Integrating drivers of nutrient biogeochemistry in riverine floodplains to inform restoration design

Abstract:

Excess nitrogen and phosphorus in receiving waters is associated with high inputs from agricultural watersheds with the majority of export occurring during late winter and spring floods. Restoring connectivity between a river and its floodplains can create optimal conditions for physical and biological retention. However, tradeoffs exist when anoxic flooded conditions promote nitrogen removal via denitrification but also release phosphorus from sorption sites that are maintained under oxic conditions. Interactions between hydrologic, geomorphic, and biogeochemical drivers affect nutrient dynamics and factors change across spatial and temporal scales. Our work aims to disentangle these factors through an integrated modeling and monitoring approach. We are using the Wabash-Tippecanoe River confluence as a model ecosystem that is representative of floodplains in the Midwestern USA: two restored prairie sites, a restored wetland, and an agricultural field. We are using a combination of in situ monitoring coupled with laboratory mesocosm experiments to characterize biogeochemical processes. We are using a novel groundwater-surface water model (ICPR, Interconnected Channel and Pond Routing) to inform event-scale drivers of biogeochemical processes and the widely applied HEC-RAS 2D model to characterize floodplain connectivity over annual timescales. Results from seasonal flooded core mesocosms suggest that inundation periods between 4-8 days created redox conditions that enhanced denitrification but did not release significant phosphorus, that was primarily held in extractable calcium, amorphous aluminum, and organic complexes. Machine learning techniques show that denitrification is strongly influenced by vegetation type, which is an integrator of hydrologic connectivity and geomorphic position (via plant type) and carbon supply. Lastly, modeling results from ICPR show how antecedent conditions can change water sources from rapid localized response of groundwater to inundation from river water, which has implications for both redox and nutrient loading. By integrating these approaches, we are better able to characterize drivers of water quality in these complex systems but also provide decision makers with guidance to match the right management tool with the scale of the question.