Flooding events and groundwater redox dynamics of coastal ecosystems

Terrestrial-aquatic interfaces develop across diverse coastal environments and are highly dynamic systems in which biogeochemical processes often vary more dramatically than in other environments. The dynamics of these interfaces are important for climate models to capture but are currently overlooked by global-scale models. There remains considerable uncertainty in our understanding of how coastal terrestrial-aquatic interfaces will influence climate change feedbacks across various spatial and temporal scales, much of which arises from redox potential dynamics influencing rates of denitrification, reduction of iron and sulfur, and methanogenesis. In particular, we lack essential data to understand how redox conditions, a key control on biogeochemical cycling and greenhouse gas fluxes, are affected by the hydrological and hydrodynamic processes controlling groundwater. Our objective was to characterize the sub-daily to monthly variability in redox potential and groundwater quality in coastal ecosystems of Lake Erie and Chesapeake Bay in response to water level fluctuations, including tidal cycles, lake-level cycles, and disturbance events. We observed temporal variability in redox potential in tandem with water level changes and infer that water level is a key control on groundwater biogeochemistry. Redox potential fluctuations displayed a distinct gradient from wetlands to uplands varying with coastal ecosystem types and hydrological drivers such as flooding intensity, frequency, water level and retention time. Fluctuations in redox potential may thus serve as a control point (sensu Bernhardt et al. 2017) linking microbial carbon and nutrient cycling processes with large-scale hydrological processes that transport materials across the terrestrial-aquatic interface.