HESS Hydrology and Earth System Sciences HESS Hydrol. Earth Syst. Sci. 1607-7938 Copernicus Publications Göttingen, Germany 10.5194/hess-17-187-2013 Seasonal stratification and property distributions in a tropical estuary (Cochin estuary, west coast, India) Shivaprasad A. 1 Vinita J. 1 Revichandran C. 1 Reny P. D. 1 Deepak M. P. 1 Muraleedharan K. R. 1 Naveen Kumar K. R. 1 National Institute of Oceanography, Regional Centre, Dr. Salim Ali road, Kochi-18, Kerala, India 21 01 2013 17 1 187 199 Copyright: © 2013 A. Shivaprasad et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://hess.copernicus.org/articles/17/187/2013/hess-17-187-2013.html The full text article is available as a PDF file from https://hess.copernicus.org/articles/17/187/2013/hess-17-187-2013.pdf

The intratidal, spring–neap and seasonal variations in stratification were examined in the Cochin estuary. The observations established a strong connection with the distribution of chemical and biological properties. The influence of tides and river discharge forcing in water column stability was quantified using potential energy anomaly (PEA) and stratification parameter. Partially mixed (neap) and well-mixed (spring) conditions during low river discharge (dry) period were altered in monsoon by the salt wedge intrusions. The ecological impact of salt wedge propagation on high tides bringing upwelled water to the system was evident from the bottom hypoxic, high chlorophyll <i>a</i> and nutrient-rich conditions. Phosphate and nitrite concentrations were higher at the bottom saline conditions but silicate and nitrate were clearly supplied by river water. However, during ebb tide this front was driven out of the estuary. The periodic advance and retreat of the salt wedge was inevitable in making the system immune from extended hypoxia/anoxia and maintaining the health of the Cochin estuary. For the seasonally varying river flow in the estuary, salt intrusion receded with increasing river flow in monsoon and rebounded with decreasing river flow in dry season. During monsoon, the intense flushing and reduction in salinity field expansion seemed to be responsible for the limited chlorophyll <i>a</i> levels along the surface of the Cochin estuary.

 References Balachandran, K. K., Laluraj, C. M., Nair, M., Joseph, T., Sheeba, P., and Venugopal, P.: Heavy metal deposition in a flow-restricted, tropical estuary, Estuar Coast Shelf Sci., 65, 361–370, 2005. Balachandran, K. K., Reddy, G. S., Revichandran, C., Srinivas, K., Vijayan, P. R., and Thottam, T. J.: Modelling of tidal hydrodynamics for a tropical ecosystem with implications for pollutant dispersion (Cochin Estuary, Southwest India), Ocean Dynam., 58, 259–273, 2008. Balchand, A. N. and Nair, S. M.: Fractionation of phosphorous in the sediments of a tropical estuary, Environ Geol., 23, 284–294, 1994. Banse, K.: Hydrography of the Arabian Sea shelf of India and Pakistan and effects on demersal fishes, Deep Sea Res., 15, 45–79, 1968. Blanton, J.: Energy dissipation in a tidal estuary, J. Geophys. Res., 74, 5460–5466, 1969. Brenkert, A. and Malone, E.: Vulnerability and resilience of India and Indian states to climate change: a first-order approximation, Joint Global Change Research Institute, 65, 2003. Dinesh Kumar, P. K., Sankaranarayanan, V. N., and Saraladevi, K.: An introduction to the system, prior studies, historical trends and future implications, Indian J. Environ. Protection., 14, 98–102, 1994. Dyer, K. R.: Estuaries: A Physical introduction, Wiley-Interscience, London, 1, 1973. Dyer, K. R.: Estuaries: A Physical introduction, 2nd Edn., John Wiley and Sons, Sydney, 1997. Geyer, W. R. and David, M. F.: Tide-Induced Variation of the Dynamics of a Salt Wedge Estuary, J. Phys. Oceanogr., 19, 1060–1072, 1989. Government of Kerala, General features, <a href="http://en.wikipedia.org/wiki/Kerala">http://en.wikipedia.org/wiki/Kerala</a>, 2005b. Grasshoff, K., Ehrhardt, M., and Kremling, K.: Methods of Seawater Analysis, Verlag Chemie, Weinheim, 419, 1983. Haralambidou, K., Sylaios, G., and Tsihrintzis, V. A.: Salt-Wedge propagation in Mediterranean micro-tidal river mouth, Estuar. Coast Shelf Sci., 90, 174–184, 2010. Joseph, S. and Ouseph, P. P.: Assessment of nutrients using multivariate statistical techniques in estuarine systems and its management implications: a case study from Cochin Estuary, India, Water Environ J., 24, 126–132, 2010. Kasai, A., Kurikawa, Y., Ueno, M., Rober, D., and Yamashita, Y.: Salt-wedge intrusion of seawater and its implication for phytoplankton dynamics in the Yura Estuary, Japan., Estuar. Coast Shelf Sci., 86, 408–414, 2010. Ketchum, B. H.: Estuaries and enclosed seas, Elsevier science publication Comp, Amsterdam, 1–14, 1983. Krishnakumar, K. N., Prasada Rao, G. S. L. H. V., and Gopakumar, C. S.: Rainfall trends in twentieth century over Kerala, India, Atmos Environ., 43, 1940–1944, 2009. Lane, R., Day, J., Marx, B. D., Reyes, E., Hyfield, E., and Day, J. N.: The effects of riverine discharge on temperature, salinity, suspended sediment and chlorophyll <i>a</i> in a Mississippi delta estuary measured using a flow-through system, Estuar. Coast Shelf Sci., 74, 145–154, 2007. Levin, L. A., Ekau, W., Gooday, A. J., Jorissen, F., Middelburg, J. J., Naqvi, S. W. A., Neira, C., Rabalais, N. N., and Zhang, J.: Effects of natural and human-induced hypoxia on coastal benthos, Biogeosciences, 6, 2063–2098, <a href="http://dx.doi.org/10.5194/bg-6-2063-2009">https://doi.org/10.5194/bg-6-2063-2009</a>, 2009. Lund-Hansen, L. C., Skyum, P., and Christiansen, C Modes of stratification in a semi-enclosed bay at the North Sea-Baltic Sea transition, Estuar. Coast Shelf Sci., 42, 45–54, 1996. Madhu, N. V., Balachandran, K. K., Martin, G. D., Jyothibabu, R., Thottathil, S. D., Nair, M, Joseph, T., and Kusum, K. K.: Short Term Variability of Water Quality and its Implications on Phytoplankton Production in a Tropical Estuary (Cochin Estuary – India), Environ. Monit. Asses., 170, 287–300, <a href="http://dx.doi.org/10.1007/s10661-009-1232-y">https://doi.org/10.1007/s10661-009-1232-y</a>, 2010. Mann, K. H. and Lazier, J. R. N.: Vertical structure of the open ocean: Biology of the mixed layer, Dynamics of marine ecosystems-Biological-physical interactions in the oceans, 2nd Blackwell, Science, 12, 56–97, 1996. Menon, N. N., Balchand, A. N., and Menon, N. R.: Hydrobiology of the Cochin estuary system a review, Hydrobiologia, 43, 149–183, 2000. Martin, G. D., Vijay, V. J., Laluraj, C. M., Madhu, N. V., Joseph, T., Nair, M., Gupta, G. V. M., and Balachandran, K. K.: Fresh water influence on nutrient stoichiometry in a tropical estuary, south west coast of India, Appl. Ecol. Environ. Res., 6, 57–64, 2008. Naqvi, S. W. A., Jayakumar, D. A., Narvekar, P. V., Naik, H., Sarma, V. S., D'Souza, W., Joseph, T., and George, M. D.: Increased marine production of N<sub>2</sub>O due to intensifying anoxia on the Indian continental shelf, Nature, 408, 346–349, 2000. Neil, M.: A method to determine which nutrient is limiting for plant growth in estuarine waters at any salinity, Marine Pollut. Bull., 50, 945–955, 2005. Nepf, H. M. and Geyer, W. R.: Intratidal variations in stratification and mixing in the Hudson estuary, J. Geophys. Res., 101, 12079–12086, 1996. Nunes Vaz, R. A., Lennon, G. W., and De Silva Samarasinghe, J. R.: The negative role of turbulence in estuarine mass transport, Estuar. Coast Shelf Sci., 28, 361–377, 1989. Qasim, S. Z.: Indian Estuaries. Allied Publication Pvt. Ltd., Heriedia Marg, Ballard estate, Mumbai, 259&nbsp;pp., 2003. Qasim, S. Z. and Gopinathan, C. K.: Tidal cycles and the environmental features of Cochin backwater (A tropical estuary), proceedings of the Indian academy of sciences, Bangalore, 1969. Qasim, S. Z. and Sankaranarayanan, V. N.: Organic Detritus of a Tropical Estuary, Mar. Biol., 15, 193–199, 1972. Ranasinghe, R. and Pattiaratchi. C.: Circulation and mixing characteristics of a seasonally open tidal inlet: a field study, Aust. J. Mar. Freshwater Res., 50, 281–290, 1999. Rippeth, T. P. and Simpson, J. H.: The frequency and duration of episodes of complete vertical mixing in the Clyde Sea, Cont. Shelf Res., 16, 933–947, 1996. Rotchord, D. J.: Nutrient environment of Australian coastal waters, Aust. J. Mar. Freshwater Res., 26, 223–243, 1975. Shetye, S. R., Gouveia, A., Shenoi, S. S. C., Sundar, D., Michael, G. S., Almeida, A. M., and Shatanam, K.: Hydrography and circulation off the west coast of India during the Southwest monsoon, J. Mar. Res., 48, 359–378, 1990. Simpson, J. H.: The shelf – sea fronts: implications of their existence and behaviour, Phil. T. Roy. Soc. Lon. A, 302, 531–546, 1981. Srinivas, K. and Dinesh Kumar, P. K.: Atmospheric forcing on the seasonal variability of sea level at Cochin, southwest coast of India, Cont. Shelf Res., 26, 1113–113, 2006. Srinivas, K., Revichandran, C., Thottam, T. J., Maheswaran, P. A., Asharaf, M., and Murukesh, N.: Currents in the Cochin estuarine system during March 2000 (Southwest coast of India), Indian J. Mar. Sci., 32, 123–132, 2003a. Srinivas, K., Revichandran, C., Maheswaran, P, A., Asharaf, M., and Murukesh N.: Propagation of tides in the Cochin estuarine system , Southwest coast of India, Indian J. Mar. Sci., 32, 14–24, 2003b. Strickland, J. D. H. and Parsons, T. R.: In: A Practical Handbook of Seawater Analysis. Bulletin of Fisheries Research Board, Canada, 2, 167–310, 1972. Udaya Varma, P., Pylee, A., and Rama Raju, V. S.: Tidal influence on the seasonal variation in current and salinity around Willington Island, Mahasagar, 4, 225–237, 1981. Uncles, R. J., Ong, J. E., and Gong, W. K.: Observations and Analysis of a stratification-destratification event in a tropical estuary, Estuar. Coast Shelf Sci., 31, 651–665, 1990.