Spatiotemporal monitoring of water quality degradation and trophic state in a severely polluted tropical urban lake in Southern India

Bellandur Lake in Bangalore, India represents one of the most severely polluted urban water bodies in South Asia, necessitating systematic monitoring for environmental management. This study employed Sentinel-2 Multispectral Instrument imagery to assess spatiotemporal variations in water quality parameters from January 2023 to December 2025. Six water quality indicators were derived using established remote sensing algorithms: chlorophyll-a (estimated using the Normalised Difference Chlorophyll Index as the primary method), turbidity, total suspended solids, coloured dissolved organic matter, Secchi depth, and floating algae index. A total of 100 Sentinel-2 images were processed using Google Earth Engine, yielding 93 valid observations across 28 months after excluding monsoon periods affected by cloud cover and confining analysis to complete years 2023-2025. Results indicated severely degraded conditions with mean NDCI-based chlorophyll-a of 71.31 µg/L (median 70.24 µg/L), consistently classifying the lake as hypereutrophic. Mann-Kendall trend analysis revealed statistically significant increasing trends in chlorophyll-a (τ = 0.144, p = 0.041) and floating algae index (τ = 0.419, p < 0.001), with concurrent significant decreasing trends in turbidity (p < 0.001) and total suspended solids (p = 0.001). Pixel-wise FDR-corrected trend analysis identified 33.7% of the lake area exhibiting significant increasing chlorophyll-a trends. The lake consistently exceeded Central Pollution Control Board standards, with 100% of observations surpassing permissible total suspended solids limits and 98.9% of observations classified as hypereutrophic. These findings are interpreted as indicative of relative spatial and temporal patterns given the absence of site-specific in situ calibration; however, they provide valuable baseline information for management interventions and demonstrate the utility of satellite remote sensing for monitoring polluted urban water bodies.