Estuarine Mixing Drives Organic Nitrogen Transformation and Bioavailability Dynamics

Abstract. Estuaries act as critical transition zones for nitrogen transport, where the dynamics of inorganic nitrogen has been extensively studied. In contrast, organic nitrogen (ON), encompassing particulate organic nitrogen (PON) and dissolved organic nitrogen (DON), is strongly influenced by estuarine mixing of freshwater and seawater. However, the mechanisms driving ON transformation and their implications for bioavailability remain poorly understood. Here, estuarine mixing experiments are conducted across salinity gradients to explore ON transformation and changes in nitrogen bioavailability driven by physicochemical and biological processes. Using tangential flow filtration, optical signatures, and stable isotopes (δ¹³C, δ¹⁵N), we quantified ON composition and molecular characteristics. DON dominated the ON pool (> 71 %) throughout the mixing process, with the low-molecular-weight (LMW) fraction accounting for 49 ± 7.8 %. Salt-induced flocculation and adsorption (i.e. physicochemical processes) preferentially transferred a large fraction (63 ± 11 %) of humic-like components, mainly terrestrial refractory compounds, into the particulate phase, thereby increasing PON. Meanwhile, biological activity promoted the degradation of residual humic-like components (especially microbial C3), producing labile LMW-DON and ammonium. This conversion was evidenced by a strong negative correlation between humic-like and protein-like components in control treatments. The isotopic enrichment (δ¹³C, δ¹⁵N) and elevated C/N ratios in PON further suggested the re-adsorption of biologically-modified and δ¹⁵N-enriched DON onto particles, enhancing PON refractoriness. Overall, estuarine mixing drives conversion of humic-like ON into refractory particulate forms, while simultaneously enhancing the bioavailability of the residual dissolved nitrogen pool, thereby influencing nitrogen cycling and eutrophication risks at the river-sea interface.