Sediment Nutrient Dynamics in a River Network of a Modified Landscape

Abstract

<p>Connectivity between the watershed and the river network drives much of the ecosystem structure and functioning in riverine ecosystems. Models of this connection are based on natural ecosystems and may not be representative of modified river systems of the Anthropocene. This thesis was one of the first studies to research how ecosystem functions differ in an Anthropocene river system compared to a natural river system. In particular, this thesis examined nutrient dynamics across the watershed-river network linkage in a modified watershed. River networks draining modified watersheds transport elevated loads of nitrogen (N) and phosphorus (P) to receiving basins where they negatively impact both aquatic biota and humans. However, excess N and P from the water column can be removed by biotic uptake from microbial communities in the river sediment, excess N can be removed through microbially-mediated denitrification, and excess P can be removed through sedimentadsorption and burial. All of these processes improve water quality and reduces downstream transport. Thus, in modified systems, it is important to understand how in-stream nutrient processes operate and how land use disturbances affect in-stream nutrient dynamics. To accomplish these objectives, new approaches to the research and management of nutrients in modified watersheds were developed and applied in this thesis. </p> <p>A new conceptual framework for the interdisciplinary research and management of nutrients in agricultural watersheds was first developed. This framework detailed how to effectively research and manage nutrients across the watershed-river network connection to reduce the effects of the anthropogenic disturbance of agricultural land use on river systems. The framework incorporated an ecosystems approach, resilience thinking, and strategic adaptive management to connect research and management of agricultural land with the research and management of the river network.</p> <p>Next, a large spatial-scale study on sediment nutrient dynamics was conducted in a modified river network. The objectives of this study were to identify hot spots of sediment N removal and P retention within the river network and assess how current land use affects these rates. No direct link was found between current watershed land use and sediment nutrient dynamics as suggested by current conceptual river models. Current land use likely influenced sediment nutrient dynamics in indirect and complex ways that were difficult to ascertain within the study design for this research. Legacy nutrients also potentially masked the impact of current land use on sediment nutrient dynamics. Land use did appear to influence the delivery and availability of nitrate at the site, which affected denitrification rates. Overall, sediment N and P dynamics were mainly associated with the small-scale drivers. Sediment N processes were associated with sediment and water column variables, and sediment P dynamics were associated with sediment variables. Fine sediments were ubiquitous throughout the river network, likely deposited during past anthropogenic land use modifications. Due to the presence of fine sediment, P hot spots were located throughout the river network and denitrification hot spots were located throughout the river network where nitrate was present. The high concentrations of P in the river network also caused N limitation in some of the most modified areas of the river network. This complex interaction is typically not seen in natural river systems. This thesis suggests that legacy effects from past anthropogenic disturbances are the main driver of sediment nutrient dynamics in the modified system.</p> <p>The data from the large spatial-scale study also were used to develop significant causal models linking current land use to sediment nutrient dynamics. This is a novel approach to researching sediment nutrient dynamics, because causal models are typically not developed for riverine ecosystem processes. These models indicated that sediments throughout the modified watershed are releasing P to the water column. To cause a regime shift where the sediment converts from a P source to a P sink, current agricultural land use will need to decrease, or current land management actions will need to increase within the watershed.</p>