Natural and anthropogenic controls on nutrient fluxes in two small rivers draining contrasting climate gradients across Southern Western Ghats, India.

Abstract

The Western Ghats of southern India, a biodiversity hotspot and major water source, host several rivers influenced by diverse climatic, geological, and land-use settings. This study investigates the spatiotemporal dynamics of Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphate (DIP), and Dissolved Silica (DSi) in two small rivers-the Bhavani and the Thuthapuzha-which drain contrasting climatic zones of the southern Western Ghats. A total of 150 water samples were collected across seasons and zones to assess nutrient fluxes and their controlling factors. Results revealed substantial spatial and seasonal differences in nutrient concentrations. In the Bhavani river, mean DIN values were 112.12 µg/l (pre-monsoon), 1432.14 µg/l (monsoon), and 240.98 µg/l (post-monsoon). The corresponding values in the Thuthapuzha river were 278.56 µg/l, 306.90 µg/l, and 122.70 µg/l, respectively. Mean DIP concentrations ranged from 8.38 to 16.59 µg/l in the Bhavani and from 2.69 to 11.01 µg/l in the Thuthapuzha. DSi levels were higher in the Bhavani river, with mean values between 6.83 and 8.73 mg/l, compared to 4.51 to 7.67 mg/l in the Thuthapuzha river. Nutrient concentrations in the Bhavani increased downstream, influenced by intensive agriculture and domestic wastewater inputs, particularly in the semi-arid zones. In the Thuthapuzha, which drains a persistently humid catchment, nutrient enrichment was also observed in areas of high land-use intensity. Seasonal monsoonal runoff significantly impacted nutrient loads, with elevated DIN and DIP during the wet season. Higher DSi levels in cultivated zones were linked to silicate weathering and soil erosion. Long-term trends suggest an increase in nutrient inputs, particularly from fertilizer use. These findings highlight the role of climate, land use and anthropogenic pressure in regulating nutrient fluxes in tropical river systems and underscore the need for integrated watershed management in rapidly transforming catchments of the Western Ghats.