Electrode measurements of the oxidation reduction potential in the Gotland Deep using a moored profiling instrumentation

The Gotland Deep Environmental Sampling Station (GODESS) operating between 30 m and 185 m was used to assess the variation of in situ redox potential (Eh) in the stratified water column of the central Baltic Sea, for a period of 56 days (November 2010-January 2011). The aim of this study was to acquire in-depth understanding of redox variations with the overall objective to identify the processes controlling Eh. At an interval of 8 h basic hydro-physical parameters were registered, including the oxidation-reduction potential, dissolved oxygen, chlorophyll a fluorescence, turbidity, temperature and conductivity. In total 170 profiles of all parameters were obtained. The measured Eh (with respect to standard hydrogen electrode, SHE) ranged from −0.055 V to 0.167 V. After temperature and pH correction of the standard reduction potentials, a comparison of the measured Eh with that calculated for the particular redox couples was carried out by applying the Nernst-equation. Furthermore, the concentrations of the most important redox elements such as Fe, Mn, N, O, C and S used for comparison were estimated by using empirical functions or were based on measurements of discrete water samples, taken at the time of deployment and recovery of the mooring. The obtained results reveal that the reduction of Fe(III) and O2 may be the main processes controlling the Eh potential in the Gotland basin. Below the redoxcline the reduction of hematite and then ferric oxyhydroxide could be related to the redox potential and somewhat deeper Fe(III)/Fe(II) was found to be the dominant redox couple. Although mixed potential theory could have been applied, the Fe couples appear to dominate over many other possible redox equilibria. This may be explained by the relatively high exchange current of Fe and by high Fe concentrations (up to 1.5 μM). Finally, a close correspondence to the Eh potentials measured in the oxic/hypoxic part of the water column was found considering the O2/O2−· and the O2−·/H2O2 couples. This suggests that one-electron transfer reactions determined the Eh under oxic/hypoxic conditions.