Continental shelves ecology

Continental shelves are the submerged margins of land around landmasses that extend outward from the intertidal zone. These shelves eventually slope away into much deeper waters, the true ocean, known as the abyssal plain. At their widest, continental shelves can underlie entire seas, such as the North Sea. While continental shelves compose just 8 percent of the entire seabed, they are extremely important marine biomes because of their plant and animal life, as well as nonliving resources.

For most humans, the experience of the marine realm is completely limited to continental shelf habitats, and the value attributed to the ocean as a whole is based on knowledge and perception of these regions. While the geological mineral composition of continental shelves varies considerably throughout their worldwide distribution, usually similar to that of the exposed terrain, they share several common features. Continental shelves are relatively shallow, less than 650 feet (100–200 meters), whereas the open ocean is typically greater than 1,312 feet (400 meters) deep. Waters overlying continental shelves are often termed shelf seas to distinguish them from the high seas, or the ocean proper. The uses made of continental shelves are also similar the world over. Not only are most marine recreational activities focused in these areas, so too are aquaculture, mining, oil extraction, and other important economic activities. Natural influences on shelf waters are also similar and include deposition, pollution, ice scour (particularly in shallower areas), currents, stratification, plumes, turbulence, and upwelling.

The Earth's landmasses are generally aligned north–south, meaning that major coastlines tend to be eastward or westward facing. There are distinct differences that arise as a result of these individual aspects, caused in part by the rotation of the Earth. Eastern continental shelves, for example, are exposed to some of the Earth's strongest currents, taking the form of gyres, such as the Pacific Gyre and the North Atlantic Gyre.

Further, sea and air interactions contrive in a complex manner, resulting in a greater variation in sea temperature, which in turn has repercussions for the communities of animals and plants that live there. Western continental shelves are notably productive, with four of the world's major upwellings occurring on western shelves; they are generally narrow in their extent, and adjacent landmasses are typically high and mountainous or fjordic (higher latitudes). Desert conditions are common on these shorelines, and riverine inputs are fewer than on eastern shelves. Western shelves of the Americas are particularly vulnerable to El Niño Southern Oscillation (ENSO) effects. These can manifest in periodically higher sea level, elevated water temperatures, and a depressed thermocline.

Habitats

Continental shelf habitats are diverse and heterogeneous, with benthic (seafloor) habitats ranging from silty sands, muds, and clays to coarser gravel, stones, and boulders, and even bare rock. Different substrata support particular types of plants and animals, with organisms specifically adapted to a certain set of conditions. Creatures that live on or in sediment thrive in muddy and sandy areas, and typical animals that might be found in such environments include worms, clams, prawns, lobsters, and flatfish. Rocky areas provide more complex physical habitats and support organisms, including lush algae, anemones, sponges, bryozoans, urchins, starfish, octopus, and more agile predatory fish. The substratum is not the only habitat in continental seas; the water column is also an important habitat for a plethora of pelagic species of fish and invertebrates such as squid, jellyfish, sardines, haddock, whiting, cod, and even larger animals such as basking shark, porpoises, dolphins, and whales.

The marine habitat that humans are most familiar with is the intertidal zone, because it is accessible and readily witnessed. Even this single marine ecotope can offer extreme ambient conditions. At their simplest, intertidal shores may be exposed or sheltered, with levels of exposure rated along a gradient. Substrates may be rocky, stony, gravelly, sandy, or muddy. Salinity changes drastically according to proximity to constant freshwater inputs and the effects of meteorological conditions. Exposure to solar radiation, wind, and predation by terrestrial organisms is periodical, according to the tides, and can change drastically in its intensity according to the season. Fringing systems range from tidal mudflats and sand flats to mangroves, wetlands, marshes, and barrier reefs.

At its oceanward extreme, where the continental shelf becomes the continental slope, the seabed declines steeply. The seafloor of the abyssal plain curves upward at the continental shelf edge at a gentler gradient, to form the deeper continental rise. During this transition from shelf to seafloor, the geology of the benthos also changes from continental crust to oceanic crust. As a habitat, continental slopes and rises tend to be rich in mineral deposits dumped there by oceanic currents, or terrestrial runoff and glacial deposition over geological time when sea level was periodically low. Conditions are much colder in these waters compared to shelf seas, light is limited, and barometric pressure is much higher. Species of the deeper slope and rise area tend to be long-lived, with slower metabolism and growth rates.

Continental Shelf Communities

The relatively shallow waters of the world's continental shelves are some of the richest regions of the ocean with respect to marine life. This is partly because of the nutrient-rich runoff from rivers and streams on land. Shallow waters also allow sunlight to penetrate to the seafloor, which makes photosynthesis possible. Marine flora then sustain invertebrates, such as crustaceans, mollusks, cnidarians (urchins, starfish, and sand dollars), and worms, which in turn sustain fish, sharks, marine mammals, and seabirds. Nature is rarely simple, however, and in the marine realm, food webs are probably more complex than anywhere else on the planet.

Phytoplankton (plant) and zooplankton (animal) form the basis of life on the continental shelf. The world of plankton consists of minute, drifting, single-celled plants and tiny animals, fish larvae and eggs, foraminifera, copepods, radiolaria, and a whole host of other strange and astonishing creatures. Among these, perhaps krill is one of the best known: Although one of the smallest animals on the planet, this crustacean is the main food source of a number of marine mammals and predatory fish by virtue of its abundance, particularly in upwelling zones. Some organisms spend their entire life as plankton (holoplankton), while others are planktonic for just the early stages of their life cycle (meroplankton). Plankton, however, is not limited to continental shelf waters. Seagrass meadows are blankets of lush green foliage that are found on continental shelves throughout the world. These delicate, habitat-forming plants are hugely important in the marine biome, both as a source of food for larger animals such as sea turtles and dugongs, and as a refuge for very rare and endangered species such as seahorses. Seagrass meadows also provide ecological services through the stabilization of sediments and the absorption of excess nutrients in eutrophic runoffs.

Kelp forests are perhaps one of the lesser known assets in coastal waters. Not the most beautiful plant, kelp is rubbery-brown seaweed with a thick stalk that attaches to the seabed, topped with thick, wavy, herbaceous fronds. Despite their lack of aesthetic appeal, kelp stands are one of the most important living habitats in higher latitude waters, both a source of shelter and nutrition for everything from marine invertebrates to otters and seals. The jewel in the crown of coastal waters is coral reefs. These living ecosystems abound with diverse and prolific species, and are typical in the cleanest, warmest waters of the world. It has been suggested that coral reefs are home to about a third of all described fish species in the world. Coral reefs are abundant along western Pacific continental shelves, with Australia's Great Barrier Reef the largest-known coral reef in the world, which is in fact a collection of close to 3,000 reefs, rather than one continuous structure. Toward its northern extreme, the Great Barrier Reef lies only a few miles from the coast, reaching several hundred miles offshore in its southern stretches.

As well as resident species, the transient vertebrate community is important on continental shelves. Marine vertebrates are numerous and diverse, ranging from fish and sharks to turtles, porpoises, whales, and dolphins to seabirds. Fish are among the most prominent life forms in continental shelf waters. They are important for a number of reasons, as prey and predators within ecosystems, as well as a valuable natural resource to humans. Continental shelf habitats are crucial to fish in a number of ways: as nursery habitats, feeding grounds and spawning grounds, and as a highway for diadromous species that spend part of their life cycle in freshwater and part in the sea. Continental shelves are also home to a plethora of marine mammal species, from sea otters to seals, sea lions, walruses, porpoises, dolphins, and whales.

The nutrient-rich waters of the shelf attract both the plankton-feeding baleen whales, as well as toothed predators such as sperm whales and orcas. Larger marine mammals are found throughout a wide range of pelagic conditions, from the shelf to slope and deeper oceanic waters, many will utilize shelf areas at least temporarily. In the northeastern region of the U.S. Continental Shelf, for example, right, humpback, and minke whale species prefer shelf environments, whereas sei and blue whales show preference for the outer slope and oceanic depths.

Living Resources

Most of the world's fisheries are concentrated along continental shelves. Fishing comes in many forms: subsistence, artisanal, commercial, and large-scale commercial. For many thousands of years, marine fish and invertebrates have been exploited for subsistence and trade. Nowadays, fish products are collected not only for human sustenance, but also in the production of several secondary products such as animal feed and fertilizer, alternative medical treatments, and for the aquarium trade. Fishing is carried out with an array of gear and technologies. Gear used in continental shelf fisheries ranges from a hand-held rod or line, to pelagic and bottom longlines, gillnets, purse seines, trawls, pots, traps, and dredges. Spear fishing and hand collection through free diving are also common practices in shallower, warmer waters. In U.S. coastal waters, groundfish have long been the most heavily exploited stocks, including fish such as cod, haddock, flounder, and sole.

For many decades, cod was the currency behind North American prosperity; it fed millions and found a fond place at the heart of Western culture. The crash of cod stocks in the northeastern U.S. and Canadian continental shelves was unexpected. For at least 500 years, cod fishing along the Newfoundland and Labrador shelves was highly profitable. It is believed that Portuguese fishermen were crossing the Atlantic to catch these fish, drying and salting them, and shipping them back to Europe even before the long-held discovery of the New World in 1492. From the Georges Bank offshore of New England to the Grand Banks off Newfoundland and further still to the Gulf of St. Lawrence, cod aggregated in huge numbers to spawn.

Fishing these shelves remained sustainable for hundreds of years, but the post–World War II arrival of hi-tech factory ships marked the end of these cod stocks. These new ships could trawl and haul huge nets in virtually any weather conditions, 24 hours a day, remaining at sea for weeks at a time, as well as process and freeze fish onboard, and were equipped with sonar, radar, and “fish-finders.” To make matters worse, at the time, these banks were considered international waters, and therefore did not fall under the jurisdiction of any one nation. Although the creation of the 200 exclusive economic zones (EEZs) in the early 1980s diminished the power of these large vessels, the damage to the stocks was already done. Catches continued to rise until 1968, and then took a turn for the worst, falling slowly until by the early 1990s, the great northeastern cod stocks were depleted.

Oil Extraction and Mining

The outer continental shelf (OCS) is the region of most interest with regard to extracting mineral resources. OCS regions are the seaward extents of the shelf that in the United States are managed under federal jurisdiction. Four regions have been demarcated in U.S. shelf waters: the Atlantic OCS, Pacific OCS, Alaskan OCS, and Gulf of Mexico OCS. The most recent data from the Bureau of Ocean Energy Management reported that there were about 3,200 functioning extraction facilities in the Gulf of Mexico OCS, and 23 facilities on the Pacific OCS, with no exploitation of the Alaskan or Atlantic shelf regions. Following a flurry of activity in the early 1980s, the overall trend in recent years has been downward, with fewer and fewer installations established. In 2020, approximately 15 percent of U.S. oil production was from OCS reserves and 2 percent of natural gas production. Marine sands and minerals are also highly prized. Since the 1990s, the United States has seen mining of marine sediments along the Atlantic and Gulf coasts for the replenishment of beaches where erosion has taken a toll. Mining activities in the United States within three nautical miles (5.56 nautical kilometers) of the shoreline are generally regulated by individual states, while those beyond fall under federal jurisdiction and are regulated by the Bureau of Ocean Energy Management (formerly the Marine Mineral Service [MMS]).

Plate Tectonics and Sea-Level Change

Evidence suggests that over geological time, sea levels have risen and fallen, both exposing and concealing areas of the continental shelf during this process. Sea level can change by two principal mechanisms, eustatic and isostatic change. Eustatic change is climate-driven and linked with ice formation and melt, while isostatic change is driven by the rise and fall of tectonic plates. As little as 21,000 years ago, sea levels were 410 feet (125 meters) lower than at present, and in places the sea would have receded to the edge of the continental slope, exposing the entire continental shelf. Evidence of such changes in sea level and the geological history of continental shelves has been demonstrated by the discovery of fossilized tree remains in deep shelf waters. Continental crust is less dense than oceanic crust, therefore, where these plates meet, the oceanic crust tends to be pushed under, or subducted by, the continental plate.

Often, this causes an oceanic trench to form along the edge of the continental shelf plate edge. The impact causes the terrestrial plate to buckle and lift up, eventually forming mountain ranges, often associated with very narrow continental shelves with steep continental slopes. Although sediment runoff can be considerable from mountain streams, these are usually carried directly offshore to quite deep water. This type of continental shelf is typical along the Pacific coast of North and South America. Earthquakes are common where plates meet, as is changing sea level. This is not because of changes in the water mass, but rather because of movements of the tectonic plates. These movements are characterized by gradual uplifting, interspersed with sudden dropping of the coastline when energy is released during seismic events. Rifts form where plates move apart; these trailing coastlines are typical of much of the African continent, as well as the eastern seaboard of North America. Here, continental shelves tend to be more extensive and more prone to sedimentation.

International Law of the Sea

Throughout history, the sea was seen as a common resource that could be navigated and exploited by all. In the early 17th century, however, the principle of Freedom of the Seas, mare liberum, was born. This was a doctrine wherein sovereign nations were given jurisdiction over continental shelf waters within 3 nautical miles (6 nautical kilometers) of shore, while the rest of the sea remained a common resource. By the mid-20th century, concerns about exploitation of fish stocks, pollution, and the potential profits available from both living and nonliving marine resources incited a number of coastal nations to unilaterally extend their claim to govern and manage adjacent waters. In 1945, U.S. President Harry Truman declared national jurisdiction over all continental shelf waters, mineral resources, and underlying geology, in a move to secure valuable oil, gas, and mineral reserves, and in doing so began a revolution in the way that shelf resources are managed. Several South American countries soon laid claim to waters within 200 nautical miles (370 nautical kilometers) of shore, while north African and Arabian Gulf nations extended their territorial claims from 3 to 12 nautical miles (6 to 22 nautical kilometers). The world over, nations were disputing claims to marine resources, from tin to diamonds, gas and oil. The 1958 United Nations Convention on the Law of the Sea (UNCLOS I), which entered into force in 1964, defined “continental shelf” in terms of its use, rather than its geological extent. Here, the continental shelf was considered the seabed and subsoil, beyond territorial seas, extending to a depth of 656 feet (200 meters), and coastal states were given exclusive sovereign rights over its exploitation.

Responding to continued international unrest, the United Nations convened the Third United Nations Conference on the Law of the Sea in 1973, a process that continued for nine years, finally ending in 1982 with the publication of the UN Convention on the Law of the Sea (UNCLOS III). The convention provided for the setting of limits, resolution of disputes, inauguration of EEZs, as well as exploitation, environmental protection, and research. Under UNCLOS III, territorial waters extended to 12 nautical miles (22 nautical kilometers) from the mean low water mark, with a further contiguous zone extending to 24 nautical miles (44 nautical kilometers). EEZs were waters extended up to 200 nautical miles (370 nautical kilometers). The degree and nature of regulation that coastal nations can impose in each of these jurisdictions varies, but EEZs demarcate the waters within which a state has sole rights to exploitation of all natural resources, both living and nonliving. In the United States, management of the continental shelf is shared by state and federal governments. In most cases, these limits are delineated by the 3-mile limit (9-mile limit in Florida), in nautical miles.

The legal definition of continental shelf was modified from that of UNCLOS I, and now refers to the natural prolongation of territory to the continental margin or 200 nautical miles (370 nautical kilometers), whichever is greater, satisfying nations with narrow natural continental shelves. To appease nations with a naturally wider shelf, the convention allows the extension of continental shelf designation of as much as 350 nautical miles (648 nautical kilometers) or more where specific geological criteria are met. A further development in UNCLOS III was the creation of the International Seabed Authority, which governs mineral exploitation beyond EEZ limits. The Commission on the Limits of the Continental Shelf was set up to resolve disputes about the extent of shelf regions and their sovereignty.