High spatial resolution analysis of the distribution of sulfate reduction and sulfide oxidation in hypoxic sediment in a eutrophic estuary.

Bottom hypoxia and consequential hydrogen sulfide (HS) release from sediment in eutrophic estuaries is a major global environmental issue. We investigated dissolved oxygen, pH and HS concentration profiles with microsensors and by sectioning sediment cores followed by colorimetric analysis. The results of these analyses were then compared with the physicochemical properties of the bottom water and sediment samples to determine their relationships with HS production in sediment. High organic matter and fine particle composition of the sediment reduced the oxidation-reduction potential, stimulating HS production. Use of a microsensor enabled measurement of HS concentration profiles with submillimetre resolution, whereas the conventional sediment-sectioning method gave HS measurements with a spatial resolution of 10 mm. Furthermore, microsensor measurements revealed HS consumption occurring at the sediment surface in both the microbial mat and the sediment anoxic layer, which were not observed with sectioning. This HS consumption prevented HS release into the overlying water. However, the microsensor measurements had the potential to underestimate HS concentrations. We propose that a combination of several techniques to measure microbial activity and determine its relationships with physicochemical properties of the sediment is essential to understanding the sulfur cycle under hypoxic conditions in eutrophic sediments.