ABSTRACT Groundwater biogeochemistry in coastal areas is spatially and temporally dynamic because fluctuations in groundwater level may cause alternate redox between distinct hydrological conditions. Recent studies have proposed connections between biogeochemistry and large-scale hydrological processes, specifically focusing on the role of redox-active compounds in changing the oxidation state during flooding and draining events. While water saturation generally results in a shift of redox-active compounds from electron donors to acceptors, the specific mechanisms underlying the transition of groundwater between oxidizing and reducing conditions in response to water level fluctuations are uncertain. To determine the effects of groundwater levels on redox dynamics, we monitored groundwater redox potential across the terrestrial-aquatic interface in Lake Erie coastal areas throughout the high and low-water seasons. In contrast to previously observed responses to flooding in soils, our results revealed patterns of oxidizing redox potentials during high-water and reducing during low-water periods. Furthermore, short-term fluctuations in water table levels significantly impacted the redox potential of groundwater when dissolved oxygen increased, and redox dynamics displayed voltage hysteresis in most events. Based on these findings, we propose that for improved predictions of microbial functions and biogeochemical cycles, redox-informed models should incorporate the antagonistic changes in groundwater redox balance compared to soils and consider the time lags in redox fluctuations. Graphical Abstract Conceptual diagram of groundwater redox fluctuations in coastal ecosystems. Large redox fluctuations are derived by dissolved oxygen inputs and smaller more frequent redox fluctuations are led by redox sensitive species leaching from topsoil.

Groundwater redox dynamics across the terrestrial-aquatic interface of Lake Erie coastal ecosystems