Nutrient and DOM distribution and transformation along a river course from headwaters to discharge: inorganic and organic nutrients

Abstract:

Transformation of nutrients and dissolved organic matter (DOM) from stream headwaters to the sea is hampered by the manpower necessary to cover the course of the river in a timeframe with similar hydroclimatic conditions. Knowledge of headwater, as well as watershed, land use practices influence on the quality of downstream waters is essential to water-resource management affecting ecosystem services for human and ecological health. The dynamic coupling of hydrological and biogeochemical processes in upland streams further controls the chemical form, timing, and longitudinal distances of nutrients/DOM transport to downstream waters. Our evaluation of nutrient data collected along a 300 mile stretch of the San Marcos-Guadalupe River freshwater (part of a 70-hour safari trip) demonstrates 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 even though salinities were still in the freshwater range (2-4). Reduced N (i.e., NH₄⁺) is present in high amounts at an outfall and concentrations are visibly decreasing downgradient until, like NO₂^-+NO₃^-, they peak again along the coastal zone. Interestingly, ratios of DOC/total dissolved nitrogen (TDN) and DOC/dissolved organic nitrogen (DON) are similar in trend to NH₄⁺, showing a gradual decrease from the outflow for ⁓20 miles downstream. Higher flow and runoff of terrestrial DOM following wet conditions, as in this study, could explain the higher C:N ratio closer to headwaters. Lower ratios throughout the middle, more stagnant stretch of the river may be explained by in-situ production of autochthonous dissolved organic matter (DOM). Higher NH₄⁺ could be explained by photodegradation of DOM, and, thus, transported downstream from point sources. Further constraints on processing and sources of DOM and N sources to the river will be evaluated via molecular analyses using the PPL-solid phase extraction and high-performance liquid chromatography coupled with Orbitrap Fusion Tribrid mass spectrometry.