A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment. Descriptively, the deep sea water column is divided into five parts—pelagic zones (from Greek πέλαγος (pélagos), 'open sea')—from the surface to below the floor, as follows: epipelagic, from the surface to 200 meters below the surface; mesopelagic, from 200 to 1000 meters below the surface; bathypelagic, from 1000 to 4000 meters below the surface; abyssopelagic, from 4000 meters below the surface to the level sea floor; hadopelagic, depressions and crevices below the level sea floor.
The concept of water column is useful since many aquatic phenomena are explained by the incomplete vertical mixing of chemical, physical or biological parameters. For example, when studying the metabolism of benthic organisms, it is the specific bottom layer concentration of available chemicals in the water column that is meaningful, rather than the average value of those chemicals throughout the water column.
Water columns are used chiefly for environmental studies evaluating the stratification or mixing of the thermal or chemically stratified layers in a lake, stream or ocean: for example, by wind-induced currents. Some of the common parameters analyzed in the water column are pH, turbidity, temperature, hydrostatic pressure, salinity, total dissolved solids, various pesticides, pathogens and a wide variety of chemicals and biota.
The term water column is also commonly used in scuba diving to describe the vertical space through which divers ascend and descend.