Pelagic communities

The oceanic pelagic zone is dominated by the activity of plankton in the euphotic surface waters, Plankton are by definition drifting or weakly swimming organisms, comprising a wide range of small to microscopic animals, protoctists and bacteria. Free-swimming pelagic organisms are collectively termed nekton. With few exceptions, the only food source for creatures in the aphotic zone is the 'rain' of organic matter (faeces, moulted crustacean exoskeletons, corpses) derived from the plankton and nekton. Both tend to concentrate along major circulation currents (gyres), contact zones and upwellings, and this causes significant local variations in diversity,

The marked vertical gradients of light, temperature, pressure, nutrient availability and salinity within the pelagic zone creates a vertical structuring of pelagic species assemblages into several zones according to depth. These zones tend to fluctuate in time and space, as well as season. Some components of the epipelagic and mesopelagic nekton perform astonishing diel migrations ascending to surface waters at night to feed and descending, sometimes over 1 km, during the day.

It has generally been assumed that biomass in the pelagic zone everywhere below the euphotic zone is low, however, recent studies have indicated that biomass of tropical mesopelagic animals may be surprisingly high. One study recognised around 160 fish genera in 30 families as important components of the fauna. Most species are small (less than 10 cm in length) and often bizarrely shaped but estimates based on a variety of surveys carried out indicate that global biomass of this stock may be large: a figure of 650 million tons (some six to seven times total current marine fisheries landings) has been suggested. Mesopelagic biomass is greatest in the northern Indian Ocean, and particularly in the northern Arabian Sea (surveys there have indicated extremely high biomass of 25-250 g per m2).