Sea

The sea normally refers to the Earth's ocean. The term sea may also refer to marginal seas, second-order sections of the oceanic sea (e.g. the Mediterranean Sea), or certain large, nearly landlocked bodies of water.

The seas have been an integral element for humans throughout history and culture. Humans harnessing and studying the seas have been recorded since ancient times, and evidenced well into prehistory, while its modern scientific study is called oceanography and maritime space is governed by the law of the sea, with admiralty law regulating human interactions at sea. The seas provide substantial supplies of food for humans, mainly fish, but also shellfish, mammals and seaweed, whether caught by fishermen or farmed underwater. Other human uses of the seas include trade, travel, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving. Many of these activities create marine pollution.

Definition

The sea is the interconnected system of all the Earth's oceanic waters, including the Atlantic, Pacific, Indian, Southern and Arctic Oceans.^[1] However, the word "sea" can also be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea. There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas or particularly as a mediterranean sea) or wholly (as inland seas) enclosed by land.^[2] However, an exception to this is the Sargasso Sea which has no coastline and lies within a circular current, the North Atlantic Gyre.^[3]: 90 Seas are generally larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake.^[4][a] The United Nations Convention on the Law of the Sea states that all of the ocean is "sea".^[8][9][b]

The law of the sea has at its center the definition of the boundaries of the ocean, clarifying its application in marginal seas. But what bodies of water other than the sea the law applies to is being crucially negotiated in the case of the Caspian Sea and its status as "sea", basically revolving around the issue of the Caspian Sea about either being factually an oceanic sea or only a saline body of water and therefore solely a sea in the sense of the common use of the word, like all other saltwater lakes called sea.

Human history

Exploration on the sea surface

Humans have travelled the seas since they first built sea-going craft. Mesopotamians were using bitumen to caulk their reed boats and, a little later, masted sails.^[11] By c. 3000 BC, Austronesians on Taiwan had begun spreading into maritime Southeast Asia.^[12] Subsequently, the Austronesian "Lapita" peoples displayed great feats of navigation, reaching out from the Bismarck Archipelago to as far away as Fiji, Tonga, and Samoa.^[13] Their descendants continued to travel thousands of miles between tiny islands on outrigger canoes,^[14] and in the process they found many new islands, including Hawaii, Easter Island (Rapa Nui), and New Zealand.^[15]

The Ancient Egyptians and Phoenicians explored the Mediterranean and Red Sea with the Egyptian Hannu reaching the Arabian Peninsula and the African Coast around 2750 BC.^[16] In the first millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean and the Black Sea.^[17] Around 500 BC, the Carthaginian navigator Hanno left a detailed periplus of an Atlantic journey that reached at least Senegal and possibly Mount Cameroon.^[18][19] In the early Mediaeval period, the Vikings crossed the North Atlantic and even reached the northeastern fringes of North America.^[20] Novgorodians had also been sailing the White Sea since the 13th century or before.^[21] Meanwhile, the seas along the eastern and southern Asian coast were used by Arab and Chinese traders.^[22] The Chinese Ming Dynasty had a fleet of 317 ships with 37,000 men under Zheng He in the early fifteenth century, sailing the Indian and Pacific Oceans.^{[3]: 12–13} In the late fifteenth century, Western European mariners started making longer voyages of exploration in search of trade. Bartolomeu Dias rounded the Cape of Good Hope in 1487 and Vasco da Gama reached India via the Cape in 1498. Christopher Columbus sailed from Cadiz in 1492, attempting to reach the eastern lands of India and Japan by the novel means of travelling westwards. He made landfall instead on an island in the Caribbean Sea and a few years later, the Venetian navigator John Cabot reached Newfoundland. The Italian Amerigo Vespucci, after whom America was named, explored the South American coastline in voyages made between 1497 and 1502, discovering the mouth of the Amazon River.^{[3]: 12–13} In 1519 the Portuguese navigator Ferdinand Magellan led the Spanish Magellan-Elcano expedition which would be the first to sail around the world.^{[3]: 12–13}

As for the history of navigational instrument, a compass was first used by the ancient Greeks and Chinese to show where north lies and the direction in which the ship is heading. The latitude (an angle which ranges from 0° at the equator to 90° at the poles) was determined by measuring the angle between the Sun, Moon or a specific star and the horizon by the use of an astrolabe, Jacob's staff or sextant. The longitude (a line on the globe joining the two poles) could only be calculated with an accurate chronometer to show the exact time difference between the ship and a fixed point such as the Greenwich Meridian. In 1759, John Harrison, a clockmaker, designed such an instrument and James Cook used it in his voyages of exploration.^[23] Nowadays, the Global Positioning System (GPS) using over thirty satellites enables accurate navigation worldwide.^[23]

With regards to maps that are vital for navigation, in the second century, Ptolemy mapped the whole known world from the "Fortunatae Insulae", Cape Verde or Canary Islands, eastward to the Gulf of Thailand. This map was used in 1492 when Christopher Columbus set out on his voyages of discovery.^[24] Subsequently, Gerardus Mercator made a practical map of the world in 1538, his map projection conveniently making rhumb lines straight.^{[3]: 12–13} By the eighteenth century better maps had been made and part of the objective of James Cook on his voyages was to further map the ocean. Scientific study has continued with the depth recordings of the Tuscarora, the oceanic research of the Challenger voyages (1872–1876), the work of the Scandinavian seamen Roald Amundsen and Fridtjof Nansen, the Michael Sars expedition in 1910, the German Meteor expedition of 1925, the Antarctic survey work of Discovery II in 1932, and others since.^[25] Furthermore, in 1921, the International Hydrographic Organization (IHO) was set up, and it constitutes the world authority on hydrographic surveying and nautical charting.^[26] A fourth edition draft was published in 1986 but so far several naming disputes (such as the one over the Sea of Japan) have prevented its ratification.

Deep sea exploration

Scientific oceanography began with the voyages of Captain James Cook from 1768 to 1779, describing the Pacific with unprecedented precision from 71 degrees South to 71 degrees North.^[3]: 14 John Harrison's chronometers supported Cook's accurate navigation and charting on two of these voyages, permanently improving the standard attainable for subsequent work.^[3]: 14 Other expeditions followed in the nineteenth century, from Russia, France, the Netherlands and the United States as well as Britain.^[3]: 15 On HMS Beagle, which provided Charles Darwin with ideas and materials for his 1859 book On the Origin of Species, the ship's captain, Robert FitzRoy, charted the seas and coasts and published his four-volume report of the ship's three voyages in 1839.^[3]: 15 Edward Forbes's 1854 book, Distribution of Marine Life argued that no life could exist below around 600 metres (2,000 feet). This was proven wrong by the British biologists W. B. Carpenter and C. Wyville Thomson, who in 1868 discovered life in deep water by dredging.^[3]: 15 Wyville Thompson became chief scientist on the Challenger expedition of 1872–1876, which effectively created the science of oceanography.^[3]: 15

On her 68,890-nautical-mile (127,580 km) journey round the globe, HMS Challenger discovered about 4,700 new marine species, and made 492 deep sea soundings, 133 bottom dredges, 151 open water trawls and 263 serial water temperature observations.^[27] In the southern Atlantic in 1898/1899, Carl Chun on the Valdivia brought many new life forms to the surface from depths of over 4,000 metres (13,000 ft). The first observations of deep-sea animals in their natural environment were made in 1930 by William Beebe and Otis Barton who descended to 434 metres (1,424 ft) in the spherical steel Bathysphere.^{[citation needed]} This was lowered by cable but by 1960 a self-powered submersible, Trieste developed by Jacques Piccard, took Piccard and Don Walsh to the deepest part of the Earth's oceans, the Mariana Trench in the Pacific, reaching a record depth of about 10,915 metres (35,810 ft),^[28] a feat not repeated until 2012 when James Cameron piloted the Deepsea Challenger to similar depths.^[29] An atmospheric diving suit can be worn for deep sea operations, with a new world record being set in 2006 when a US Navy diver descended to 2,000 feet (610 m) in one of these articulated, pressurized suits.^[30]

At great depths, no light penetrates through the water layers from above and the pressure is extreme. For deep sea exploration it is necessary to use specialist vehicles, either remotely operated underwater vehicles with lights and cameras or crewed submersibles. The battery-operated Mir submersibles have a three-person crew and can descend to 20,000 feet (6,100 m). They have viewing ports, 5,000-watt lights, video equipment and manipulator arms for collecting samples, placing probes or pushing the vehicle across the sea bed when the thrusters would stir up excessive sediment.^[31]

Bathymetry is the mapping and study of the topography of the ocean floor. Methods used for measuring the depth of the sea include single or multibeam echosounders, laser airborne depth sounders and the calculation of depths from satellite remote sensing data. This information is used for determining the routes of undersea cables and pipelines, for choosing suitable locations for siting oil rigs and offshore wind turbines and for identifying possible new fisheries.^[32]

Ongoing oceanographic research includes marine lifeforms, conservation, the marine environment, the chemistry of the ocean, the studying and modelling of climate dynamics, the air-sea boundary, weather patterns, ocean resources, renewable energy, waves and currents, and the design and development of new tools and technologies for investigating the deep.^[33] Whereas in the 1960s and 1970s, research could focus on taxonomy and basic biology, in the 2010s, attention has shifted to larger topics such as climate change.^[34] Researchers make use of satellite-based remote sensing for surface waters, with research ships, moored observatories and autonomous underwater vehicles to study and monitor all parts of the sea.^[35]

Industrial aspects

Seafood

Since prehistory, artisan fishing methods has been employed, which include rod and line, harpoons, skin diving, traps, throw nets and drag nets. Traditional fishing boats are powered by paddle, wind or outboard motors and operate in near-shore waters. The Food and Agriculture Organization is encouraging the development of local fisheries to provide food security to coastal communities and help alleviate poverty.^[36]

Fish and other fishery products are among the most widely consumed sources of protein and other essential nutrients.^[37] In 2009, 16.6% of the world's intake of animal protein and 6.5% of all protein consumed came from fish.^[37] In order to fulfill this need, coastal countries have exploited marine resources in their exclusive economic zone, although fishing vessels are increasingly venturing further afield to exploit stocks in international waters.^[38] In 2011, the total world production of fish, including aquaculture, was estimated to be 154 million tonnes, of which most was for human consumption.^[37] The harvesting of wild fish accounted for 90.4 million tonnes, while annually increasing aquaculture contributes the rest.^[37] The north west Pacific is by far the most productive area with 20.9 million tonnes (27 percent of the global marine catch) in 2010.^[37] In addition, the number of fishing vessels in 2010 reached 4.36 million, whereas the number of people employed in the primary sector of fish production in the same year amounted to 54.8 million.^[37]

Modern fishing vessels include fishing trawlers with a small crew, stern trawlers, purse seiners, long-line factory vessels and large factory ships which are designed to stay at sea for weeks, processing and freezing great quantities of fish. The equipment used to capture the fish may be purse seines, other seines, trawls, dredges, gillnets and long-lines and the fish species most frequently targeted are herring, cod, anchovy, tuna, flounder, mullet, squid and salmon. Overexploitation has become a serious concern; it does not only cause the depletion of fish stocks, but also substantially reduce the size of predatory fish populations.^[39] It has been estimated that "industrialized fisheries typically reduced community biomass by 80% within 15 years of exploitation."^[39] In order to avoid overexploitation, many countries have introduced quotas in their own waters.^[40] However, recovery efforts often entail substantial costs to local economies or food provision.

Trade

Maritime trade has existed for millennia. The Ptolemaic dynasty had developed trade with India using the Red Sea ports, and in the first millennium BC, the Arabs, Phoenicians, Israelites and Indians traded in luxury goods such as spices, gold, and precious stones.^[41] The Phoenicians were noted sea traders and under the Greeks and Romans, commerce continued to thrive. With the collapse of the Roman Empire, European trade dwindled but it continued to flourish among the kingdoms of Africa, the Middle East, India, China and southeastern Asia.^[42] From the 16th to the 19th centuries, over a period of 400 years, about 12–13 million Africans were shipped across the Atlantic to be sold as slaves in the Americas as part of the Atlantic slave trade.^{[43][44]: 194}

Large quantities of goods are transported by sea, especially across the Atlantic and around the Pacific Rim. A major trade route passes through the Pillars of Hercules, across the Mediterranean and the Suez Canal to the Indian Ocean and through the Straits of Malacca; much trade also passes through the English Channel.^[45] Shipping lanes are the routes on the open sea used by cargo vessels, traditionally making use of trade winds and currents. Over 60 percent of the world's container traffic is conveyed on the top twenty trade routes.^[46] Increased melting of Arctic ice since 2007 enables ships to travel the Northwest Passage for some weeks in summertime, avoiding the longer routes via the Suez Canal or the Panama Canal.^[47]

Seaborne trade carries more than US$4 trillion worth of goods each year.^[48] Bulk cargo in the form of liquids, powder or particles are carried loose in the holds of bulk carriers and include crude oil, grain, coal, ore, scrap metal, sand and gravel.^[49] Other cargo, such as manufactured goods, is usually transported within standard-sized, lockable containers, loaded on purpose-built container ships at dedicated terminals.^[50] Before the rise of containerization in the 1960s, these goods were loaded, transported and unloaded piecemeal as break-bulk cargo. Containerization greatly increased the efficiency and decreased the cost of moving goods by sea, and was a major factor leading to the rise of globalization and exponential increases in international trade in the mid-to-late 20th century.^[51]

Power generation

The sea offers a very large supply of energy carried by ocean waves, tides, salinity differences, and ocean temperature differences which can be harnessed to generate electricity.^[52] Forms of sustainable marine energy include tidal power, ocean thermal energy and wave power.^[52][53] Electricity power stations are often located on the coast or beside an estuary so that the sea can be used as a heat sink. A colder heat sink enables more efficient power generation, which is important for expensive nuclear power plants in particular.^[54]

Tidal power uses generators to produce electricity from tidal flows, sometimes by using a dam to store and then release seawater. The Rance barrage, 1 kilometre (0.62 mi) long, near St Malo in Brittany opened in 1967; it generates about 0.5 GW, but it has been followed by few similar schemes.^{[3]: 111–112}

The large and highly variable energy of waves gives them enormous destructive capability, making affordable and reliable wave machines problematic to develop. A small 2 MW commercial wave power plant, "Osprey", was built in Northern Scotland in 1995 about 300 metres (980 feet) offshore. It was soon damaged by waves, then destroyed by a storm.^[3]: 112

Offshore wind power is captured by wind turbines placed out at sea; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore.^[55] The first offshore wind farm was installed in Denmark in 1991,^[56] and the installed capacity of worldwide offshore wind farms reached 34 GW in 2020, mainly situated in Europe.^[57]

Resource extraction

The seabed contains large reserves of minerals which can be exploited by dredging. This has advantages over land-based mining in that equipment can be built at specialised shipyards and infrastructure costs are lower. Disadvantages include problems caused by waves and tides, the tendency for excavations to silt up and the washing away of spoil heaps. There is a risk of coastal erosion and environmental damage.^[58]

Seafloor massive sulphide deposits are potential sources of silver, gold, copper, lead and zinc and trace metals since their discovery in the 1960s. They form when geothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers". The ores are of high quality but prohibitively costly to extract.^[59]

There are large deposits of petroleum and natural gas, in rocks beneath the seabed. Offshore platforms and drilling rigs extract the oil or gas and store it for transport to land. Offshore oil and gas production can be difficult due to the remote, harsh environment.^[60] Drilling for oil in the sea has environmental impacts. Animals may be disorientated by seismic waves used to locate deposits, and there is debate as to whether this causes the beaching of whales.^[61] Toxic substances such as mercury, lead and arsenic may be released. The infrastructure may cause damage, and oil may be spilt.^[62]

Large quantities of methane clathrate exist on the seabed and in ocean sediment, of interest as a potential energy source.^[63] Also on the seabed are manganese nodules formed of layers of iron, manganese and other hydroxides around a core. In the Pacific, these may cover up to 30 percent of the deep ocean floor. The minerals precipitate from seawater and grow very slowly. Their commercial extraction for nickel was investigated in the 1970s but abandoned in favour of more convenient sources.^[64] In suitable locations, diamonds are gathered from the seafloor using suction hoses to bring gravel ashore. In deeper waters, mobile seafloor crawlers are used and the deposits are pumped to a vessel above. In Namibia, more diamonds are now collected from marine sources than by conventional methods on land.^[65]

The sea holds large quantities of valuable dissolved minerals.^[66] The most important, Salt for table and industrial use has been harvested by solar evaporation from shallow ponds since prehistoric times. Bromine, accumulated after being leached from the land, is economically recovered from the Dead Sea, where it occurs at 55,000 parts per million (ppm).^[67]

Desalination is the technique of removing salts from seawater to leave fresh water suitable for drinking or irrigation. The two main processing methods, vacuum distillation and reverse osmosis, use large quantities of energy. Desalination is normally only undertaken where fresh water from other sources is in short supply or energy is plentiful, as in the excess heat generated by power stations. The brine produced as a by-product contains some toxic materials and is returned to the sea.^[68]

Leisure

Use of the sea for leisure developed in the nineteenth century, and became a significant industry in the twentieth century.^[69] Maritime leisure activities are varied, and include self-organized trips cruising, yachting, powerboat racing^[70] and fishing;^[71] commercially organized voyages on cruise ships;^[72] and trips on smaller vessels for ecotourism such as whale watching and coastal birdwatching.^[73]

Sea bathing became the vogue in Europe in the 18th century after William Buchan advocated the practice for health reasons.^[74] Surfing is a sport in which a wave is ridden by a surfer, with or without a surfboard. Other marine water sports include kite surfing, where a power kite propels a rider on a board across the water,^[75] windsurfing, where the power is provided by a fixed, manoeuvrable sail^[76] and water skiing, where a powerboat is used to pull a skier.^[77]

Beneath the surface, freediving is necessarily restricted to shallow descents. Pearl divers can dive to 40 feet (12 m) with baskets to collect oysters.^[78] Human eyes are not adapted for use underwater but vision can be improved by wearing a diving mask. Other useful equipment includes fins and snorkels, and scuba equipment allows underwater breathing and hence a longer time can be spent beneath the surface.^[79] The depths that can be reached by divers and the length of time they can stay underwater is limited by the increase of pressure they experience as they descend and the need to prevent decompression sickness as they return to the surface. Recreational divers restrict themselves to depths of 100 feet (30 m) beyond which the danger of nitrogen narcosis increases. Deeper dives can be made with specialised equipment and training.^[79]

Political aspects

Law

"Freedom of the seas" is a principle in international law dating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought in international waters.^[80] Today, this concept is enshrined in the United Nations Convention on the Law of the Sea (UNCLOS), the third version of which came into force in 1994. Article 87(1) states: "The high seas are open to all states, whether coastal or land-locked." Article 87(1) (a) to (f) gives a non-exhaustive list of freedoms including navigation, overflight, the laying of submarine cables, building artificial islands, fishing and scientific research.^[80] The safety of shipping is regulated by the International Maritime Organization. Its objectives include developing and maintaining a regulatory framework for shipping, maritime safety, environmental concerns, legal matters, technical co-operation and maritime security.^[81]

UNCLOS defines various areas of water. "Internal waters" are on the landward side of a baseline and foreign vessels have no right of passage in these. "Territorial waters" extend to 12 nautical miles (22 kilometres; 14 miles) from the coastline and in these waters, the coastal state is free to set laws, regulate use and exploit any resource. A "contiguous zone" extending a further 12 nautical miles allows for hot pursuit of vessels suspected of infringing laws in four specific areas: customs, taxation, immigration and pollution. An "exclusive economic zone" extends for 200 nautical miles (370 kilometres; 230 miles) from the baseline. Within this area, the coastal nation has sole exploitation rights over all natural resources. The "continental shelf" is the natural prolongation of the land territory to the continental margin's outer edge, or 200 nautical miles from the coastal state's baseline, whichever is greater. Here the coastal nation has the exclusive right to harvest minerals and also living resources "attached" to the seabed.^[80]

Warfare

Control of the sea is important to the security of a maritime nation, and the naval blockade of a port can be used to cut off food and supplies in time of war. Battles have been fought on the sea for more than 3,000 years. In about 1210 B.C., Suppiluliuma II, the king of the Hittites, defeated and burned a fleet from Alashiya (modern Cyprus).^[82] In the decisive 480 B.C. Battle of Salamis, the Greek general Themistocles trapped the far larger fleet of the Persian king Xerxes in a narrow channel and attacked vigorously, destroying 200 Persian ships for the loss of 40 Greek vessels.^[83] At the end of the Age of Sail, the British Royal Navy, led by Horatio Nelson, broke the power of the combined French and Spanish fleets at the 1805 Battle of Trafalgar.^[84]

With steam and the industrial production of steel plate came greatly increased firepower in the shape of the dreadnought battleships armed with long-range guns. In 1905, the Japanese fleet decisively defeated the Russian fleet, which had travelled over 18,000 nautical miles (33,000 km), at the Battle of Tsushima.^[85] Dreadnoughts fought inconclusively in the First World War at the 1916 Battle of Jutland between the Royal Navy's Grand Fleet and the Imperial German Navy's High Seas Fleet.^[86] In the Second World War, the British victory at the 1940 Battle of Taranto showed that naval air power was sufficient to overcome the largest warships,^[87] foreshadowing the decisive sea-battles of the Pacific War including the Battles of the Coral Sea, Midway, the Philippine Sea, and the climactic Battle of Leyte Gulf, in all of which the dominant ships were aircraft carriers.^[88][89]

Submarines became important in naval warfare in World War I, when German submarines, known as U-boats, sank nearly 5,000 Allied merchant ships,^[90] including the RMS Lusitania, which helped to bring the United States into the war.^[91] In World War II, almost 3,000 Allied ships were sunk by U-boats attempting to block the flow of supplies to Britain,^[92] but the Allies broke the blockade in the Battle of the Atlantic, which lasted the whole length of the war, sinking 783 U-boats.^[93] Since 1960, several nations have maintained fleets of nuclear-powered ballistic missile submarines, vessels equipped to launch ballistic missiles with nuclear warheads from under the sea. Some of these are kept permanently on patrol.^[94][95]

Cultural aspects

The sea appears in human culture in contradictory ways, as both powerful but serene and as beautiful but dangerous.^[3]: 10 It has its place in literature, art, poetry, film, theatre, classical music, mythology and dream interpretation.^[96] The Ancients personified it, believing it to be under the control of a being who needed to be appeased, and symbolically, it has been perceived as a hostile environment populated by fantastic creatures; the Leviathan of the Bible,^[97] Scylla in Greek mythology,^[98] Isonade in Japanese mythology,^[99] and the kraken of late Norse mythology.^[100]

The sea and ships have been depicted in art ranging from simple drawings on the walls of huts in Lamu^[96] to seascapes by Joseph Turner. In Dutch Golden Age painting, artists such as Jan Porcellis, Hendrick Dubbels, Willem van de Velde the Elder and his son, and Ludolf Bakhuizen celebrated the sea and the Dutch navy at the peak of its military prowess.^[101][102] The Japanese artist Katsushika Hokusai created colour prints of the moods of the sea, including The Great Wave off Kanagawa.^[3]: 8

Music too has been inspired by the ocean, sometimes by composers who lived or worked near the shore and saw its many different aspects. Sea shanties, songs that were chanted by mariners to help them perform arduous tasks, have been woven into compositions and impressions in music have been created of calm waters, crashing waves and storms at sea.^{[103]: 4–8}

As a symbol, the sea has for centuries played a role in literature, poetry and dreams. Sometimes it is there just as a gentle background but often it introduces such themes as storm, shipwreck, battle, hardship, disaster, the dashing of hopes and death.^[103]: 45 In his epic poem the Odyssey, written in the eighth century BC,^[104] Homer describes the ten-year voyage of the Greek hero Odysseus who struggles to return home across the sea's many hazards after the war described in the Iliad.^[105] The sea is a recurring theme in the Haiku poems of the Japanese Edo period poet Matsuo Bashō (松尾 芭蕉) (1644–1694).^[106] In the works of psychiatrist Carl Jung, the sea symbolizes the personal and the collective unconscious in dream interpretation, the depths of the sea symbolizing the depths of the unconscious mind.^[107]

Indigenous peoples of the sea

Several nomadic indigenous groups in Maritime Southeast Asia live in boats and derive nearly all they need from the sea. The Moken people live on the coasts of Thailand and Burma and islands in the Andaman Sea.^[108] Some Sea Gypsies are accomplished free-divers, able to descend to depths of 30 metres (98 ft), though many are adopting a more settled, land-based way of life.^[109][110]

The indigenous peoples of the Arctic such as the Chukchi, Inuit, Inuvialuit and Yup'iit hunt marine mammals including seals and whales,^[111] and the Torres Strait Islanders of Australia include the Great Barrier Reef among their possessions. They live a traditional life on the islands involving hunting, fishing, gardening and trading with neighbouring peoples in Papua and mainland Aboriginal Australians.^[112]

Environmental impact aspects

The environmental issues that affect the sea can loosely be grouped into those that stem from marine pollution, from over exploitation and those that stem from climate change. They all impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.^[113] An overview of environmental issues is shown below:

• Marine pollution: Pathways of pollution include direct discharge, land runoff, ship pollution, atmospheric pollution and, potentially, deep sea mining. The types of marine pollution can be grouped as pollution from marine debris, plastic pollution, including microplastics, nutrient pollution, toxins and underwater noise.
• Over exploitation and biodiversity loss: overfishing, habitat loss, introduction of invasive species
• Effects of climate change on the sea: an increase in sea surface temperature as well as ocean temperatures at greater depths, more frequent marine heatwaves, a reduction in pH value, a rise in sea level from ocean warming and ice sheet melting, sea ice decline in the Arctic, increased upper ocean stratification, reductions in oxygen levels, increased contrasts in salinity (salty areas becoming saltier and fresher areas becoming less salty),^[114] changes to ocean currents including a weakening of the Atlantic meridional overturning circulation, and stronger tropical cyclones and monsoons.^[115]

Marine pollution

Many substances enter the sea as a result of human activities. Combustion products are transported in the air and deposited into the sea by precipitation. Industrial outflows and sewage contribute heavy metals, pesticides, PCBs, disinfectants, household cleaning products and other synthetic chemicals. These become concentrated in the surface film and in marine sediment, especially estuarine mud. The result of all this contamination is largely unknown because of the large number of substances involved and the lack of information on their biological effects.^[116] The heavy metals of greatest concern are copper, lead, mercury, cadmium and zinc which may be bio-accumulated by marine organisms and are passed up the food chain.^[117]

Much floating plastic rubbish does not biodegrade, instead disintegrating over time and eventually breaking down to the molecular level. Rigid plastics may float for years.^[118] In the centre of the Pacific gyre there is a permanent floating accumulation of mostly plastic waste^[119] and there is a similar garbage patch in the Atlantic.^[120] Foraging sea birds such as the albatross and petrel may mistake debris for food, and accumulate indigestible plastic in their digestive systems. Turtles and whales have been found with plastic bags and fishing line in their stomachs. Microplastics may sink, threatening filter feeders on the seabed.^[121]

Most oil pollution in the sea comes from cities and industry.^[122] Oil is dangerous for marine animals. It can clog the feathers of sea birds, reducing their insulating effect and the birds' buoyancy, and be ingested when they preen themselves in an attempt to remove the contaminant. Marine mammals are less seriously affected but may be chilled through the removal of their insulation, blinded, dehydrated or poisoned. Benthic invertebrates are swamped when the oil sinks, fish are poisoned and the food chain is disrupted. In the short term, oil spills result in wildlife populations being decreased and unbalanced, leisure activities being affected and the livelihoods of people dependent on the sea being devastated.^[123] The marine environment has self-cleansing properties and naturally occurring bacteria will act over time to remove oil from the sea. In the Gulf of Mexico, where oil-eating bacteria are already present, they take only a few days to consume spilt oil.^[124]

Run-off of fertilisers from agricultural land is a major source of pollution in some areas and the discharge of raw sewage has a similar effect. The extra nutrients provided by these sources can cause excessive plant growth. Nitrogen is often the limiting factor in marine systems, and with added nitrogen, algal blooms and red tides can lower the oxygen level of the water and kill marine animals. Such events have created dead zones in the Baltic Sea and the Gulf of Mexico.^[122] Some algal blooms are caused by cyanobacteria that make shellfish that filter feed on them toxic, harming animals like sea otters.^[125] Nuclear facilities too can pollute. The Irish Sea was contaminated by radioactive caesium-137 from the former Sellafield nuclear fuel processing plant^[126] and nuclear accidents may also cause radioactive material to seep into the sea, as did the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011.^[127]

The dumping of waste (including oil, noxious liquids, sewage and garbage) at sea is governed by international law. The London Convention (1972) is a United Nations agreement to control ocean dumping which had been ratified by 89 countries by 8 June 2012.^[128] MARPOL 73/78 is a convention to minimize pollution of the seas by ships. By May 2013, 152 maritime nations had ratified MARPOL.^[129]

See also

• Ocean surface topography – Shape of the ocean surface relative to the geoid
• List of seas
• Bay
• Gulf

Notes

References

External links

• National Oceanic and Atmospheric Administration Archived 24 April 2013 at the Wayback Machine

This page was last edited on 5 December 2023, at 12:06