The water column may refer to two slightly different concepts. One is a formal way to refer to the various habitats that occur over any given piece of oceanic or lacustrine real estate. The other is as an informal catch-all term that means "in the ocean, and not on the sea floor".
Officially, the water column consists of five pelagic zones:
- Epipelagic: from the surface to 200 meters below the surface. This is brightly lit by the sun, and so is where most photosynthesis takes place. As a result, this is also the most biologically dense part of the water column, and where most primary production of nutrients takes place.
- Mesopelagic: from 200 to 1000 meters below the surface. This is often called the twilight zone. It is darker, and is sometimes formally defined not by depth but as the areas from which only 1% of the sunlight reaches down to the zone at which no sunlight reaches. Most nutrients come from matter falling down from the surface zone, although the massive diurnal vertical migration means that many organisms don't find their nutrients here at all -- they move to the surface to feed at night, and then sink into the depths by day to avoid predation and slow their metabolism in the cooler depths.
- Bathypelagic: AKA the Bathyal zone. From 1000 to 4000 meters below the surface. Not much happens here. Nothing is produced here, most of the detritus from the epipelagic zone has been fed on multiple times, the temperature is near freezing, the pressure is high, and it is as dark as night. Many animals here are slow moving and eyeless, or are only visitors, such as whales or squid. The bathypelagic zone goes right down to the continental slope in many areas, and many sponges, sea stars, corals, and other benthic creatures live in these depths. This zone, together with the abyssopelagic, is sometimes called the midnight zone.
- Abyssopelagic: AKA the abyssal zone. From 4000 meters below the surface to the level sea floor. This would be a very dead zone, with very low oxygen levels, within a few degrees of freezing, and crushing pressures... except we have finally reached the bottom. This is where all the nutrients from the surface finally end up, and we also get an alternate source of energy from hydrothermal vents. There are many odd and wonderful creatures here, mostly clustered within a few meters of the seafloor.
- Hadopelagic: AKA the hadal zone. Depressions and crevices below the level sea floor. As you move deeper, some organisms are excluded due to pressure; from about 6,000 to 7,000 meters you see a shift in the ecosystem as larger animals are excluded, and smaller creepy-crawlies take over. Another major factor is at play here; as these zones are below the rest of the sea floor, they are essentially isolated. Globally there are only 46 known large trenches on the seafloor that act as isolated valleys, resulting in speciation and the development of unique biomes.