The influence of spatial and temporal hydrologic variability on nutrient fluxes and transformations, San Pedro River, Arizona, USA

Abstract

Pre-monsoon (June) and monsoon (August) sampling campaigns were conducted on a gaining and a losing reach in the San Pedro River in 2005 to quantify the effects of variability in hydrologic conditions on carbon and nitrogen cycling in an N-limited semi arid stream. Nutrient concentrations decrease along the gaining reach, while new groundwater inputs, rapid N03-N uptake and net production of organic DOC and DON occurs. In contrast, the losing reach experiences little spatial variability suggesting the presence of a large, well-mixed alluvial aquifer as a single source. However, so/ concentrations and S04:Cl ratios decrease between gaining and losing systems with isotope mixing models indicating uptake of S04 2- and suggesting biological sulfate reduction between reaches. Diurnal cycles of increases in nighttime stage, er and nutrients indicate change in riparian vegetation transpiration demand and bank drainage of relatively N rich water at night. Monsoon stream flow becomes a roughly equal mixture of groundwater and monsoon precipitation with nutrient concentrations exhibiting little spatial variability. However, a decrease in DON concentrations and DON between the two reaches indicates nutrient processing occurs at much larger spatial scales. These data illustrate several examples of how hydro logic variability at different spatial and temporal scales controls nutrient fluxes and transformations.