Abstract
Rapid urbanization processes and indiscriminate disposal of urban wastewaters are major causes for anthropogenic lake-sediment deposition and eutrophication. However, information about the spatial and temporal variation of macrophyte and phytoplankton distribution as indicators for water contamination is limited. To gain insights into the dynamics, we analyzed lake-cover changes of Bellandur and Varthur Lake in the S-Indian megacity of Bengaluru for the post-rainy seasons of the years 2002–2019. Supervised maximum likelihood classifications were conducted on 62 freely available, true-color satellite images in order to distinguish between macrophytes, algae, and free water surface. The image-derived results were verified by supervised classification and manual mapping of two simultaneously recorded multispectral satellite images (Sentinel-2 and WorldView-2). Seasonal interrelations between macrophytes and algae distribution were similar for both lakes. The increase in macrophyte cover during post-rainy season negatively correlated with algal abundance. Macrophyte expansion progressively suppressed algae development at both lakes, reflective of increasing eutrophication caused by on-going wastewater input. Seasonal variation in precipitation, wind direction, and temperature seemed to trigger intra-annual shifts of macrophytes and algae while similar macrophyte spread intensities during the post-monsoon season indicated independence of nutrient loads in the lake water.
Highlights
Urban lakes and associated wetlands are threatened by landfills following sprawling settlements and by discharge of urban waste water and pollutants [1,2,3], which is critically diminishing ecosystem functions for large parts of the local population
The lake cover dynamics of the Bellandur–Varthur lake system depend on many factors, which reshape constantly as part of the rural–urban transformation environment
We did not study the local distribution of the increasing macrophyte cover within the waterbodies, the images of our analysis suggest that perennial plant accumulation happened mainly in the shallower western areas of the wetlands
Summary
Urban lakes and associated wetlands are threatened by landfills following sprawling settlements and by discharge of urban waste water and pollutants [1,2,3], which is critically diminishing ecosystem functions for large parts of the local population. This is a frequent occurrence in India, where many people rely on both direct and indirect ecosystem services conferred by regional aquatic systems [4,5]. The few, but expensive, governmental rejuvenation programs focused on the lakes’ recreational purposes rather than on their ecological functions, severely hampering the traditional public uses of these lakes [4,10,17] and excluding previous local users [15]
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