Estuarine mixing drives organic nitrogen transformation and bioavailability dynamics

Summary

Estuaries act as critical transition zones for nitrogen transport, where the dynamics of inorganic nitrogen have 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 %. The results suggest that estuarine mixing drives two co-occurring ON transformation processes with distinct consequences for nitrogen fate. First, 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. 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. Second, biological activity promoted the degradation of residual humic-like components (especially microbial C3), producing labile LMW-DON and ammonium; a strong negative correlation between humic-like and protein-like fluorescence in control treatments evidenced this conversion. Overall, estuarine mixing generates a dual response: physicochemical processes channel humic-like ON into refractory particulate pools, while biological processing simultaneously shifts the residual dissolved pool toward a more labile composition. This mechanistic framework advances our understanding of non-conservative ON behavior across estuarine salinity gradients and has implications for assessing nitrogen bioavailability in receiving coastal waters.