Biogeochemical controls on nutrient fluxes in small tropical mountainous river basins: Mechanisms for P-limitation and Si-rich conditions

Abstract. Small scale mountainous rivers with their quick flowing mechanisms provide a clear understanding on the nutrient flux dynamics in assessing their role in biogeochemical cycles and coastal nutrient budgets. The present study examines the spatio-temporal variability of dissolved inorganic nutrients in the Karamana River Basin (KRB) and Vamanapuram River Basin (VRB), flowing through Western Ghats, which emphasis the hydro geochemistry, segment-wise nutrient fluxes, and their biogeochemical cycle implications. The results reveal marked spatio-temporal variability in nutrient fluxes similar to the hydrochemistry, with higher fluxes generally recorded during the MON due to enhanced runoff and weathering. The segment-wise average fluxes of DIN, DIP, and DSi in the KRB were estimated at 6.84, 0.05, and 127.25 kg ha^-1 yr^-1, respectively. In comparison, the corresponding values for the VRB were 9.44, 0.07, and 81.66 kg ha⁻¹ yr⁻¹, respectively. A clear indication of low concentration conditions in the upstream followed by slight enrichment in the mid and downstream regions further highlight pristine environment and the role of land use and anthropogenic influence, respectively. The stagnant conditions after reaching the downstream regions with favorable tropical climate conditions promoting the consumption of nutrients through in-situ production. The DSi flux of VRB (127.25 kg ha^-1 yr^-1) is comparable to that of large global rivers such as the Amazon (108.6 kg ha^-1 yr^-1) and Mississippi (118.21 kg ha^-1 yr^-1), further supports the claim of intense chemical weathering derived silica-rich conditions. Overall, the study highlights the critical role of climate and topography in regulating nutrient fluxes and confirms that nutrient inputs from these small-scale mountainous rivers have a relatively limited influence on coastal eutrophication in the receiving zones.