North Atlantic Ocean

• Category: Marine and Oceanic Biomes.
• Geographic Location: Atlantic Ocean north of the equator.
• Summary: The complex environment of the North Atlantic has supported human populations for centuries, but the seemingly inexhaustible fisheries of the past are threatened by human activity.

The North Atlantic Ocean, the portion of the Atlantic that lies north of the equator, is a vast and growing expanse of water with a complex ecology due to numerous connections with adjoining oceans, seas, gulfs, bays, and currents that help set the pace of the Earth's thermohaline circulation—a temperature- and salinity-driven system that exerts a strong influence on the planet's climate.

As a whole, the Atlantic is the Earth's second-largest ocean but is expanding, while the largest oceanthe Pacificis shrinking as a result of plate tectonics. On its west, the North Atlantic is bordered by the continents of South and North America and by the Caribbean Sea, Gulf of Mexico, Bay of Fundy, and Gulf of St. Lawrence. On its north, the North Atlantic Ocean is bounded by Greenland, Iceland, and the Greenland and Norwegian Seas. On its east, the North Atlantic Ocean is bounded by the continents of Europe and Africa; the British Isles; the North, Scottish, and Irish Seas; the Bristol and English Channels; the Bay of Biscay, Mediterranean Sea, and Gulf of Guinea.

The North Atlantic is slightly larger in area than the South Atlantic16,020,000 square miles (41,490,000 square kilometers) compared to 15,550,000 square miles (40,270,000 square kilometers) respectively. However, the North Atlantic has a lower average depth at 11,545 feet (3,519 meters) compared to the South Atlantic's 13,034 feet (3,973 meters). The North Atlantic also has a smaller volume at 35,027,263 cubic miles (146,000,000 cubic kilometers) compared to the South Atlantic Ocean's volume at 38,386,041 cubic miles (160,000,000 cubic kilometers).

Zonal Characteristics

There are numerous ways to classify the environmental zones of the oceans. One is by type of environment, whether pelagic or benthicsea bottom. The dominant residents of the pelagic environment are either plankton, free-floating organisms that are usually microscopic in size; or nekton, free-swimming organisms that are typically fish, but include sea mammals such as whales, and reptiles such as sea turtles. The dominant residents of the benthic zone are invertebrates such as hard and soft corals, polychaetes, crustaceans, and mollusks.

The pelagic environment can be subdivided into two provinces. One region is the neritic, which extends from the high tide line out to the edge of the continental shelves at about 660 feet (200 meters) deep, and is subject to influence from adjoining terrestrial environments. The other region is the oceanic, which encompasses the deepwater environment beyond the edge of the continental shelves and is less influenced by the input of nutrients, sediment, and other pollutants from land.

The pelagic environment can be further subdivided into zones based on depth. The epipelagic zone is the surface layer down to 660 feet (200 meters). By definition, all of the neritic provinces would fall into the epipelagic zone. The mesopelagic zone extends from 660 feet down to 3,300 feet (1,000 meters). The bathypelagic zone extends from 3,300 feet down to 13,000 feet (4,000 meters). Below 13,000 feet lies the abyssopelagic zone.

The benthic, or sea-bottom, environment can likewise be subdivided into two provinces. One province is the subneritic, which encompasses the continental shelves down to about 660 feet (201 meters). The other province is the suboceanic, which encompasses the sea bottom in the deeper waters beyond the edges of the continental shelves. As with the pelagic environment, the benthic environment can be divided into zones based on depth. The littoral zone corresponds with the edge of the sea—the area between the high- and low-tide lines. The sublittoral zone encompasses the rest of the seafloor of the subneritic province, from the low-tide line down to about 660 feet (201 meters).

The Sunlight Factor

This littoral zone can be further subdivided into inner and outer sublittoral zones. The inner sublittoral is defined as the portion where sufficient light reaches the ocean floor to permit the growth of photosynthetic marine algae, such as kelp, attached to the bottom. The maximum depth of the inner sublittoral varies based on the amount of solar radiation reaching the ocean surface as well as the turbidity, or cloudiness, of the water, but is usually about 160 feet (50 meters).

The bathyal zone corresponds with the location of the continental slope—a drop-off from the edge of the continental shelf down to the abyssal plains of the deep ocean—from 660 feet down to 13,000 feet. The abyssal zone is the seafloor in waters between 13,000 feet and 20,000 feet (6,000 meters) in depth. Below 20,000 feet lies the hadal zone.

Another classification system is based on what is arguably the single most important variable influencing the distribution of marine life—sunlight. The euphotic zone has sufficient light for photosynthesis to occur. It extends from the surface down to about 330 feet (100 meters). The primary photosynthetic organisms are phytoplanktonmicroscopic organisms such as diatoms, coccolithophores, dinoflagellates, and cyanobacteriaalong with fixed algae, attached to the bottom in shallower waters. Because of the incredible productivity made possible by the energy in sunlight, the euphotic zone is home to the vast majority of all marine life.

Below the euphotic zone is the disphotic zone, which extends to about 3,300 feet in depth. While not sufficient to permit photosynthesis, the disphotic zone has measurable quantities of light that may have significant ecological effects, such as concealing prey from predators or revealing prey to predators. The aphotic zone lies below the disphotic zone. No sunlight penetrates those depths. Any light in the aphotic zone is the product of bioluminescence—light produced by the organisms.

North Atlantic Gyre

Centuries ago, oceanographers realized that a series of currents flowed through and subdivided the North Atlantic. The first of these currents to be discovered was the Gulf Stream, known to European explorers such as Juan Ponce de León as early as the beginning of the 16th century and named and mapped by Benjamin Franklin in 1770.

The Gulf Stream originates off the tip of Florida where the Caribbean Current and Antilles Current—both offshoots of the Atlantic North Equatorial Current—rejoin one another. Atmospheric circulation drives the current northward along the east coast of North America and east into the Atlantic south of Greenland. There it splits into two currents. The first of these currents is the North Atlantic Drift, which flows northeastward along the coast of Scandinavia into the Arctic Ocean. The other current is the Canary Current, which flows east and south along the western coasts of the Iberian Peninsula, which includes Spain and Portugal, and North Africa. The Canary Current turns west off the coast of Africa, becoming the Atlantic North Equatorial Current and completing the final leg of a clockwise-rotating system known as the North Atlantic Gyre.

By bringing warm tropical water into the higher latitudes, the Gulf Stream helps make northern and western Europe warmer than they would otherwise be. It also permits the development of an extensive coral reef system around Bermuda, which at 32 degrees north latitude is the most northerly expanse of coral reefs in the entire Atlantic.

Where the Gulf Stream comes into contact with the colder Labrador Current, which flows southward between Canada and Greenland into the northwestern North Atlantic, it creates potentially stormy conditions, but it also creates some of the richest fisheries on the planet. One such contact is off the Grand Banks, an undersea rise southeast of Newfoundland; another is off the Outer Banks barrier island system of North Carolina.

The wind-driven surface currents of the North Atlantic Gyre are connected with the density-driven, deep currents of the thermohaline circulation—where colder and saltier, hence denser, water sinks to form strong currents that circulate through the Earth's ocean basins—to form the global oceanic "conveyor belt ultimately." The conveyor begins in the North Atlantic off Greenland and Iceland, where cold water sinks to form North Atlantic Deep Water, which bends southward along the continental slopes of the Americas to join with Antarctic waters that, in turn, spread into the Indian and Pacific Ocean basins.

By its interactions with the atmosphere as well as all the Earth's seas, the global ocean conveyor helps act as a thermostat on the Earth's climate, keeping global temperatures relatively stable. Disruptions in the North American portion of the conveyor—such as a massive influx of cold freshwater from rapidly melting glaciers in Greenland—could trigger sudden and devastating climate change.

Fauna Profile

The rich fisheries off the east coast of North America have significantly impacted the history of Europe and the Americas. Arguably, the most important was that of the North Atlantic cod, which drew European sailors across the Atlantic hundreds of years before Columbus's 1492 voyage and fueled 16th-century Europe's ambitions for colonizing the northeastern United States and Canada. Cod survived intense fishing pressure for 400 years until technological innovations allowed commercial fishers to so thoroughly deplete the population that the fishery collapsed throughout the species' range.

While abundant fisheries have historically been found off the east coast of North America and on the northwestern fringes of Europe, inside the great loop of the North Atlantic Gyre, it is a biological wasteland. The nutrient-poor waters have relatively depauperate animal populations, except where upwelling currents bring cold, nutrient-laden water up from the depths.

What is found in abundance inside the gyre is floating plastic debris. Plastic refuse, discarded on land, washes into coastal waters and is picked up by the currents circulating the edges of the gyre. As a result of the wind effect known as Ekman transport, surface waters, and the pollution they carry, are deflected to the right of the direction of the prevailing winds that drive the gyre's currents in the Northern Hemispherein the Southern Hemisphere, the deflection is to the left. This transport concentrates plastic debris in the gyre's center, where the debris frequently enters marine food webs after ingestion by aquatic animals. The debris cloud surrounding the gyre is known as the North Atlantic Garbage Patch. While experts do not know how much garbage is in the North Atlantic Garbage Patch, in 2022, they estimated that eighty percent of the garbage was plastic. There may be as many as 125,000 pieces per square mile (200,000 per square kilometer).

In addition to cod, the cooler waters of the western North Atlantic are home to Atlantic herring, whose populations have also suffered from overfishing in recent decades. Among notable mammalian species are the beluga whalea formerly heavily hunted species now suffering from water pollution and diseasesthe endangered northern right whale, the fin whale, the humpback whale, and the grey seal. The northern right whale, whose population was initially depleted by hunting, began to rebound in the late twentieth and early twenty-first centuries. However, after 2010, it went into decline once more due to fatalities caused by entanglement in various types of commercial fishing gear. It was estimated in 2021 that fewer than 400 individuals exist.

The warmer waters of the western North Atlantic, under the influence of the Gulf Stream, support a rich temperate fish fauna—which includes snappers, groupers, grunts, and porgies—with a significant number of residents more typical of tropical water, such as wrasses and damselfish. This region's significant environmental concerns are development, pollution, and introduced species and diseases.

The northern European waters offer largely shallow water environments with areas of upwelling that bring cool, nutrient-rich waters to the surface that have traditionally supported robust fisheries in species such as Arctic cod, Arctic char, and capelin. They are also home to porbeagle shark; several cetacean species, including the blue whale, minke, killer, beluga, narwhal, bowhead, and northern bottlenose; and the grey and harbor seal. Overfishing and pollution are the primary environmental threats to the region.

Tropical Populations

While the tropical North Atlantic coral reefs are less diverse than their Indo-Pacific counterparts, they are diverse enough to support major feeding grounds that draw a wide range of species from the temperate North Atlantic. Among the more important residents are parrotfish, wrasses, Cherub fish, and large grouper, including Nassau grouper, drum, and conch. The region serves as a breeding haven for the humpback whale, offers crucial habitat for such littoral fauna as the American crocodile and manatees, and hosts a robust array of sea turtles, including loggerhead, green, hawksbill, leatherback, olive ridley, and Kemp's ridley. Loss of habitat from development and disturbance, epidemic disease, coral bleaching, and pollution threaten many reefs in the region.

The waters of the northeast Brazil shelf offer a diversity of habitats, including mangroves and coral reefs, that are home to a number of sea turtle species as well as breeding grounds for humpback whales and smalleye hammerhead sharks. Development, pollution, and destructive fishing practices are significant environmental concerns here.

Several areas of upwelling occur where the Canary Current approaches the coast of southern Europe and northwestern Africa. These areas support significant fisheries for deep-sea fish and lobsters. The Canary Current region is also home to several economically important speciessardine, Spanish sardine, and pilchardhorse mackerel, hake, green and olive ridley sea turtles, and West African manatee. It serves as a breeding ground for humpback whales. As with other marine provinces near areas of dense human habitation, development, and pollution are significant concerns.

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