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The pelagic zone contrasts with the benthic and demersal zones at the bottom of the sea. The benthic zone is the ecological region at the very bottom, including the sediment surface and some subsurface layers. Marine organisms such as clams and crabs living in this zone are called benthos. Just above the benthic zone is the demersal zone.
The distinction between demersal species of fish and pelagic species is not always clear cut. The Atlantic cod (Gadus morhua) is a typical demersal fish, but can also be found in the open water column, and the Atlantic herring (Clupea harengus) is predominantly a pelagic species but forms large aggregations near the seabed when it spawns on banks of gravel.
Marine habitats can be broadly divided into pelagic and demersal habitats. Pelagic habitats are the habitats of the open water column, away from the bottom of the ocean. Demersal habitats are the habitats that are near or on the bottom of the ocean. An organism living in a pelagic habitat is said to be a pelagic organism, as in pelagic fish.
Pelagic fish species include some anchovy, mahi-mahi, marlins, swordfish, amberjack and Atlantic mackerel. Well-known and ecologically important benthic fish associate with the surface when young, including species of: lefteye flounder, blenny, goby, seahorses, seadragons and pipefish. Deep-sea fish with surface larvae include viperfish and ...
Consequently, these species have short pelagic larval durations and do not disperse long distances. Planktotrophic larvae feed while they are in the water column and can be over a long time pelagic and so disperse over long distances. This disperse ability is a key adaptation of benthic marine invertebrates. [3]
[176] [177] Hypoxic systems tend to lose many long-lived higher organisms and biogeochemical cycles typically become dominated by benthic bacterial processes and rapid pelagic turnover. [178] However, if hypoxia does not occur, benthic fauna tends to increase in biomass with eutrophication. [179] [180] [181] [2]
Benthic-pelagic coupling are processes that connect the benthic zone and the pelagic zone through the exchange of energy, mass, or nutrients. These processes play a prominent role in both freshwater and marine ecosystems and are influenced by a number of chemical, biological, and physical forces that are crucial to functions from nutrient cycling to energy transfer in food webs.
All photosynthetic life needs light to grow and how far out into the ocean light can still penetrate through the water column to the floor or benthic zone is what defines the neritic zone. That photic zone , or area where light can penetrate through the water column, is usually above ~100 meters (~328 feet).