Isotopic compositions of organic and inorganic nitrogen reveal processing and source dynamics at septic influenced...

Historically, dissolved organic nitrogen (DON) has not been characterized in the nitrogen profiles of most estuaries despite its significant contribution to total nitrogen and projected increase in loading. The characterization of dissolved inorganic nitrogen (DIN) and DON processing from groundwater to surface water also remains unconstrained. This study attempts to fill in these knowledge gaps by quantifying the DON pool and potential sources in a semiarid, low inflow estuary (Baffin Bay, Texas) using stable isotope techniques. High NO₃^- and DON concentrations, and high δ¹⁵N-NH₄⁺ (+55.0 ± 56.7 ‰), δ¹⁵N-NO₃^- (+23.9 ± 8.6 ‰) and δ¹⁵N-DON (+22.3 ± 6.5 ‰) were observed in groundwaters of a septic-influenced estuarine area, indicating coupled septic contamination and nitrification/denitrification. In contrast, groundwater of an undeveloped area provided evidence of inundation by bay water through high NH₄⁺ concentrations and δ¹⁵N-NH₄⁺ (+8.4 ± 3.0 ‰) resembling estuary porewater. NH₄⁺ was the dominant nitrogen species in porewater of both areas and δ¹⁵N-NH₄⁺ indicated production via organic nitrogen mineralization and dissimilatory nitrate reduction to ammonium. Surface water had similar nitrogen profiles (DON constituted ∼98 % of dissolved nitrogen pool) and potential source contributions, despite distinct nitrogen processing and profiles found in each water table. This was attributed to low nitrogen removal rates and prolonged mixing associated with long residence time. This study emphasizes the importance of DON in a low-inflow estuary and the isotopic approach to comprehensively examine both inorganic and organic N processing and sources serving as a guide to investigate N cycling in high DON estuaries globally.