Nitrogen cycling in a high-nutrient tidal river revealed through high-resolution sampling and nitrate isotope mixing models

Tidal freshwater zones (TFZs) serve as critical transition areas between riverine and estuarine systems, where external nutrient inputs and internal biological transformations drive nitrogen cycling. We investigated nitrate sources and cycling along the Hawkesbury River TFZ in Eastern Australia under varying hydrological conditions, using high-resolution stable isotope analysis (δ 15 N-NO 3 and δ 18 O-NO 3 ) combined with conservative mixing models. Over 650 unique isotope measurements were collected, providing unprecedented resolution of nitrogen dynamics in a high-nutrient, multi-source system. δ 15 N-NO 3 values were significantly enriched beyond those typical of agricultural and urban catchments, indicating widespread non-conservative nitrate behaviour. Mixing-model results revealed deviations from conservative nitrate mixing under contrasting hydrological conditions. During wet conditions, enhanced hydrological connectivity increased external inputs of dissolved inorganic nitrogen to the TFZ. High discharge and short residence times, limited nitrate accumulation and isotopic enrichment. In contrast, under dry conditions, lower discharge and extended residence times, combined with weaker groundwater inflows, constrained external nitrate inputs, enhancing the role of in-stream processing and strengthening isotope–concentration relationships, particularly in the upper TFZ. Enrichment patterns, deviations from conservative mixing, and Δ(15,18) diagnostics collectively indicate that internal nitrate cycling played a crucial role in regulating nitrate dynamics, with denitrification serving as an important nitrate removal pathway in the upper river reaches during dry-period low-flow conditions. During the wet period, negative Δ(15,18) values were consistent with the nitrification of groundwater-derived NH 4 , indicating a strong coupling between hydrology and nitrogen cycling. Overall, our results demonstrate that discharge and residence time exerted primary control over nitrate retention and processing in the TFZ, mediating the balance between external inputs and internal cycling. Our study highlights the importance of combining high-resolution sampling with isotope-based mixing models to resolve nitrogen transformation processes in dynamic environments such as TFZs, where internal cycling strongly influences nitrogen speciation, isotopic composition, and downstream export.