A five-year study by the Centre for Aquatic Resource Management and Conservation, Kerala University of Fisheries and Ocean Studies, has revealed a highly degraded, dysfunctional, and debilitated Vembanad wetland system that affects the lives and livelihood of around eight million people in Alappuzha, Pathanamthitta, Kottayam, and Ernakulam districts.
The study released on Saturday says the wetland system faces declining ecosystem services and degradation in functional abilities from anthropogenic and natural disturbances. “The probability of a natural revival of the system is limited and therefore science and technology-based interventions are recommended,” says a brief for policymakers.
The weland system, a Ramsar site, is home to vast wetland areas, including the unique Kuttanad polders (55,000 ha), most of which lie at or below the sea level. The system has seen its water retention capacity reduced by 85.3% from 2,617.5 million cubic metres in 1990 to 384.66 MCM in 2020. The sharp decline in water holding capacity of the lake is due directly to the significant reduction in (43.5%) area of the lake (from 365 sq km in 1900 to the present 206.30 sq km).
Rivers Achankovil, Pampa, Manimala, and Meenachil in the south; Muvattupuzha in the central; and the Periyar and Chalakudy rivers in the north drain into Vembanad Lake. The lake also features the Thanneermukkom barrage, Thottappally spillway, as well as the Andhakaranazhi (near Cherthala), Cochin and Munambam barmouths.
The wetland system has seen what the study called “alarming depth shrinkage” of the south and central Vembanad Lake. The average 8.5 m depth in the 1930s has been reduced to the present average of 1.8 m. in the south and to 2.87 m. in the central portion of the lake. Around 82% of the lake falls below the two-m depth and 10.5% area has only depth ranging between two and three metres.
The study says the current fish diversity in the lake is represented by 90 species, a 40% decrease in diversity as 150 species were reported in the 1980s.
The study has reported the presence of 30 different compounds of pesticides in the surface sediments in the south and central Vembanad Lake. These include organochlorine pollutants, organophosphate pollutants, urea derivatives (diuron), and other pesticides.
Though pollution levels are below the hazard levels of the World Health Organization, the presence of diuron, mainly in the central lake area, “is a cause for concern as it limits the growth of microalgal groups chlorophyceae and cyanophyceae, reduces the abundance of clams, and, therefore, has the potential to alter the trophic structure of the estuary”, the study says.
Plastic pollution of the lake is much higher than reported from most estuaries world over. The study estimates the presence of 3,005 tonnes dry weight of macroplastics in the surface one-metre sediment of the lake. The study found microplastic contamination of the body tissue of black clams from south and central Vembanad lake. The invasive mussel, locally called Kallumakai, appears to ingest more microplastics. Microplastic ingestion can reduce the growth rate of bivalves.