Marine organisms often occur in belts or depth zones because of gradients in environmental factors. Above the high water zone, there is the supralittoral zone. The bottom of the sea is divided in a littoral zone (intertidal) a sublittoral zone (between low water and approx. 200 m). Below the 200 m. there is the bathyal zone down to 2000 m, the abyssal zone down to 6000 m and the hadal zone to the greatest depths. The water masses from the low water mark to the greatest depth we call the pelagic and is also divided into the neritic (near shore) waters roughly to 200 m depth mark (mostly the shelf edge) near continents and oceanic waters in the open ocean. The oceanic waters itself are divided into epipelagic waters (the photic zone) down to about 200 m, the mesopelagic (aphotic) to 2000 m, the bathypelagic (aphotic) to 6000 m, and the abyssopelagic (also aphotic) to the greatest depth. Temperature is the main factor for zonation in deep water. The salinity is the dominant ecological factor in zonation in estuaries.

Alternative form for zonation : zone.