Short-Term Groundwater\nLevel Fluctuations Drive Subsurface\nRedox Variability

As global change processes modify the extent and functions\nof terrestrial–aquatic\ninterfaces, the variability of critical and dynamic transitional zones\nbetween wetlands and uplands increases. However, it is still unclear\nhow fluctuating water levels at these dynamic boundaries alter groundwater\nbiogeochemical cycling. Here, we used high-temporal resolution data\nalong gradients from wetlands to uplands and during fluctuating water\nlevels at freshwater coastal areas to capture spatiotemporal patterns\nof groundwater redox potential (<i>E</i><sub>h</sub>). We\nobserved that topography influences groundwater <i>E</i><sub>h</sub> that is higher in uplands than in wetlands; however,\nthe high variability within TAI zones challenged the establishment\nof distinct redox zonation. Declining water levels generally decreased <i>E</i><sub>h</sub>, but most locations exhibited significant <i>E</i><sub>h</sub> variability, which is associated with rare\ninstances of short-term water level fluctuations, introducing oxygen.\nThe <i>E</i><sub>h</sub>-oxygen relationship showed distinct\nhysteresis patterns, reflecting redox poising capacity at higher <i>E</i><sub>h</sub>, maintaining more oxidizing states longer\nthan the dissolved oxygen presence. Surprisingly, we observed more\nfrequent oxidizing states in transitional areas and wetlands than\nin uplands. We infer that occasional oxygen entering specific wetland–upland\nboundaries acts as critical biogeochemical control points. High-resolution\ndata can capture such rare yet significant biogeochemical instances,\nsupporting redox-informed models and advancing the predictability\nof climate change feedback.