Diurnal oscillation in dissolved oxygen at sediment-water interface fuels denitrification-driven N removal in Ganga River

Abstract

Human activities have substantially enhanced riverine nitrogen loads in many parts of the world. The processes such as denitrification remove a large fraction of N added to the system, and thus, are very helpful in mitigating the effects of increasing human N loads in aquatic ecosystems. No systematic studies, so far, are available on current magnitude and factors regulating efficiency of N removal by the Ganga River. Using three sub-sets of studies- main river course, two tributary confluence trajectories and two point source trajectories along 520 km middle stretch, we generated high spatio-temporal resolution data to show that spatially targeted river reaches with higher nitrate concentrations are acting as ‘hotspots’ of N removal in the Ganga River. The static core method was used to measure the rate of denitrification. Intact sediment core samples were incubated in the laboratory condition and efficiency adjusted. Pollution-impacted point sources showed higher rates of denitrification (0.40 mg N/m2/hr to 2.48 mg N/m2/hr) relative to main stream sites (0.32 mg N/m2/hr to 2.09 mg N/m2/hr). Our results indicate that increased level of carbon (C) and phosphorus (P), which often accompany nitrogen (N) inputs from human activities, have enhanced N removal by the river. Periodic oxic-hypoxic cycle at sediment-water interface (DOsw), which might link nitrification with denitrification, has stimulated N removal at study sites in an opportunistic manner. These results suggest that data on this natural process of N removal (denitrification), and factors thereof, especially those accounting for interactive effects at spatially targeted locations, should be taken into account to formulate policies for the management of nutrient and organic pollution.