Lagoons are generally divided into coastal lagoons and atoll lagoons. However, there is ambiguity over the distinction between an estuary and a lagoon. For this purpose the definitions given by Cameron and Pritchard (1963) are the most commonly accepted definitions. While an estuary is an inland river valley or coastal plain inundated by sea water during the Holocene sea-level rise, with tidal waves as a feature, coastal lagoons are different. Coastal lagoons have as their distinctive feature separation from the ocean by a barrier, and are connected to the ocean by one or more restricted inlets.
Although the formation of most coastal lagoons is attributed to Holocene or Pleistocene sea level rise, lagoons are not subject to tidal mixing and salinity can vary. Sediment transport processes are responsible for the existence of coastal lagoons. Sediment deposits of tides, currents, waves, rivers and wind which are assisted by the slowing of currents by various factors such as aquatic plants are responsible the formation of coastal lagoons. Continuous sediment deposits are required to maintain lagoons, against the erosion of lagoon barriers by waves and wind. Lagoons aren’t usually deep, and depth may not exceed more than a few meters.
The most prominent lagoons in India include the Chilika lake, the Kaliveli lake, the Kerala backwaters, Pulicat lake and Vembanad lake.
Located in the state of Odisha, Chilika is the largest coastal lagoon in India and the second-largest lagoon in the world. The largest coastal lagoon in the world is the New Caledonian Barrier reef in New Caledonia. Chilika lake in winter becomes the largest winter ground for migratory birds in the Indian subcontinent. The lake hosts a number of endangered species of plants and animals.
According to Prateep K. Nayak in his paper ‘The Chilika Lagoon Social-Ecological System: An Historical Analysis’, published in Ecology and Society (2014), the two major threats to the lagoon’s ecological system are intensive commercial shrimp aquaculture by fishermen and sea-mouth alteration, initiated due to the blockage of the natural sea-mouth, as reported in 1908. The new mouth that had been artificially dredged worked as intended to flush out sediments, but also led to an extra influx of sea water, with adverse ecological and livelihood impacts. These have led to a change in species composition in the lagoon coupled with altered food webs. Cumulatively, there has been observed a variability in the events associated with the lagoon, altering ecosystems and local lifestyles and livelihoods.
Located between Andhra Pradesh and Tamil Nadu, Pulikat Lake and is India’s second-largest brackish lagoon. The lagoon houses the Pulicat Lake Bird Sanctuary. But, despite its rich biodiversity, today it is a major commercial fishing site. Siltation is also closing the lagoon’s barriers leading to reduced sea-water exchange, which is reducing water levels.
According to Raj, Tilak and Kalaimani in their paper ‘Experiments in restoration of benthic biodiversity in Pulicat lake, south India’, published by ‘The marine Biological Association of India’ (2002), the key ecological crises faced by Pulicat lake include siltation, lake-mouth closure, dryness in northern regions of the lake, over-fishing, aquafarms, agricultural use and encroachment and industrial pollution. The lake mouth, where fresh tides bring in fresh marine organisms, is the place with maximum biodiversity, whereas the biodiversity declines as one travels inland. As such there are distinct species compositions at the southern region, closer to the sea-mouth, and the northern region, where many areas are protected by the stewardship of fishermen.
The Kerala Backwaters and Vembanad Lake
The Kerala backwaters are a network of five lakes connected by canals that are both man-made and natural and are supplied by rivers. The largest of these, the Vembanad lake, is protected by the Ramsar Convention for sustainable use of wetlands. Due to freshwater from the rivers meeting the saline water, a barrage has been built near Thanneermukkom to preserve the unique ecosystems of the lagoon and the rivers.
According to Krishnakumar, Ali, Pereira and Raghavan, in their paper ‘Unregulated aquaculture and invasive alien species: a case study of the African Catfish Clarias gariepinus in Vembanad lake’, published in the Journal of Threatened Taxa (2011), Vembanad lake has witnessed indiscriminate and illegal introduction of several exotic species of marine life for the purposes of commercial aquaculture.
The radically altered ecosystem of Vembanad Lake poses a severe challenge to policy makers that is made difficult by inadequate documentation of marine populations in Vembanad lake and the activities as regards the influx of invasive exotic species. The lack of data, the authors claim, is often used as a caveat by officials in not addressing the problem correctly. Their study undertakes the documentation of exotic African Catfish species in the lagoon, which can be seen as a step towards proper documentation of the marine ecosystem in the lagoon.
Located in Tamil Nadu, Kaliveli Lake is among the largest wetlands in peninsular India and functions as an important flyby point for migratory birds. As the Lake is not accessible by road for much of its shores, it is abundant in biodiversity of flora and fauna. However, recently human encroachment is reported in its ecosystem due to agriculture, poaching, deforestation and commercial prawn fishing.
Coastal lagoons provide a variety of habitats such as salt marshes and mangroves, which are rich in biodiversity due to abundant water near terrestrial habitats. Biodiversity in a lagoon is also enriched by offering native species a protected space from predation and by acting as an enclosed nursery with feeding habitats for many species. Most importantly, due to low flushing rates, lagoons provide an ecosystem for primary producers such as phytoplankton and aquatic plants to flourish, and thus offer higher rates of secondary production than most other aquatic ecosystems.