Changes in biogeochemical supply and demand during storm events alter the role of river networks in controlling downstream exports

Abstract:

River systems can be important regulators of downstream carbon and nutrient exports. The capacity for river networks to regulate fluxes is defined by the balance between biogeochemical supply (inputs), and the aggregate demand for various carbon and nutrient forms throughout river networks. Extreme events such as high runoff events can have complex effects on the balance of supply and demand. We present a conceptual framework for understanding the regulating role of river networks in terms of supply and demand that can be used to understand responses to extreme events and other environmental changes. The framework emphasizes integration of river network models with field observations, including next generation of in situ high frequency sensors that can be used to reduce uncertainty in observations for model testing. Storms increase transport of materials, even when concentrations dilute, increasing supply, while biological demand is likely more constant and may even decline due to scouring. However, at certain flow thresholds, demand can also increase due to increased connection to additional habitats (e.g. floodplains). We highlight research priorities for improved understanding of extreme events. The framework allows general assessment of multiple factors that influence supply and demand in river networks including storm flows, terrestrial nutrient loads, biological activity, nutrient saturation, spatial location of loads, exchange with different habitat patches, and geomorphology.