Nitrogen accumulation and attenuation in the Ganges-Brahmaputra-Meghna river system: An evaluation with multiple stable isotopes and microbiota

We examined dissolved inorganic nitrogen (DIN) accumulation and attenuation in the lower stream and estuary of the Ganges-Brahmaputra-Meghna River system. In the lower stream of Ganges, Meghna and Brahmaputra rivers, nitrate (NO₃⁻) was the dominant component in the DIN pool apart from the site near an industrial center. Concentrations of NO₃⁻ displayed minor differences between surface and bottom water, accounting for >90 % of the riverine DIN pool. Sources of NO₃⁻ were likely to be municipal wastewater and fertilizer based on signals of ¹⁵N-NO₃⁻ and ¹⁸O-NO₃⁻. In the Meghna River, ammonium concentration in river water increased due to sewage discharge from local industrial centers. In the estuary, likely due to the high-abundance nitrifiers, nitrification rates overwhelmed removal rates and led to NO₃⁻ accumulation. Towards coastal ocean, DIN concentrations decreased due to seawater dilution and biological assimilation, indicating a tight linkage between the riverine input and ecological stability in the receiving water.