Transport and fate of organic nitrogen in a montane river ecosystem

Nitrogen is an essential nutrient for biotic communities. Studies of the nitrogen cycle in inland waters have focused primarily on inorganic nitrogen. The ecological role of dissolved organic nitrogen (DON) has scarcely been considered.; Patterns in the chemical fractionation of organic nitrogen were investigated in watersheds that receive low amounts of atmospheric N deposition (ca. 3 kg/ha/y). Approximately 65% of the annual nitrogen export in stream water was organic. DON accounted for >90% of nitrogen export during growing seasons, at which time inorganic N declined below limits of detection (5 μg/L). Chromatographic separation indicated that DON consists of fractions that undergo large intraannual changes in proportions and chemical composition. Treatment of DON as a single pool may be misleading from the viewpoint of understanding ecosystem processes related to its source and biological reactivity.; A significant portion of DON (15–71%) exported from watersheds with low supplies of N was bioavailable. Bioavailable DON was 2–16 times more abundant than inorganic N during growing seasons. The uptake of DON by microbes in sediments followed different seasonal patterns than the uptake of dissolved organic carbon (DOC). Uptake of DON was strongly related to its chemical composition, and declined when inorganic N was added to incubations and when inorganic N increased seasonally in stream water. Results suggest that when a nitrogen-deficient ecosystem is enriched with inorganic nitrogen, biotic use of the natural supply of DON declines. Organic N is probably an important component of the N cycle in streams experiencing low rates of N enrichment from anthropogenic sources.; The effect of N enrichment on export of inorganic and organic N was documented. Export of inorganic N was high in residential segments of a montane river draining areas with septic systems. A technique was developed to detect and quantify inorganic nitrogen enrichment from dispersed sources by analyzing N isotope ratios in algae. Annual estimates indicated that between 19 and 23% of the nitrate export from residential segments was derived from septic systems. N from septic systems reached a peak of 60% during the tourist season; estimates were validated independently with mass balance. Analysis of N isotopes and concentrations of N fractions were applied along a transect from headwaters of the montane river to its entry into Lake Dillon, a major supply of drinking water. The study showed that substantial amounts of inorganic N derived from human waste are present in groundwater, streams, and the lake. The biotic capacity of headwater ecosystems to assimilate inorganic N from wastewater may be minimal in watersheds experiencing rapid population growth.