https://doi.org/10.4236/ijg.2015.65037.

La Violette, P.E. 1967. Temperature, Salinity and Density of the World’s Seas: Bay of Bengal and Andaman Sea. Informal Report 67-57, Naval Oceanographic Office, Washington, DC, 81 pp, http://www.dtic.mil/dtic/tr/fulltext/u2/820709.pdf.

Leatherwood, S., C.B. Peters, R. Santerre, M. Santerre, and J.T. Clarke. 1984. Observations of cetaceans in the northern Indian Ocean Sanctuary, November 1980–May 1983. Reports of the International Whaling Commission 34:509–520.

Madhu, N.V., P.A. Maheswaran, R. Jyothibabu, V. Sunil, V. Ravichandran, T. Balasubramanian, T.C. Gopalakrishnan, and K.K.C. Nair. 2002. Enhanced biological production off Chennai triggered by October 1999 super cyclone (Orissa). Current Science 82:1,472–1,479.

Maneesha, K., V.V.S.S. Sarma, N.P.C. Reddy, Y. Sadhuram, T.V.R. Murty, V.V. Sarma, and M.D. Kumar. 2011. Meso-scale atmospheric events promote phytoplankton blooms in the coastal Bay of Bengal. Journal of Earth System Sciences 120:773–782, https://doi.org/10.1007/s12040-011-0089-y.

Murty, V.S.N., G.V.M. Gupta, V.V. Rao, B.P. Rao, D. Jyothi, P.N.M. Shastri, and Y. Supraveena. 2002. Effect of vertical stability and circulation on the depth of the chlorophyll maximum in the Bay of Bengal during May–June 1996. Deep Sea Research Part I 47:859–873, https://doi.org/​10.1016/S0967-0637(99)00071-0.

Naqvi, S.W.A., D.A Jayakumar, M. Nair, M.D. Kumar, and M.D. George. 1994. Nitrous oxide in the western Bay of Bengal. Marine Chemistry 47:269–278, https://doi.org/10.1016/0304-4203(94)90025-6.

Naqvi, S.W.A., M.S. Shailaja, M.D. Kumar, and R.S. Gupta. 1996. Respiration rates in subsurface waters of the northern Indian Ocean: Evidence for low decomposition rates of organic matter within the water column in the Bay of Bengal. Deep Sea Research Part I 43:73–81, https://doi.org/10.1016/0967-0645(95)00080-1.

NASA Aquarius Project. 2015. Aquarius Official Release Level 2 Sea Surface Salinity. Ver. 4.0. PODAAC, CA, https://doi.org/10.5067/AQR40-2SOCS.

Ocean Biology Processing Group. 2003. MODIS Aqua Level 3 Global Daily Mapped 4 km Chlorophyll a. Ver. 6. PODAAC, CA, USA. Data set accessed 2014-12-06.

Prasanna Kumar, S., P.M. Muraleedharan, T.G. Prasad, M. Gauns, N. Ramaiah, S.N. De Souza, D.S. Sardesai, and M. Madhupratap. 2002. Why is the Bay of Bengal less productive during summer monsoon compared to the Arabian Sea? Geophysical Research Letters 29, 2235, https://doi.org/10.1029/2002GL016013.

Prasanna Kumar, S., M. Nuncio, J. Narvekar, A. Kumar, S. Sardesai, S.N. De Souza, M. Gauns, N. Ramaiah, and M. Madhupratap. 2004. Are eddies nature’s trigger to enhance biological productivity in the Bay of Bengal? Geophysical Research Letters 31, L07309, https://doi.org/10.1029/2003GL019274.

Ramaiah, N., V. Fernandes, J.T. Paul, R. Jyothibabu, M. Gauns, and E.A. Jayraj. 2010. Seasonal variability in biological carbon biomass standing stocks and production in the surface layers of the Bay of Bengal. Indian Journal of Marine Science 39:369–379.

Rao, C.K., S.W.A. Naqvi, M.D. Kumar, S.J.D. Varaprasad, D.A. Jayakumar, M.D. George, and S.Y.S. Singbal. 1994. Hydrochemistry of the Bay of Bengal: Possible reasons for a different water-column cycling of carbon and nitrogen from the Arabian Sea. Marine Chemistry 47:279–290, https://doi.org/10.1016/0304-4203(94)90026-4.

Reilly, S.B. 1990. Seasonal changes in distribution and habitat differences among dolphins in the eastern tropical Pacific. Marine Ecology Progress Series 66:1–11. 

Roy, R., R. Chitari, V. Kulkarni, M.S. Krishna, V.V.S.S. Sarma, and A.C. Anil. 2015. CHEMTAX-derived phytoplankton community structure associated with temperature fronts in the northeastern Arabian Sea. Journal of Marine Systems 144:81–91, https://doi.org/10.1016/j.jmarsys.2014.11.009.

Sarin, M.M., S. Krishnaswami, K. Dilli, B.L.K. Somayajulu, and W.S. Moore. 1989. Major ion chemistry of the Ganga-Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal. Geochemica et Cosmochimica Acta 53:997–1,009, https://doi.org/10.1016/0016-7037(89)90205-6.

Sarma, V.V.S.S. 2002. An evaluation of physical and biogeochemical processes regulating the oxygen minimum zone in the water column of the Bay of Bengal. Global Biogeochemical Cycles 16:1–46, https://doi.org/10.1029/2001GB001461.

Sarma, V.V.S.S., M.S. Krishna, R. Viswanadham, G.D. Rao, V.D. Rao, B. Sridevi, B.S.K. Kumar, V.R. Prasad, Ch.V. Subbaiah, T. Acharyya and D. Bandopadhyay, 2013. Intensified oxygen minimum zone on the western shelf of Bay of Bengal during summer monsoon: Influence of river discharge. Journal of Oceanography 69:45–55, https://doi.org/10.1007/s10872-012-0156-2.

Shetye, S.R. 1993. The movement and implications of the Ganges-Brahmaputra runoff on entering the Bay of Bengal. Current Science 64:32–38.

Shetye, S.R., S.S.C. Shenoi, A.D. Gouveia, G.S. Michael, D. Sundar, and G. Nampoothiri. 1991. Wind-driven coastal upwelling along the western boundary of the Bay of Bengal during the southwest monsoon. Continental Shelf Research 11:1,397–1,408, https://doi.org/10.1016/0278-4343(91)90042-5.

UNESCO. 1979. Discharge of Selected Rivers of the World: A Contribution to the International Hydrological Decade. Paris, 104 pp.

Uitz, J., H. Claustre, A. Morel, and S.B. Hooker. 2006. Vertical distribution of phytoplankton communities in open ocean: An assessment based on surface chlorophyll. Journal of Geophysical Research 111, C08005, https://doi.org/10.1029/2005JC003207.

Unger, D., V. Ittekkot, P. Schafer, J. Tiemann, and S. Reschke. 2003. Seasonality and interannual variability of particle fluxes to the deep Bay of Bengal: Influence of riverine input and oceanographic processes. Deep Sea Research Part II 50:897–923, https://doi.org/10.1016/S0967-0645(02)00612-4.

Varkey, M.J., V.S.N. Murty, and A. Suryanarayana. 1996. Physical oceanography of the Bay of Bengal and Andaman Sea oceanography and marine biology. Pp. 1–70 in Oceanography and Marine Biology: An Annual Review, vol. 34. A.D. Ansell, R.N. Gibson, and S. Barnes, eds, CRC Press.

Vidya, P.J., and S. Prasanna Kumar. 2013. Role of mesoscale eddies on the variability of biogenic flux in the northern and central Bay of Bengal. Journal of Geophysical Research 118:5,760–5,771, https://doi.org/10.1002/jgrc.20423.

Vipin, P., K. Sarkar, S.G. Aparna, D. Shankar, V.V.S.S. Sarma, D.G. Gracias, M.S. Krishna, G. Srikanth, R. Mandal, E.P. Rama Rao, and N. Srinivasa Rao. 2015. Evolution and sub-surface characteristics of a sea-surface temperature filament and front in the northeastern Arabian Sea during November–December 2012. Journal of Marine Systems 150:1–11, https://doi.org/10.1016/​j.jmarsys.2015.05.003

Copyright & Usage

This is an open access article made available under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution, and reproduction in any medium or format as long as users cite the materials appropriately, provide a link to the Creative Commons license, and indicate the changes that were made to the original content. Images, animations, videos, or other third-party material used in articles are included in the Creative Commons license unless indicated otherwise in a credit line to the material. If the material is not included in the article’s Creative Commons license, users will need to obtain permission directly from the license holder to reproduce the material.