- Our understanding of nitrogen (N) and phosphorus (P) variability in forested stream systems with minimal disturbance is confounded by the complex interactions of hydrologic and biogeochemical controls. In this thesis, our studies evaluate the variability of stream water N and P in a forested, headwater system in the Oregon Coast Range. This type of system is underrepresented in the literature, though it serves as a reference against which more disturbed systems are compared.
In the first study we evaluate instream N and P responses to precipitation across four catchments, Gus, Pothole, Rock, and Upper Main, using ~300 storm samples collected from a 1-year period. We characterized seasonal patterns and modeled relationships with cumulative precipitation and discharge for N and P species, nitrate-nitrogen (NO3--N), ammonium-nitrogen (NH4+-N), total dissolved nitrogen (TDN), soluble reactive phosphorus (SRP), and total dissolved phosphorus (TDP). There was significant spatial and temporal variability for all nutrient species concentrations with the exception of NH4+-N which showed little variability storm-to-storm. Maximum NO3--N concentration occurred at the onset of the wet season while SRP had the lowest concentration during the same period. Exports for all nutrient species across all catchments generally increased with cumulative precipitation. However, evidence was weak for relationships between nutrient concentrations and discharge.
In the second study we evaluated instream N and P spatial variability in early summer with landscape characteristics using “snapshot” samples across 11 sub-catchments throughout the four catchments for eight years. Samples were taken during the end of the rainy season in June, a period in which the watershed is transitioning from wet to dry and during which nutrient relationships with landscape characteristics have not been specifically evaluated in the literature. We generated a series of simple linear regression models to assess the influence of landscape characteristics (geology, vegetation, sub-catchment slope, geology, elevation, and opportunities for biogeochemical activity) on each of the nutrient species evaluated in the first study, as well as dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP). To more fully illustrate the range of variability in stream water N and P, we modeled the mean, maximum, and coefficient of variation for each nutrient species. Results for means and maximums were comparable to each other. P concentrations varied with geology and elevation across catchments, with catchments composed of more intrusive geology showing less SRP and TDP. SRP and TDP also showed negative relationships with elevation. Mean distance from the hillslope to the stream and specific conductance, which was assumed to increase with increasing water residence time, served as metrics representing opportunities for biogeochemical activity. P species and, to a lesser degree, NH4+-N, varied with these metrics (positive relationships). NO3--N, DON, and TDN varied with Red alder (Alnus rubra) coverage on the hillslope (positive relationships) and was the only explanatory variable of significance for these nutrient species. Evidence was lacking for the importance of metrics summarizing vegetation in the riparian zone for any nutrient species. Similarly, evidence was weak to support the importance of sub-catchment slope for any nutrient species. Overall, there was little evidence to support the importance of the landscape characteristics on coefficient of variability. Our results suggest that characteristics influencing transportability of nutrients, such as sorptive properties, may play an important role in how landscape characteristics influence stream water nutrients during this season. This may be linked to the ease with which nutrient species are able to be flushed, which may affect the timing and degree in which drivers are related to stream water nutrients.