Impacts of faulty grey infrastructures on the bathymetry and salinity of Vembanad Lake, a tropical estuary along the south west coast of India.

The fast degradation in bathymetry, floodwater retention capacity, and changes in salinity regimes of Vembanad-the largest wetland system in South India-and its impacts on the ecosystem services are assessed using high-resolution data sets on bathymetry and hydrography. Strengthening the outer banks of Kuttanad polders to promote paddy cultivation led to a sharp decline in its sediment (suspended) filtration potential, promoting heavy siltation and depth shrinkage of Vembanad Lake. With an area of 315 sq.km and an average depth of 8.5 m (south sector)/7.87 m (central sector), the lake had a retention capacity of 2617.5 MCM floodwaters in 1930 which was reduced to 384.7 MCM by 2020 (85.3% loss in retention capacity) due to area (162.5 sq.km) and depth (average 1.8 m in the south sector and 2.87 m in the central sector) shrinkages. This resulted in the prolonged inundation of low-lying areas and increased incidences of flash floods. Both sectors of the lake maintain limnetic and well-mixed conditions during the monsoon whereas during non-monsoon season, the central sector turns to polyhaline/mesohaline and oligohaline conditions with stratified, partially mixed, and well-mixed salinity stratifications unlike the south sector which perennially remain limnetic and well mixed. The observed shift in salinity regimes appears to promote hypertrophic conditions, biodiversity loss, and a sharp reduction in the black clam abundance of the south sector, which before the commissioning of Thannermukkom Barrage (TMB) was a major clam ground.