Redox-network reconfiguration inferred from the ln[O2]-Eh relationship under mixed-potential conditions in a shallow pond time series

Kyoko Morimoto, Mayumi Seto, Katsutoshi Ito, Mana Ito

Oxidation-reduction potential (Eh) is widely used as an in situ indicator of redox conditions in aquatic environments, and field electrodes typically record a mixed potential generated by multiple concurrent interfacial redox reactions. Beyond a simple Nernstian interpretation based on a single redox couple, here we ask what mechanistic information can be extracted from an observed relationship between Eh and a single chemical species under mixed-potential conditions, focusing on dissolved oxygen. Using a linearized mixed-potential formulation, we show that the sensitivity ∂Eh/∂(ln a_x) can be decomposed into (i) Nernstian contributions and (ii) kinetic contributions from multiple reactions. Consequently, an approximately constant log-linear ln[O2]-Eh sensitivity does not require dominance by a single couple (e.g., O2/H2O); it can also arise when the effective reaction set contributing to the mixed potential and their relative weights remain approximately invariant, suggesting that this relationship can serve as a compact indicator of redox-network stability.

To examine whether such slope stability and breakdown are observable in the field, we apply this interpretation to a 21-month, multi-site time series from a constructed shallow pond in Japan, where dissolved oxygen and electrode redox potential were co-measured at weekly fixed depths and along biweekly vertical profiles. Channel excavation produced a pond-wide electrical conductivity anomaly, and change-point detection was used to define pre- and post-disturbance regimes. During the pre-disturbance regime, the ln[O2]-Eh slope was relatively stable across sites. After disturbance, the inflow-proximal site exhibited a weakened slope and systematically elevated Eh relative to the pre-disturbance baseline; notably, baseline-referenced Eh deviations peaked after the EC anomaly had largely relaxed, and a follow-up survey in February 2025 indicated partial recovery. Co-located Eh and oxygen measurements can thus provide a simple, system-level indicator of disturbance-driven redox-network reconfiguration and recovery, while recognizing that comprehensive speciation remains necessary to identify the dominant redox couples.