These compartments and transport processes comprise an aquatic biogeochemical cycle for mercury and illustrate the important mechanisms involved in controlling the concentration and movement of mercury in an aquatic ecosystem. Among the more important processes regulating mercury in an aquatic ecosystem are scavenging onto particles and particle settling to the sediments, which removes mercury from the water column, and microbial transformation and subsequent incorporation into the food chain. Mono-methyl mercury enters the water column from the sediment, where it is incorporated into phytoplankton during growth. Once in the food chain, the concentration of mercury in aquatic organisms increases as larger organisms consume smaller ones. This can occur through the formation of photoactive organic complexes of mercury.
Atmospheric mercury is oxidized, then returns to the water surface in rain.