pelagic

Pelagic describes the open-water component of the ocean, away from the coast and seabed. It encompasses the water column from the surface to the deep sea and excludes coastal shelf regions and benthic habitats. The pelagic realm contains both planktonic and nektonic organisms. Plankton include phytoplankton that perform photosynthesis near the surface and zooplankton that feed on phytoplankton or on each other; nekton comprises actively swimming animals such as fishes, squids, marine mammals, and reptiles, capable of moving substantial distances.

The pelagic environment is traditionally subdivided by depth: the epipelagic, or sunlight zone, extends from the surface to about 200 meters and hosts most primary production. The mesopelagic, or twilight zone, ranges roughly from 200 to 1000 meters and is characterized by dim light and abundant bioluminescence. Deeper zones—the bathypelagic (about 1000 to 4000 meters) and abyssopelagic (approximately 4000 to 6000 meters), and in some schemes the hadopelagic in trenches—are cold, dark, and food-scarce, with organisms adapted to high pressure and low temperatures.

Many pelagic species perform diel vertical migrations, rising toward the surface at night to feed and descending during the day. Common pelagic adaptations include streamlined bodies, countershading, fins and tails for sustained swimming, and sensory systems tuned to open-water life. Large pelagic predators such as tuna, marlin, swordfish, sharks, and some marine mammals roam vast oceanic distances in search of prey, while countless smaller planktonic and nektonic organisms form the base of open-ocean food webs.

Human activities, including fishing and climate-driven changes in circulation and productivity, affect pelagic ecosystems, which are highly dynamic and dependent on nutrient supply and ocean currents.