Hydrobiology of the Cochin backwater system - a review

On the south west coast of India, there is an extensive estuarine system of backwaters, of which Vembanad Lake is the largest. The backwaters of Keral a support as much biological productivity and diversity as tropical rain fo rests. They are responsible for the rich fisheries potential of Kerala. Coc hin backwaters situated at the tip of the northern Vembanad lake is a tropi cal positive estuarine system extending between 9 degrees 40' and 10 degree s 12'N and 76 degrees 10' and 76 degrees 30' E with its northern boundary a t Azheekode and southern boundary at Thannirmukham bund. The lake has a len gth of 80 km and the width varies between 500 and 4000 m. A channel, about 450 m wide at Cochin gut and another at Azheekode, make permanent connectio ns with the Arabian Sea. The depth of the estuary varies considerably. Whil e the shipping channels are maintained at a depth of 10-13 m, the major por tion of the estuary has a depth range of 2-7 m. Water from two major rivers viz., Periyar and Muvattupuzha drain into this estuary. During south west monsoon, the estuary is virtually converted into a freshwater basin even in areas around barmouth where salt water penetration occurs below 5 m depth only. The major hydrological variable in the Cochin backwaters is salinity, similar to the situations encountered in estuaries with a gradual declensi on of salinity from 30 at the entrance of the estuary to 0.2 at the point o f entry of the rivers. Salinity gradient in the Cochin backwaters supports diverse species of flora and fauna depending on their capacity to tolerate oligohaline, mesohaline or marine conditions. Low lying swamps and tidal cr eeks, dominated by sparse patches of mangroves with their nutrient rich phy sical environment, support larvae and juveniles of many economically import ant species. Backwaters also act as nursery grounds of commercially importa nt prawns and fishes. The fields around the backwater are suitable for aqua culture. These areas support traditional, seasonal and perennial prawn fish ery. The changes in the hydrology controlled by the seasons play an importa nt role in regulating the migrant fauna of the estuary. The Cochin backwate r supports a well established endemic fauna. The nutrients and pollutants i ntroduced into the estuary control to a great extent the distribution and a bundance of less tolerant species in ecologically sensitive areas in the ba ckwaters. Cochin backwaters, widely regarded as one of the polluted estuari es in India, receive contaminated freshwater inputs and discharges of efflu ents and partially treated sewage from many points throughout its tidally m ixed zone. Recently, changes brought about in the estuary like reclamation and consequent shrinkage of the backwaters and the discharge of pollutants have made an adverse impact on the potential of aquatic ecosystems that use d to support high levels of bioproductivity and biodiversity. The construct ion of Thannirmukham bund and Thottapally spillway to prevent salt water pe netration into the paddy fields during pre-monsoon has led to serious ecolo gical problems by interrupting the natural ebb and flow of tides. The hydro graphy, floral and faunal composition - its spatial and temporal variation plus assessments of the impact of the anthropogenic activities are presente d in this review. An attempt to critically evaluate the status of the estua ry from the biological and pollutional stand point is also done.