Coastal seas

Coastal seas are extremely important for biodiversity and ecosystems. Coastal seas can be defined in terms of the continental shelves, as understood geologically, and by reference to international conventions. In geological terms, the continental shelf is the extended perimeter of the continent and the associated coastal plain. The continental margin, which is located between the continental shelf and the abyssal plain, consists of a steep continental slope followed by a flatter continental rise. Sediment from the continent descends down the slope and accumulates at the continental rise located at the base of the slope.

Under the United Nations Convention on the Law of the Sea (UNCLOS), the continental shelf is the seabed adjacent to the shores of a country. UNCLOS states that a country's continental shelf extends to the limit of the continental margin, but no less than 200 nautical miles (370 kilometers) from the baseline. The waters above the continental shelf, also known as territorial waters, are subject to the sovereignty and control of coastal nations. The legal definition of a continental shelf under UNCLOS differs significantly from the geological definition. Inhabited volcanic islands, such as the Azores and the Canaries, which have no continental shelf geologically, have a legal continental shelf.

Understanding Types of Coastal Seas

Some coastal seas are pelagic seas, located along an open ocean. Examples of pelagic seas include the North Sea and the South China Sea. Others are enclosed seas or semi-enclosed seas. Examples of enclosed seas include the Black Sea, the Baltic Sea, and the Mediterranean Sea, while semi-enclosed seas include the Gulf of Mexico, the Gulf of Carpentaria, and Hudson Bay. Some seas may have multiple characteristics over time, such as the Wadden Sea, which was a pelagic sea that became enclosed over time through human action. Lastly, polar seas that are adjacent to the Arctic and Antarctic coasts can be considered a unique type of coastal sea because of their extreme environmental conditions and predominantly pristine nature.

Coastal seas, particularly estuaries and adjacent coasts, have played a critical role in human development for most of human history, and are the most affected by human activities. Historically, coastal seas were primary commercial fishing grounds. Due to their depth and seafloor, they can provide protection from moderate storms and extreme events. Coastal seas contain abundant life because of the way sunlight diffuses through the shallow waters over the continental shelves, in contrast to the sparser life of the deeper abyssal plains. Most commercial exploitation from the sea, such as mining and hydrocarbon development, also takes place on continental shelves.

Increasing numbers of the world's people live in coastal regions adjacent to coastal seas. Many of the world's major cities have been built near natural harbors and have port facilities. The coast is also a frontier for military invaders, smugglers, and migrants. Coastal beaches and warm waters can be important tourist attractions. As a result of these increasing populations and uses, coastal seas face many environmental challenges from human-induced impacts. While human activity and the associated adverse impacts have been affecting coastal seas since early times, the decline of coastal seas has significantly accelerated in the last few centuries. For example, records from all coastal seas illustrate that seagrass habitats have been destroyed, water quality has decreased significantly, and marine species and diversity have lessened.

A perfect storm of overexploitation, nutrient pollution, and climate change is creating an uncertain future for coastal seas. Land-based and marine pollution includes petrochemicals and biological waste, while climate change contributes to sea-level rise, which threatens coastal ecosystems. Jeremy Jackson of the Scripps Institution of Oceanography has warned that “synergistic effects of habitat destruction, overfishing, introduced species, warming, acidification, toxins, and massive runoff of nutrients are transforming once complex ecosystems like coral reefs and kelp forests into monotonous level bottoms, transforming clear and productive coastal seas into anoxic dead zones, and transforming complex food webs topped by big animals into simplified, microbially dominated ecosystems with boom and bust cycles of toxic dinoflagellate blooms, jellyfish, and disease.” Knowledge of coastal seas and their original ecosystems is important for understanding the current and possible future ecosystems. Managing and conserving the world's coastal seas and their ecosystems now and in the future will require science-based approaches, commitment, and resources for managing these seas and human activities.

Climate change is of particular concern to coastal seas. A report issued in 2022 from the National Oceanic and Atmospheric Administration and other agencies found that, if the current rate of warming continues, about 1 foot of sea level rise is expected by 2050. The United States will experience as much sea level rise in the next thirty years as it had over the last century. Experts contend that while reducing greenhouse gas emissions is important for the planet, the sea level rise will occur regardless of greenhouse gas emissions. The result is that coastal seas will flood much more often, affecting the 130 million people who live within 60 miles of the ocean.

Pelagic Seas

The North Sea is located on the European continental shelf and connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Sea in the north. The North Sea is an important shipping lane and supports major fisheries. The sea is popular for tourism and recreation and is the site of significant hydrocarbon production and wind- and wave-energy generation. The coasts adjacent to the North Sea are subject to high populations, industrialization, and intense uses of the sea and coasts. Environmental impacts on the sea include overfishing, industrial and agricultural pollution, dredging, and dumping. Climate change is affecting its food web and encouraging its fisheries to migrate northward, while new species enter from southern waters.

The European Union has been implementing coastal zone management and marine spatial planning in the North Sea, and there is extensive regional cooperation. The Wadden Sea is adjacent to the North Sea and borders Denmark, Germany, and the Netherlands. It is a pelagic sea that has been modified through a system of dikes and causeways to become a semi-enclosed sea. The Wadden Sea has long been an important source of food and transport for adjacent communities. Eutrophication and nutrient pollution have extended for over one hundred years. Given its significance, the Joint Declaration on the Protection of the Wadden Sea was agreed to by the three adjacent countries in 1982, and the Trilateral Wadden Sea Plan was adopted in 1997. The United Nations Educational, Scientific and Cultural Organization (UNESCO) declared the Wadden Sea Area a biosphere reserve in 1986 and added the Dutch and German conservation areas to its list of World Heritage sites in 2009. Most of the Danish conservation area was added five years later.

The South China Sea is bordered by China to the north; the Philippines to the east; Malaysia, Singapore, Indonesia, and Brunei to the south; and Thailand, Cambodia, and Vietnam to the west. The sea lies above a drowned continental shelf that was above the sea during the prior ice ages, when the global sea level was hundreds of meters lower. The South China Sea contains many small islands, atolls, cays, shoals, reefs, and sandbars, many of which are uninhabited and below water at high tide. One of these archipelagos is the Spratly Islands. Jurisdiction over the South China Sea is disputed by the adjacent countries, which advocate different management regimes. The sea includes the second-most-used sea lane in the world, extensive proven oil reserves, and significant fishing reserves.

The South China Sea contains an estimated one-third of the world's marine biodiversity, making it an important ecosystem that includes mangrove forests as well as coral reefs and seagrass beds. Environmental threats include land-based pollution, overfishing, destructive fishing practices, the loss of mangrove forests, coral-reef degradation, and damage to seagrasses and wetlands. For example, most mangrove forests have been lost to shrimp farms, industrialization, or tourism.

Enclosed Seas

The Baltic Sea is a brackish inland sea. The sea resembles a riverbed with two tributaries, the Gulf of Finland and the Gulf of Bothnia, and can be understood as the estuary of all rivers flowing into it. As a result of these inflows, its salinity is lower than that of the Atlantic Ocean. Baltic Sea water flows are complex: brackish surface waters discharge into the North Sea, while heavier saline waters enter from the North Sea and move in the opposite direction. The Baltic Sea is stratified, with the denser saline waters remaining near the bottom, resulting in a shifting dead zone along up to a quarter of the seafloor, in which only bacteria flourishes. Freshwater and marine species reside in the Baltic Sea, but there is limited diversity. Algal blooms have occurred every summer for decades. Agricultural runoff from adjacent areas has encouraged these blooms and increased eutrophication. Climate change is likely to increase temperature and vertical stratification of the Baltic Sea, and the extent and duration of dead zones and algal blooms. Since the demise of the Soviet Union, all countries around the Baltic Sea are collaborating more extensively in the management of the sea and its watershed and the use of its resources.

The Mediterranean Sea is connected to the Atlantic Ocean and almost entirely enclosed by the Mediterranean region, which is characterized by hot, dry summers and cool, wet winters. Being enclosed affects many aspects of the Mediterranean Sea. For example, tides are very limited as a result of the narrow connection with the Atlantic Ocean. Evaporation exceeds precipitation and river flows, driving water flows. Evaporation is especially high in the eastern half, causing the water level to decrease and salinity to increase eastward.

The Mediterranean Sea has been very important throughout human history, with extensive agriculture and industrial development. The sea and the adjacent region are very vulnerable to climate change. Under the changing climate regime, sea-surface temperatures and salinity will increase. Depending on local characteristics, erosion, sediment deposition, drought, desertification, and flooding may intensify or shift. Coastal and beach tourism is an important source of income in the Mediterranean and south Atlantic regions, and the ongoing economic viability of these regions and local communities may hinge on the maintenance of the coastal and marine ecosystems on which tourism, as well as other activities such as commercial fishing, depend.

Semi-enclosed Seas

The Gulf of Mexico is a coastal sea, largely surrounded by the United States, Mexico, and Cuba. It is connected to the Atlantic Ocean through the Straits of Florida between Florida, Cuba, and the Bahamas, and to the Caribbean Sea through the Yucatán Channel between Mexico and Cuba. The outer margins of the continental shelves of Yucatán and Florida receive cooler, nutrient-rich waters from the deep by a process known as upwelling, which stimulates plankton growth and attracts fish, shrimp, and squid. The Gulf Stream is a strong, warm Atlantic current that originates in the Gulf of Mexico and warms the coasts of eastern North America and western Europe. A number of rivers flow into the gulf, including the Mississippi River and the Rio Grande in the north and the Grijalva and Usumacinta Rivers in the south. The land around the gulf is mostly low lying and includes barrier islands, marshes, swamps, and sandy beaches.

Human activities along the coast include fishing, hydrocarbon development, shipping, petrochemical processing and storage, military use, paper manufacture, and tourism. Major environmental threats are agricultural pollution and hydrocarbon activities. There are frequent red-tide algal blooms that kill fish and marine mammals and cause health problems in humans. The gulf also contains a hypoxic dead zone that is aggravated by agricultural pollutants. In November 2020, Environmental Health News (EHN) found that are 55,315 oil and natural gas wells in the Gulf of Mexico. Of those, 28,232, or 52.6 percent, are permanently abandoned while 3,444, or 6 percent are temporarily abandoned.

The 2010 Deepwater Horizon oil spill in the Gulf of Mexico was the largest accidental marine oil spill to date and caused extensive damage to marine and wildlife species, ecosystems, fishing, and tourism throughout the gulf. The oil spill illustrated the vulnerability of coastal seas to offshore activities when those activities are not properly regulated and implemented. Subsequently, US president Barack Obama issued a directive to improve the regulation of offshore drilling, but his successor, Donald Trump, signed an executive order in June 2018 repealing those regulations. However, in January 2021, President Joe Biden signed an executive order revoking Trump's order promoting offshore drilling. Biden's order reinstated protections, withdrawing some areas from oil and gas drilling.

Hudson Bay is a coastal sea in northern Canada that drains more than one-third of North America. The bay is shallow, with an average depth of 0.06 mile (100 meters), being 851 miles (1,370 kilometers) long and 656 miles (1,050 kilometers) wide. It connects to the Atlantic Ocean through the Hudson Strait, and to the Arctic Ocean's Foxe Basin through the Foxe Channel. Hudson Bay is part of the north Atlantic Ocean, but it is also viewed as the southern extension of the Arctic Ocean. It has a lower average salinity than ocean water because of its low rate of evaporation, the large volume of freshwater entering the bay, and the limited circulation of this brackish water with the Atlantic Ocean. This lower salinity layer also decreases the overall time that the bay is free of ice.

Because of its size, the Hudson Bay marine ecosystem has many coastal ecozones and varied habitats that are used year-round by local peoples and Arctic and sub-Arctic species, and seasonally by migratory fishes, marine mammals, and birds. There are few settlements and limited industrial activity in the bay, so it is mostly affected by global environmental and climate change.

The Gulf of Carpentaria is a warm, shallow sea in northern Australia that measures 115,830 square miles (300,000 square kilometers) and is fed by the flows of over twenty rivers. The gulf is enclosed on the west by Arnhem Land, in Australia's Northern Territory, and on the east by Cape York Peninsula, Queensland. The gulf floor is the continental shelf common to Australia and New Guinea, with a very low gradient. Prawn fishing developed rapidly in the gulf in the late 1960s. Sport fishing is also prevalent. Manganese and bauxite deposits are exploited around the gulf. As a result of these economic developments, settlement on the shores and islands of the gulf has increased from traditional indigenous populations to several thousand people. The Gulf of Carpentaria Commonwealth Marine Reserve, now the Gulf of Carpentaria Marine Park, was established over an area of 23,771 square kilometers (9,178 square miles) in 2012 to protect important turtle nesting and bird breeding areas, submerged coral reefs, and high concentrations of marine life.

Polar Seas

The Beaufort Sea is a complex marine ecosystem within the Arctic Ocean. The nearshore productivity of the sea has been an important resource, supporting human occupation for thousands of years. The circulation system is dominated by the Beaufort Gyre, a clockwise rotating surface current that results in large-scale movements of sea ice and surface waters. Ice usually begins to form in October. Most of the marine system is covered by ice until spring breakup, limiting biological production. The Mackenzie River significantly influences the Beaufort Sea; its freshwaters mix with marine waters, and a relatively fresh, mixed layer forms along the coastal areas and stratifies the ocean waters, allowing numerous species to use the area. Marine mammals, fish, and birds aggregate and use the adjacent area of the sea as migratory routes and seasonal breeding and feeding areas. Open-water areas in the ice known as polynyas, such as the Cape Bathurst polynya, are most important habitats and attract high densities of birds, benthic organisms, and marine mammals.

The Beaufort Sea is relatively pristine. There are large hydrocarbon reserves underlying the sea, but they have not yet been extensively developed. Renewable resources, such as fish and whales, are important for both subsistence uses by local peoples and tourism, but there are no commercial fisheries. Ecotourism is increasing in popularity. In Canada, the Beaufort Sea Large Ocean Management Area is one of five priority areas that have been identified for integrated ocean management planning. This area is located in the northwestern corner of Canada and includes the marine portion of the Inuvialuit Settlement Region, established under an indigenous land claim agreement.

The Ross Sea is one the most pristine marine ecosystems on Earth. Located adjacent to Antarctica and without human occupation, it has not been subject to pollution, invasive species, mining, or overfishing. The Ross Sea is the most productive stretch of water in the Southern Ocean and has high concentrations of wildlife, including large predatory fish, whales, seals, and penguins. Because of its pristine nature and abundance, the Ross Sea provides a unique opportunity to understand the functioning of healthy marine ecosystems. In 2016 the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), the international body that manages living marine resources around Antarctica, established a Ross Sea marine park of more than 1.5 million square kilometers (579,000 square miles)—the world's largest marine park—of which 1.1 million square kilometers is a "general protection zone" in which no fishing is allowed. Because its protections expire after thirty-five years, the Ross Sea marine park does not qualify as a "marine protected area" under International Union for Conservation of Nature and Natural Resources (IUCN) rules, which require permanent protection.

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