Spatiotemporal controls on nutrient source and transport in Puget Sound rivers

Abstract

Understanding the factors controlling nitrogen (N) and phosphorus (P) in rivers is critical to preserve ecosystem functioning from land to the coast yet challenging given the complexity of climate and landscape interactions. These interactions operate across multiple spatial and temporal scales, often manifesting differently across rivers depending on watershed-scale controls, underscoring the need to study river N and P dynamics across diverse systems. My dissertation research investigates river N and P sources and transport in a human-influenced, mountainous region in western Washington state, aiming to disentangle the roles of climate and landscape variation in driving nutrient entry and delivery in coastal rivers. Using two large regional datasets at different temporal scales (i.e., seasonal, multi-decadal), I demonstrate the strong influence of regional and macro-scale climate patterns in driving seasonal river nutrient transport regimes, while highlighting the distinct role of watershed topography modifying river nutrient responses to hydroclimatic change. Further, by using stable isotopes to trace N sources in rivers, I show the origins of N entering these human-influenced, coastal rivers are closely linked to upland soils in the forested landscape, the flushing of which is dependent on seasonal climate. To mitigate the effects of climate and landscape alterations on riverine ecosystems, there is a pressing need to understand how the interaction of ecological controls will play out across river systems. Findings from my dissertation research elucidate these interactions, demonstrating the strength of climate and the physical environment in driving river nutrient delivery to the coastal environment, and enhancing the predictability of river nutrient transport in mountainous regions facing increasing impacts from climate change and human activity.