A lagoon is a shallow water body separated from a larger water body by Islands or reefs or shallow ridge. Chilika lake is the largest brackish water lagoon in India and is also the estuary of River Daya draining into the Bay of Bengal. A 32 km long channel forms the mouth of the river joining the sea. This estuarine region is a unique combination of freshwater, brackish water and marine ecosystems. The wetland ecosystem of Chilika is the first Indian wetland of international importance under the Ramsar Convention.
The Ramsar Convention on Wetlands is an international treaty on Wetlands as waterfowl habitat, of importance around the world. Chilika is home to a large variety of resident birds and is also the wintering ground for numerous migratory birds. Apart from the avian species, it is also the home to a wide range of floral and faunal species among which many are under the threatened and endangered category.
This brackish water lagoon is unique in different ways. The estuarine and the coastal nature along with the vastness has tended to different characteristics to this wetland ecosystem. Four ecological sectors depending on the depth and salinity of the water have been marked within this lagoon – the northern zone, central zone, the southern zone and the outer channel.
The tidal inflow of saline water and the drainage of the freshwater changes in accordance with the period of the year. During the dry months, the lagoon experiences a major inflow of the marine water while during the monsoon the flooded river joined by many other drains into the sea and thus with this continuous siltation process the river mouth shifts annually.
The floral life here is broadly classified as terrestrial island vegetation and aquatic vegetation. Phytoplankton and seaweeds with numerous subspecies form the major part of the algal vegetation in the water. A few mangroves and other coastal scrubs form the terrestrial island vegetation. There are many inhabited and uninhabited islands within this lagoon among which the known names are Krushnaprasad, Nalaban, Kalijai, Somolo, Honeymoon, Breakfast and Birds Island.