BHUBANESWAR: Chilika Lake, the largest brackish water lake in Asia and the second largest coastal lagoon in the world, is both shrinking and expanding at an alarmingly high rate, posing a serious threat to its rich and unique biodiversity.
The lagoon has experienced a remarkably high erosion rate of 19.87 metre per year and accretion of 16.91 metre per year from 1990 to 2020, a global study has found.
The study on ecological ramifications of tropical storms along the Eastern Coast and shifting of tidal inlets of the lake revealed Chilika had an accretion pattern from 1990 to 2000 and erosion between 2000 and 2020.
After 2000, the erosion was greater than the accretion. The shoreline observed net erosion in around five-year intervals from 2000 to 2020, and net deposition from 1990 to 1995 and 1995 to 2000. The erosion rate which intensified from 2005 to 2011, was predominantly caused by two cyclones Bijli and Aila in 2009.
Researchers from four universities in India and Brazil studied the ecological impact on the lake. They observed the tidal inlets of Chilika lake and the periods of opening, closing and shifting.
Associate Professor of Berhampur University Manoranjan Mishra said the configuration of tidal inlets plays a critical role in the composition and richness of existing species in the lake. "The landfall of cyclonic storms modifies the morphodynamics of tidal inlet by opening/closing/shifting its mouth, disrupting the delicate balance of the lagoon’s ecosystem. It also leads to disturbing the salinity gradient and can be detrimental to the health and survival of all the species that Chilika lake supports," Mishra, an author of the study said.
The consequences of closing or narrowing down of the tidal inlets can be damaging as it would restrict entry of marine water and flushing out of freshwater besides sediment deterrence. The reduction in the diversity and constricted tidal inlet would likely influence the arrival of migratory birds and fish population and overall health of the lagoon affecting the livelihood of the dependent population.
Changing erosion and accretion pattern of Chilika a threat to its biodiversity
Lack of sediment supply by Mahanadi river, which is the source of fresh water, may have resulted in recent erosion along the Chilika shoreline. The river has been experiencing a progressive decline in sediment load due to construction of dams in its catchment areas.
While the rising sea level is globally accelerating shoreline erosion, further reduction in sediment supply exacerbates the process, the study found. "The ecology and environment of the lagoon should also be investigated under multiple tidal inlet conditions to maintain the health of the lagoon ecosystem as the number of tidal inlets and their cross-sectional area may influence the salinity and sediment outflow from the lagoon," Mishra suggested.
Chilika lake is one of the most dynamic shallow ecosystems of about 65 km long, spreading parallel to the coastline. It is fed by 52 rivers and rivulets and its area varies between 900 and 1,165 sq km during summer and monsoon, respectively.
The lagoon was enlisted as threatened ecosystem in 1993 by the Ramsar Convention due to its changing ecological characteristics. However, it was removed from the list in 2002 after the successful restoration of the lagoon by the Chilika Development Authority (CDA).