In oceanography and limnology, the sediment–water interface is the boundary between bed sediment and the overlying water column. The term usually refers to a thin layer (approximately 1 cm deep, though variable) of water at the very surface of sediments on the seafloor. In the ocean, estuaries, and lakes, this layer interacts with the water above it through physical flow and chemical reactions mediated by the micro-organisms, animals, and plants living at the bottom of the water body. The topography of this interface is often dynamic, as it is affected by physical processes (e.g. currents causing rippling or resuspension) and biological processes (e.g. bioturbation generating mounds or trenches). Physical, biological, and chemical processes occur at the sediment-water interface as a result of a number of gradients such as chemical potential gradients, pore water gradients, and oxygen gradients. Definition The location of the top of the sediment-water interface in the water column is defined as the break in the vertical gradient of some dissolved component, such as oxygen, where the concentration transitions from higher concentration in the well-mixed water above to a lower concentration at the sediment surface. This can include less than 1 mm to several mm of the water column. Physical processes Physical movement of water and sediments alter the thickness and topography of the sediment-water interface. Sediment resuspension by waves, tides, or other disturbing forces (e.g. human feet at a beach) allows sediment pore water and other dissolved components to diffuse out of the sediments and mix with the water above. For resuspension to occur the movement of water has to be powerful enough to have a strong critical shear stress that is greater than the bed shear stress. For example, a very consolidated bed would only be resuspended under a high critical shear stress, while a "fluff layer" of very loose particles could be resuspended under a low critical shear stress. Depending on the type of lake, there can be a number of mixing events each year that can influence the sediment interface. Amictic lakes are permanently stratified, similarly, meromictic lakes do not mix. Polymictic lakes undergo frequent mixing and dimictic lakes mix twice a year. This type of lake mixing is a physical process that can be driven by overlaying winds, temperature differences, or shear stress within the lake. Physical processes that affect the sediment-water interface include, but are not limited to: Resuspension Deposition Rippling (either