Nernstian stability of the Eh–O2 relationship reveals redox structural shifts

Kyoko Morimoto, Mayumi Seto, Katsutoshi Ito, Mana Ito

Oxidation-reduction potential (Eh) offers a compact descriptor of aquatic redox status, yet its interpretation is obscured by the many co-occurring electron-transfer reactions that determine it. We tested the hypothesis that the persistence of a linear, Nernstian relationship between Eh and ln[O2] reflects the robustness of the underlying redox structure. Using 18 months of depth profiles and high-frequency measurements in a disturbance-prone pond, we found that Eh increased linearly with ln[O₂] across depths and seasons (slope = 0.079 V; R2 = 0.38), indicating a persistent Nernstian slope. A sediment-inflow event temporarily collapsed this slope at one site, revealing a reorganization of the local redox network. High-frequency observations further showed that rainfall-driven oxygen spikes scarcely affected sediment Eh, demonstrating that short-term hydrological disturbances do not alter the prevailing redox structure. Additionally, causal analysis (EchoNet) showed that Eh was driven primarily by temperature rather than dissolved oxygen, indicating that oxygen’s influence is dispersed across many intertwined reaction pathways. Together, these results show that, although the Eh–O2 relationship cannot resolve detailed reaction pathways, co-located and continuous Eh–O2 monitoring provides a robust indicator of redox structural stability and a practical tool for detecting structural redox shifts in shallow aquatic systems.