The role of stream network nutrient uptake kinetics and groundwater exchange in modifying the timing, magnitude, and form of watershed export

Abstract

In this PhD dissertation research we sought to elucidate stream network biological and physical influences on hydrological and biogeochemical signatures observed along stream networks and at watershed outlets. Our research indicates that stream nutrient uptake and groundwater exchange processes can modify inputs from terrestrial sources and influence the timing and signature of watershed fluxes. We determined that stream nutrient uptake followed Michaelis-Menten kinetics across a broad range of systems and that land use / land cover change can alter stream nutrient uptake magnitudes. Additionally, we found that watershed structure and network geometry exerted strong controls over sourcewater contributions and streamwater compositions along stream networks and at watershed outlets. Combined, this PhD research suggests that uptake kinetics and hydrologic turnover exert strong controls over streamwater composition and sourcewater contributions, and that physical and biological contributions to total nutrient retention and the dynamic and concentration dependent nature of biological uptake combine to control solute and nutrient signatures. We suggest accurate assessment of total retention across stream reaches and stream networks requires quantification of physical retention and the concentration dependent nature of biological uptake, understanding necessary to help mitigate the potentially deleterious influences elevated nutrient export can have on downstream ecosystems.