Marine Biology Terms Starting With N
Marine Biology Glossary: N
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Nekton
/ NEK-ton / · Greek nektos (swimming)
Nekton are aquatic organisms capable of swimming independently against currents, including fish, squid, marine mammals, sea turtles, and large crustaceans, distinguished from passively drifting plankton by their ability to control their position in the water column.
The boundary between nekton and plankton is defined by swimming ability relative to ambient current speed. Nekton can actively choose their depth and horizontal position, migrate across ocean basins, and pursue or evade other organisms. Atlantic bluefin tuna (Thunnus thynnus) cross the entire Atlantic Ocean during seasonal migrations, covering thousands of kilometers under their own power.
Nekton dominate the upper and mid-water pelagic zones and include the commercially harvested fish species that support global fisheries worth hundreds of billions of dollars annually.
Some gelatinous animals occupy an ambiguous middle ground between nekton and plankton. Moon jellyfish (Aurelia aurita) can pulse their bells to swim vertically but cannot overcome horizontal currents, placing them in a transitional category called meroplankton or, by some classifications, micronekton depending on their life stage and size.
All ocean animals are plankton drifting with currents. Strong swimmers such as tuna and dolphins are nekton.
Swordfish (Xiphias gladius) are nekton that actively hunt prey in open water rather than drifting with currents. Tagged individuals have been recorded diving to depths exceeding 550 meters during foraging bouts, demonstrating the vertical range nekton can command through sustained swimming.
Neritic Zone
/ neh-RIT-ik zohn / · Greek Nereid (sea nymph) + zone
Neritic Zone is the shallow coastal ocean overlying the continental shelf, extending from the low-tide mark to approximately 200 meters depth, and containing the most biologically productive marine waters on Earth.
The neritic zone receives sufficient sunlight for photosynthesis throughout much of the water column, and nutrient input from rivers, estuaries, and coastal upwelling keeps primary productivity high year-round. Major commercial fisheries, including those targeting Atlantic herring (Clupea harengus), Pacific cod (Gadus macrocephalus), and Peruvian anchoveta (Engraulis ringens), concentrate in neritic waters where food availability is greatest. Although the neritic zone covers only a small fraction of the ocean surface, it supports a large majority of global marine fish catches because shallow, nutrient-rich shelf waters concentrate plankton, benthic prey, and spawning habitat.
Shallow depths also mean that dead organic matter sinks quickly to the seafloor, where benthic communities recycle nutrients back into the water column.
The neritic zone was the site of one of the most dramatic fisheries collapses in history. The Grand Banks cod fishery off Newfoundland, which had supported harvests for centuries, collapsed in 1992 after decades of industrial trawling reduced the northwest Atlantic cod (Gadus morhua) population to less than 1 percent of its historical biomass.
The open ocean is where most fisheries occur. Many major fisheries are in neritic waters over continental shelves.
The Grand Banks off Newfoundland are a neritic zone extending over the continental shelf to depths of roughly 200 meters. Cold, nutrient-rich water there once supported cod populations so dense that early European explorers reported lowering baskets to scoop fish from the sea, and twentieth-century industrial catches exceeded 800,000 metric tons in some years before the 1992 collapse.
Nutrient Upwelling
/ NOO-tree-ent UP-wel-ing / · Latin nutrire (to nourish) + Old English up + well
Nutrient Upwelling is the wind-driven rise of cold, nutrient-rich deep water to the ocean surface, fertilizing surface waters and triggering phytoplankton blooms that support highly productive marine food webs.
Along eastern ocean boundaries such as the coasts of Peru and Namibia, persistent trade winds push surface water offshore through Ekman transport, drawing cold water up from 100 to 300 meters depth. This upwelled water carries nitrate, phosphate, and silicate that have accumulated from the decomposition of sinking organic matter, supporting phytoplankton productivity orders of magnitude higher than the surrounding open ocean. The Peruvian upwelling system, which covers less than 1 percent of the ocean surface, produces about 10 percent of the world’s marine fish catch, largely through enormous schools of Peruvian anchoveta (Engraulis ringens).
During El Niño years, warm water suppresses upwelling along the Peruvian coast, and anchoveta populations can crash within a single season.
During the 1972 El Niño event, the collapse of Peruvian upwelling caused the anchoveta catch to fall from roughly 12 million metric tons in 1970 to under 2 million metric tons by 1973, triggering a global shortage of fishmeal that drove up the price of poultry and pork in markets as far away as Europe.
Surface waters always contain enough nutrients for plankton. Many sunlit surface waters are nutrient-poor because phytoplankton rapidly consume available nitrogen and phosphorus, and only mixing or upwelling replenishes the supply.
Off the coast of Namibia, the Benguela Current upwelling system draws cold water from depths of up to 300 meters to the surface. This upwelling supports one of the world's richest fisheries, including large populations of Cape anchovy (Engraulis encrasicolus) and Cape hake (Merluccius capensis), which together account for a substantial share of southern Africa's commercial fish landings.