Why are wetland mud and bottom waters often low in oxygen?

Wetland mud and bottom waters are often low in oxygen primarily because of the physical properties of water and the biological processes occurring within these environments.

In wetlands, the persistent saturation or flooding of the soil creates an environment where atmospheric oxygen cannot be effectively replenished. Because water limits the diffusion of gases, the oxygen present in the soil is quickly consumed, leading to anaerobic conditions. This lack of oxygen is a defining characteristic of hydric soils, which in turn influences the chemical composition of the environment, such as the reduction of iron and manganese by anaerobic bacteria.12

Regarding bottom waters, oxygen levels are determined by the water's source and its distance from that source. Bottom water is typically formed at the surface, where it absorbs dissolved oxygen before sinking due to its high density. As this water moves along the seafloor, it becomes isolated from the atmosphere, and the oxygen it carries is gradually depleted by the respiration of benthic organisms. Consequently, the further the water travels from its point of origin, the lower its oxygen concentration becomes.3