Longitudinal trends in concentration and composition of dissolved organic nitrogen (DON) in a largely unregulated river system

Dissolved organic nitrogen (DON) can comprise up to 80% of the dissolved N pool in riverine ecosystems, but concentration and compositional responses to catchment conditions has received limited attention. We examined the suite of nitrogenous nutrients along the length of the Ovens River, Victoria, Australia, a river with identifiable regions of native vegetation, agricultural activity and floodplain forest connection, carrying out longitudinal surveys in winter during a period of high flow and in summer during a period of stable base flow. We examined: the concentrations of DON, the proportion of DON that occurs as dissolved combined amino acids (DCAAs), whether concentration and DCAA composition varied between flow and whether land-use and tributaries have an impact upon nutrient concentration and DON composition. DON concentrations were greater than dissolved inorganic nitrogen under both base flow and high flow conditions. Under base flow DON exhibited a continuous increase in concentration downstream (ranging from 50 to 300 μg/L), compared to a much larger increase under high flow (150–600 μg/L) coupled with a major discrete increase of ~ 350 μg/L at a tributary input (King River). Concentrations of NOₓ (oxides of nitrogen) species were much higher under high flow conditions (range 50–250 μg/L) compared to 0–50 μg/L at base flow, and showed a significant increase in concentration with distance downstream. A discrete change in NOₓ concentrations was also observed at the King River confluence under high flow, although in this case causing a decrease in concentration of ~ 100 μg/L. DCAA concentrations varied little along the length of the river at base flow but increased with distance downstream at high flow. The DCAA concentrations were of the same order of magnitude as ammonium at both base and high flows and nitrate concentrations at base flow. The proportion of DON that was in the form of DCAA was reasonably uniform during high flow (3–6%), but highly variable at base flow (5–44%). The amino acid (AA) composition of the DCAA varied along the river and differed between flow regimes (except below the confluence with the King River where AA composition under the two flow conditions converged) suggesting a strong influence of land use. We show that DON is potentially a large component (4–81%) of the total N budget and given that 5–23% is in the form of peptide/protein, represents an important source of N. DON and more specifically DCAAs should therefore