Nitrogen Distribution Along a River Course from Spring-fed Headwaters to the Sea

The dynamic coupling of land use practices, hydrological, and biogeochemical processes in upland streams controls the chemical species, timing, and longitudinal distances of nutrient transport to downstream waters. A snapshot of nutrient loads to rivers from stream headwaters to the sea is hampered by the manpower necessary to cover the course of the river in a timeframe that captures similar hydroclimatic conditions. In this study, nutrient, and stable isotope data (18OH2O, DH2O, 13CDIC) were collected along a 300 mile stretch of the San Marcos-Guadalupe River during a 70-hour Texas Water Safari race. Preliminary investigation of the results indicates that oxic forms of nitrogen (i.e., NO2-+NO3-) and dissolved organic carbon (DOC) are prevalent throughout its course associated with noticeable increases at the headwater and nearing the river mouth. Concentrations decrease significantly in the bay sites as salinities become slightly brackish (2-4). Reduced N (i.e., NH4+) is present in high amounts at an outfall and concentrations visibly decrease downgradient until, like NO2-+NO3-, they peak again along the coastal zone. Interestingly, ratios of DOC/dissolved organic nitrogen (DON) are similar in trend to NH4+, showing a gradual decrease from the outflow for 20 miles downstream. Higher C:N ratio observed near the headwaters are likely explained by DOM input following the wet conditions preceding this study. Lower ratios throughout the middle, more stagnant stretch of the river may be explained While not strongly correlated, 18OH2O shows an inverse trend with NO3- and no relationship with NH4+. If depleted 18OH2O, as measured at the spring-fed headwaters, are an indicator of groundwater inputs, higher NO3- may be groundwater sourced. Surface runoff or more stagnant stretches of the river are likely to be characterized by more enriched 18OH2O (and DH2O) and removal of NO3- via denitrification or consumption by primary production. Further constraints on N sources will be evaluated in the context of land use, tributary confluences, hydrological alterations, and stable isotopes of 13CDIC.