Nitrate sources and dynamics in a salinized river and estuary – a δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻ isotope approach

Abstract. To trace NO₃⁻ sources and assess NO₃⁻ dynamics in salinized rivers and estuaries, three rivers (Haihe River: HH River, Chaobaixin River: CB River and Jiyun River: JY River) and two estuaries (HH Estuary and CJ Estuary) along the Bohai Bay (China) have been selected to determine dissolved inorganic nitrogen (DIN: NH₄⁺, NO₂⁻ and NO₃⁻. Upstream of the HH River, NO₃⁻ was removed 30.9 ± 22.1% by denitrification, resulting from effects of the floodgate: limiting water exchange with downstream and prolonging water residence time to remove NO₃⁻. Downstream of the HH River NO₃⁻ was removed 2.5 ± 13.3% by NO₃⁻ turnover processes. Conversely, NO₃⁻ was increased 36.6 ± 25.2% by external N source addition in the CB River and 34.6 ± 35.1% by in-stream nitrification in the JY River. The HH and CY Estuaries behaved mostly conservatively excluding the sewage input in the CJ Estuary. Hydrodynamics in estuaries has been changed by the ongoing reclamation projects, aggravating the loss of the attenuation function of NO₃⁻ in the estuary.