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