High-resolution monitoring of seasonal hypoxia dynamics using a capacitive potentiometric sensor: Capacitance amplifies redox potential

Hypoxia is a long-standing environmental problem in coastal areas worldwide, but technical and economic difficulties impede accurate and continuous spatiotemporal monitoring. This study aims to monitor seasonal hypoxia dynamics at high-resolution by developing a novel capacitive potentiometric sensor. The underlying hypothesis of this study was that (1) the capacitive carbon electrode charges redox energy and creates an overvoltage; (2) the overvoltage reflects the redox energy as an amplified signal. A viability of the capacitive potentiometric sensor for seasonal hypoxia was investigated from summer to autumn in Fukuyama inner bay, Japan. The study area was a brackish water with strong stratification of upper fresh water and lower saline water. In the water surface, which is a redox-equilibrium environment, the capacitive potential increased to 0.7 V with overvoltage, which corresponds to amplifying the redox energy of dissolved oxygen by 35 times. In contrast, in the bottom layer, the capacitive potential responded in a Nernstian manner, confirming that diffusion of hydrogen sulfide was the direct cause of the hypoxic water mass in the bottom of the study area. The vertical discontinuity layer of the redox reactions was defined as 0.05 V of the capacitive potential. This threshold value intuitively illustrates the spatiotemporal dynamics of the seasonal hypoxia. A principal component analysis confirmed that dissolved oxygen concentration was a major determinant of the capacitive potential. Furthermore, this novel potentiometric sensor overcomes the limitations of conventional redox potential sensors, which fail to capture weakly poised redox couples. The capacitive potential exhibited that the stratification protected environments for photosynthesis (surface water temperature and aerobic condition), thus regularly supplies dissolved oxygen to seabed with tide and suppressed full-depth hypoxia. In conclusion, the capacitive potential provides spatiotemporal information on the chemical activity of dissolved oxygen, which is a novel approach to elucidate the mechanisms of hypoxia dynamics.