Redox gradients form in water columns and their sediments. Varying levels of oxygen (oxic, suboxic, hypoxic) within the water column alter redox chemistry and which redox reactions can occur. Development of oxygen minimum zones also contributes to formation of redox gradients.
Benthic sediments exhibit redox gradients produced by variations in mineral composition, organic matter availability, structure, and sorption dynamics. Limited transport of dissolved electrons through subsurface sediments, combined with varying pore sizes of sediments creates significant heterogeneity in benthic sediments. Oxygen availability in sediments determines which microbial respiration pathways can occur, resulting in a vertical stratification of redox processes as oxygen availability decreases with depth.