Hydrologic influence on stream water nitrogen to phosphorus ratios

Watersheds and their complex network of flow paths regulate the movement of water and nutrients to aquatic ecosystems. Overland and subsurface (soil and ground water) flow paths transport nitrogen (N) and phosphorus (P) with differing levels of efficiency; P transport occurs via surface flow paths almost exclusively and N transport can utilize both overland and subsurface flow paths. Stream water N:P ratios influence ecosystem community composition, nutrient limitation of primary production, and trophic dynamics. Regional patterns of stream water N:P ratios have been identified, however, the controls driving the pattern remains poorly understood. This dissertation hypothesizes that watershed flow paths control the regional variation of stream water N:P ratios. Three independent lines of evidence were pursued to address the hypothesis: (i) a field study that quantified watershed flow paths and stream water N:P ratios in two watersheds with distinctly different hydrology; (ii) a watershed modeling exercise that simulated many different precipitation regimes and recorded the resulting flow paths and N:P ratios; (iii) a data synthesis of 58 minimally-impacted watersheds across the United States that focused on the relationship between stream water N:P ratios and stream discharge.; The results from the three studies all indicate that watershed flow paths influence stream water N:P ratios. Furthermore, the results from all lines of evidence identify a possible scaling relationship between N:P ratios and stream discharge across large temporal and spatial ranges. The results suggest that watershed connectivity with the stream, defined as the sum of the active watershed flow paths, is a fundamental condition that helps explain the variability of N:P ratios, and has important implications for the watershed transport of other elements. Such information allows more informed management of aquatic ecosystems in a changing environment.