Asea is a largebody ofsalt water. There areparticular seas andthe sea.The sea commonly refers to theocean, the interconnected body ofseawaters that spans most of Earth. Particular seas are eithermarginal seas, second-order sections of the oceanic sea (e.g. theMediterranean Sea), or certain large, nearly landlocked bodies of water.
The ocean moderates Earth'sclimate and has important roles in thewater,carbon, andnitrogen cycles. The surface of the water interacts with the atmosphere, exchanging properties such asparticles and temperature, as well ascurrents.Surface currents are the water currents that are produced by theatmosphere's currents and itswinds blowing over the surface of the water, producingwind waves, setting up throughdrag slow but stable circulations of water, as in the case of the ocean sustainingdeep-seaocean currents. Deep-sea currents, known together as theglobal conveyor belt, carry cold water from near the poles to every ocean and significantly influence Earth's climate.Tides, the generally twice-daily rise and fall ofsea levels, are caused by Earth's rotation and the gravitational effects of theMoon and, to a lesser extent, of theSun. Tides may have a very highrange inbays orestuaries.Submarine earthquakes arising fromtectonic plate movements under the oceans can lead to destructivetsunamis, as can volcanoes, hugelandslides, or the impact of largemeteorites.
Thelaw of the sea has at its centre the definition of theboundaries of the ocean, clarifying its application inmarginal seas. But what bodies of water other than the sea the law applies to is being crucially negotiated in the case of theCaspian Sea and its status as "sea", basically revolving around the issue of the Caspian Sea about either being factually anoceanic 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 othersaltwater lakes called sea.[citation needed]
A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand (‰ or per mil), and the open ocean has about 35 grams (1.2 oz) solids per litre, a salinity of 35 ‰. The Mediterranean Sea is slightly higher at 38 ‰,[22] while the salinity of the northern Red Sea can reach 41‰.[23] In contrast, some landlockedhypersaline lakes have a much higher salinity, for example, theDead Sea has 300 grams (11 oz) dissolved solids per litre (300 ‰).
While the constituents of table salt (sodium andchloride) make up about 85 percent of the solids in solution, there are also other metal ions such asmagnesium andcalcium, and negative ions includingsulphate,carbonate, andbromide. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout the world's oceans.[24][25] Seawater is too saline for humans to drink safely, as thekidneys cannot excrete urine as salty as seawater.[26]
Although the amount of salt in the ocean remains relatively constant within the scale of millions of years, various factors affect the salinity of a body of water.[27] Evaporation and by-product of ice formation (known as "brine rejection") increase salinity, whereasprecipitation, sea ice melt, and runoff from land reduce it.[27] TheBaltic Sea, for example, has many rivers flowing into it, and thus the sea could be considered asbrackish.[28] Meanwhile, theRed Sea is very salty due to its high evaporation rate.[29]
Sea temperature depends on the amount of solar radiation falling on its surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature inequilibrium with the sea ice is about −2 °C (28 °F). There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. Deep seawater has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.[30]
Seawater with a typical salinity of 35 ‰[31] has a freezing point of about −1.8 °C (28.8 °F).[32] When its temperature becomes low enough,ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known asfrazil. In calm conditions, this freezes into a thin flat sheet known asnilas, which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to formfloes. In the process of freezing, salt water and air are trapped between the ice crystals. Nilas may have a salinity of 12–15 ‰, but by the time thesea ice is one year old, this falls to 4–6 ‰.[33]
Seawater is slightlyalkaline and had an averagepH of about 8.2 over the past 300 million years.[34] More recently,climate change has resulted in an increase of thecarbon dioxide content of the atmosphere; about 30–40% of the added CO2 is absorbed by the oceans, formingcarbonic acid and lowering the pH (now below 8.1[34]) through a process calledocean acidification.[35][36][37] The extent of further ocean chemistry changes, including ocean pH, will depend onclimate change mitigation efforts taken by nations and their governments.[38]
The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, includingphytoplankton, with somevascular plants such asseagrasses. In daylight, thephotosynthetic activity of these plants produces oxygen, which dissolves in the seawater and is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea, where insufficient light penetrates for plants to grow, there is very little dissolved oxygen. In its absence, organic material is broken down byanaerobic bacteria producinghydrogen sulphide.[39]
Climate change is likely to reduce levels of oxygen in surface waters since the solubility of oxygen in water falls at higher temperatures.[40]Ocean deoxygenation is projected to increasehypoxia by 10%, and triple suboxic waters (oxygen concentrations 98% less than the mean surface concentrations), for each 1 °C of upper-ocean warming.[41]
The amount of light that penetrates the sea depends on the angle of the sun, the weather conditions and theturbidity of the water. Much light gets reflected at the surface, and red light gets absorbed in the top few metres. Yellow and green light reach greater depths, and blue and violet light may penetrate as deep as 1,000 metres (3,300 ft). There is insufficient light for photosynthesis and plant growth beyond a depth of about 200 metres (660 ft).[42]
Over most of geologic time, the sea level has been higher than it is today.[3]: 74 The main factor affecting sea level over time is the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.[43] At thelast glacial maximum, some 20,000 years ago, the sea level was about 125 metres (410 ft) lower than in present times (2025).[44]
For at least the last 100 years,sea level has been rising at an average rate of about 1.8 millimetres (0.071 in) per year.[45] Most of this rise can be attributed to an increase in the temperature of the sea due toclimate change, and the resulting slight thermal expansion of the upper 500 metres (1,600 ft) of water. Additional contributions, as much as one quarter of the total, come from water sources on land, such asmelting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.[46]
Wind blowing over the surface of a body of water formswaves that are perpendicular to the direction of the wind. The friction between air and water caused by a gentle breeze on a pond causesripples to form. A strong blow over the ocean causes larger waves as the moving air pushes against the raised ridges of water. The waves reach their maximum height when the rate at which they are travelling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in the Southern Hemisphere in theRoaring Forties, long, organised masses of water calledswell roll across the ocean.[3]: 83–84 [47][48][d] If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land. The size of the waves depends on thefetch, the distance that the wind has blown over the water and the strength and duration of that wind. When waves meet others coming from different directions, interference between the two can produce broken, irregular seas.[47]Constructive interference can cause individual (unexpected)rogue waves much higher than normal.[49] Most waves are less than 3 m (10 ft) high[49] and it is not unusual for strong storms to double or triple that height;[50]offshore construction such aswind farms andoil platforms usemetocean statistics from measurements in computing the wave forces (due to for instance thehundred-year wave) they are designed against.[51] Rogue waves, however, have been documented at heights above 25 meters (82 ft).[52][53]
The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength. The wave is pushed across the surface of the sea by the wind, but this represents a transfer of energy and not a horizontal movement of water. As waves approach land andmove into shallow water, they change their behavior. If approaching at an angle, waves may bend (refraction) or wrap rocks and headlands (diffraction). When the wave reaches a point where its deepest oscillations of the water contact theseabed, they begin to slow down. This pulls the crests closer together and increases thewaves' height, which is calledwave shoaling. When the ratio of the wave's height to the water depth increases above a certain limit, it "breaks", toppling over in a mass of foaming water.[49] This rushes in a sheet up the beach before retreating into the sea under the influence of gravity.[47]
A tsunami is an unusual form of wave caused by an infrequent powerful event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption or a collapse of land into the sea. These events can temporarily lift or lower the surface of the sea in the affected area, usually by a few feet. The potential energy of the displaced seawater is turned into kinetic energy, creating a shallow wave, a tsunami, radiating outwards at a velocity proportional to the square root of the depth of the water and which therefore travels much faster in the open ocean than on a continental shelf.[54] In the deep open sea, tsunamis have wavelengths of around 80 to 300 miles (130 to 480 km), travel at speeds of over 600 miles per hour (970 km/h)[55] and usually have a height of less than three feet, so they often pass unnoticed at this stage.[56] In contrast, ocean surface waves caused by winds have wavelengths of a few hundred feet, travel at up to 65 miles per hour (105 km/h) and are up to 45 feet (14 metres) high.[56]
As a tsunamimoves into shallower water its speed decreases, its wavelength shortens and its amplitude increases enormously,[56] behaving in the same way as a wind-generated wave in shallow water but on a vastly greater scale. Either the trough or the crest of a tsunami can arrive at the coast first.[54] In the former case, the sea draws back and leaves subtidal areas close to the shore exposed which provides a useful warning for people on land.[57] When the crest arrives, it does not usually break but rushes inland, flooding all in its path. Much of the destruction may be caused by the flood water draining back into the sea after the tsunami has struck, dragging debris and people with it. Often several tsunami are caused by a single geological event and arrive at intervals of between eight minutes and two hours. The first wave to arrive on shore may not be the biggest or most destructive.[54]
Wind blowing over the surface of the sea causesfriction at the interface between air and sea. Not only does this cause waves to form, but it also makes the surface seawater move in the same direction as the wind. Although winds are variable, in any one place they predominantly blow from a single direction and thus a surface current can be formed. Westerly winds are most frequent in the mid-latitudes while easterlies dominate the tropics.[58] When water moves in this way, other water flows in to fill the gap and a circular movement of surface currents known as agyre is formed. There are five main gyres in the world's oceans: two in the Pacific, two in the Atlantic and one in the Indian Ocean. Other smaller gyres are found in lesser seas and a single gyre flows aroundAntarctica. These gyres have followed the same routes for millennia, guided by thetopography of the land, the wind direction and theCoriolis effect. The surface currents flow in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The water moving away from the equator is warm, and that flowing in the reverse direction has lost most of its heat. These currents tend to moderate the Earth's climate, cooling the equatorial region and warming regions at higher latitudes.[59] Global climate andweather forecasts are powerfully affected by the world ocean, soglobal climate modelling makes use ofocean circulation models as well as models of other major components such as theatmosphere, land surfaces, aerosols and sea ice.[60] Ocean models make use of a branch of physics,geophysical fluid dynamics, that describes the large-scale flow of fluids such as seawater.[61]
The global conveyor belt shown in blue with warmer surface currents in red
Surface currents only affect the top few hundred metres of the sea, but there are also large-scale flows in the ocean depths caused by the movement of deep water masses. A main deep ocean current flows through all the world's oceans and is known as thethermohaline circulation or global conveyor belt. This movement is slow and is driven by differences in density of the water caused by variations in salinity and temperature.[62] At high latitudes the water is chilled by the low atmospheric temperature and becomes saltier as sea ice crystallizes out. Both these factors make it denser, and the water sinks. From the deep sea near Greenland, such water flows southwards between the continental landmasses on either side of the Atlantic. When it reaches the Antarctic, it is joined by further masses of cold, sinking water and flows eastwards. It then splits into two streams that move northwards into the Indian and Pacific Oceans. Here it is gradually warmed, becomes less dense, rises towards the surface and loops back on itself. It takes a thousand years for this circulation pattern to be completed.[59]
Besides gyres, there are temporary surface currents that occur under specific conditions. When waves meet a shore at an angle, alongshore current is created as water is pushed along parallel to the coastline. The water swirls up onto the beach at right angles to the approaching waves but drains away straight down the slope under the effect of gravity. The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current.[63] These currents can shift great volumes of sand or pebbles, createspits and make beaches disappear and water channels silt up.[59] Arip current can occur when water piles up near the shore from advancing waves and is funnelled out to sea through a channel in the seabed. It may occur at a gap in asandbar or near a man-made structure such as agroyne. These strong currents can have a velocity of 3 ft (0.9 m) per second, can form at different places at different stages of the tide and can carry away unwary bathers.[64] Temporary upwelling currents occur when the wind pushes water away from the land and deeper water rises to replace it. This cold water is often rich in nutrients and creates blooms of phytoplankton and a great increase in the productivity of the sea.[59]
High tides (blue) at the nearest and furthest points of the Earth from the Moon
Tides are the regular rise and fall in water level experienced by seas and oceans in response to thegravitational influences of the Moon and the Sun, and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level. As the water recedes, it uncovers more and more of theforeshore, also known as the intertidal zone. The difference in height between the high tide and low tide is known as thetidal range or tidal amplitude.[65][66]
Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes. This is half the 24 hours and 50 minute period that it takes for the Earth to make a complete revolution and return the Moon to its previous position relative to an observer. The Moon's mass is some 27 million times smaller than the Sun, but it is 400 times closer to the Earth (and the strength of gravity is proportional to the square of the distance between the centres of mass of the objects in question, further compensating for the smaller mass of the Moon).[67]Tidal force or tide-raising force decreases rapidly with distance, so the moon has more than twice as great an effect on tides as the Sun.[67] A bulge is formed in the ocean at the place where the Earth is closest to the Moon because it is also where the effect of the Moon's gravity is stronger. On the opposite side of the Earth, the lunar force is at its weakest and this causes another bulge to form. As the Moon rotates around the Earth, so do these ocean bulges move around the Earth. The gravitational attraction of the Sun is also working on the seas, but its effect on tides is less powerful than that of the Moon, and when the Sun, Moon and Earth are all aligned (full moon and new moon), the combined effect results in the high "spring tides". In contrast, when the Sun is at 90° from the Moon as viewed from Earth, the combined gravitational effect on tides is less causing the lower "neap tides".[65]
Astorm surge can occur when high winds pile water up against the coast in a shallow area and this, coupled with a low-pressure system, can raise the surface of the sea at high tide dramatically.
The Earth is composed of a magnetic centralcore, a mostly liquidmantle and a hard rigid outer shell (orlithosphere), which is composed of the Earth's rockycrust and the deeper mostly solid outer layer of the mantle. On land the crust is known as thecontinental crust while under the sea it is known as theoceanic crust. The latter is composed of relatively densebasalt and is some five to ten kilometres (three to six miles) thick. The relatively thin lithosphere floats on the weaker and hotter mantle below and is fractured into a number oftectonic plates.[68] In mid-ocean, magma is constantly being thrust through the seabed between adjoining plates to formmid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart. Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known assubduction. Deeptrenches are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat is produced andmagma is forced up creating underwater mountains, some of which may form chains of volcanic islands near to deep trenches. Near some of the boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed. As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity.[69][70]
The Earth's deepest trench is theMariana Trench which extends for about 2,500 kilometres (1,600 miles) across the seabed. It is near theMariana Islands, a volcanicarchipelago in the West Pacific. Its deepest point is 10.994 kilometres (nearly 7 miles) below the surface of the sea.[71]
Seaside inBudelli, Italy. Budelli beach is famous for the color of its sand, which is pink due to the presence of fragments of a microorganism calledMiniacina miniacea.[72]Praia da Marinha inAlgarve, Portugal
The zone where land meets sea is known as thecoast and the part between the lowest spring tides and the upper limit reached by splashing waves is theshore. Abeach is the accumulation of sand orshingle on the shore.[73] Aheadland is a point of land jutting out into the sea and a largerpromontory is known as acape. The indentation of a coastline, especially between two headlands, is abay, a small bay with a narrow inlet is acove and a large bay may be referred to as agulf.[74] Coastlines are influenced by several factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence. Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as theswash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normalweathering processes such as the action of frost follows, causing further destruction. Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion.[73]
Material worn from the margins of the land eventually ends up in the sea. Here it is subject toattrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on the seabed causingdeltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents.[73] Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of the land by the building ofbreakwaters,seawalls,dykes and levees and other sea defences. For instance, theThames Barrier is designed to protect London from a storm surge,[75] while the failure of the dykes and levees aroundNew Orleans duringHurricane Katrina created ahumanitarian crisis in the United States.
The sea plays a part in thewater or hydrological cycle, in which waterevaporates from the ocean, travels through the atmosphere as vapour,condenses, falls asrain or snow, thereby sustaining life on land, and largely returns to the sea.[76] Even in theAtacama Desert, where little rain ever falls, dense clouds of fog known as thecamanchaca blow in from the sea and support plant life.[77]
In central Asia and other large land masses, there areendorheic basins which have no outlet to the sea, separated from the ocean by mountains or other natural geologic features that prevent the water draining away. TheCaspian Sea is the largest one of these. Its main inflow is from theRiver Volga, there is no outflow and the evaporation of water makes it saline as dissolved minerals accumulate. TheAral Sea in Kazakhstan and Uzbekistan, andPyramid Lake in the western United States are further examples of large, inland saline water-bodies without drainage. Some endorheic lakes are less salty, but all are sensitive to variations in the quality of the inflowing water.[78]
Oceans contain the greatest quantity of actively cycled carbon in the world and are second only to thelithosphere in the amount of carbon they store.[79] The oceans' surface layer holds large amounts ofdissolved organic carbon that is exchanged rapidly with the atmosphere. The deep layer's concentration ofdissolved inorganic carbon is about 15 percent higher than that of the surface layer[80] and it remains there for much longer periods of time.[81]Thermohaline circulation exchanges carbon between these two layers.[79]
Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted intocarbonic acid,carbonate, andbicarbonate:[82]
CO2(gas) ⇌ CO2(aq)
CO2(aq) + H2O ⇌ H2CO3
H2CO3 ⇌ HCO3− + H+
HCO3− ⇌ CO32− + H+
It can also enter through rivers as dissolved organic carbon and is converted by photosynthetic organisms into organic carbon. This can either be exchanged throughout the food chain or precipitated into the deeper, more carbon-rich layers as dead soft tissue or in shells and bones ascalcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or being returned to surface waters through thermohaline circulation.[81]
The oceans are home to a diverse collection of life forms that use it as a habitat. Since sunlight illuminates only the upper layers, the major part of the ocean exists in permanent darkness. As the different depth and temperature zones each provide habitat for a unique set of species, the marine environment as a whole encompasses an immense diversity of life.[83] Marine habitats range from surface water to the deepestoceanic trenches, including coral reefs,kelp forests,seagrass meadows,tidepools, muddy, sandy and rocky seabeds, and the openpelagic zone. The organisms living in the sea range fromwhales 30 metres (98 feet) long to microscopic phytoplankton andzooplankton, fungi, and bacteria. Marine life plays an important part in thecarbon cycle as photosynthetic organisms convert dissolved carbon dioxide into organic carbon and it is economically important to humans for providingfish for use as food.[84][85]: 204–229
Life may have originated in the sea and all themajor groups of animals are represented there. Scientists differ as to precisely where in the sea life arose: theMiller-Urey experiments suggested a dilute chemical "soup" in open water, but more recent suggestions include volcanic hot springs, fine-grained clay sediments, or deep-sea "black smoker" vents, all of which would have provided protection from damaging ultraviolet radiation which was not blocked by the early Earth's atmosphere.[3]: 138–140
Marine habitats can be divided horizontally into coastal and open ocean habitats. Coastal habitats extend from the shoreline to the edge of thecontinental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only 7 percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf. Alternatively, marine habitats can be divided vertically intopelagic (open water),demersal (just above the seabed) andbenthic (sea bottom) habitats. A third division is bylatitude: from polar seas with ice shelves, sea ice and icebergs, to temperate and tropical waters.[3]: 150–151
Coral reefs, the so-called "rainforests of the sea", occupy less than 0.1 percent of the world's ocean surface, yet their ecosystems include 25 percent of all marine species.[86] The best-known aretropical coral reefs such as Australia'sGreat Barrier Reef, but cold water reefs harbour a wide array of species including corals (only six of which contribute to reef formation).[3]: 204–207 [87]
Marineprimary producers – plants and microscopic organisms in the plankton – are widespread and very essential for the ecosystem. It has been estimated that half of the world's oxygen is produced by phytoplankton.[88][89] About 45 percent of the sea'sprimary production of living material is contributed bydiatoms.[90] Much larger algae, commonly known asseaweeds, are important locally;Sargassum forms floating drifts, whilekelp form seabed forests.[85]: 246–255 Flowering plants in the form of seagrasses grow in "meadows" in sandy shallows,[91]mangroves line the coast in tropical and subtropical regions[92] andsalt-tolerant plants thrive in regularly inundatedsalt marshes.[93] All of these habitats are able to sequester large quantities of carbon and support abiodiverse range of larger and smaller animal life.[94]
Light is only able to penetrate the top 200 metres (660 ft) so this is the only part of the sea where plants can grow.[42] The surface layers are often deficient in biologically active nitrogen compounds. The marinenitrogen cycle consists of complex microbial transformations which include thefixation of nitrogen, its assimilation,nitrification,anammox and denitrification.[95] Some of these processes take place in deep water so that where there is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present, plant growth is higher. This means that the most productive areas, rich in plankton and therefore also in fish, are mainly coastal.[3]: 160–163
There is a broader spectrum of higher animaltaxa in the sea than on land, many marine species have yet to be discovered and the number known to science is expanding annually.[96] Somevertebrates such asseabirds,seals andsea turtles return to the land to breed but fish,cetaceans andsea snakes have a completely aquatic lifestyle and many invertebratephyla are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats.[96] One of these is the surface film which, even though tossed about by the movement of waves, provides a rich environment and is home to bacteria,fungi,microalgae,protozoa, fish eggs and various larvae.[97]
The pelagic zone containsmacro- andmicrofauna and myriad zooplankton which drift with the currents. Most of the smallest organisms are the larvae of fish andmarine invertebrates which liberateeggs in vast numbers because the chance of any one embryo surviving to maturity is so minute.[98] The zooplankton feed on phytoplankton and on each other and form a basic part of the complex food chain that extends through variously sized fish and othernektonic organisms to largesquid,sharks,porpoises,dolphins andwhales.[99] Some marine creatures make large migrations, either to other regions of the ocean on a seasonal basis or vertical migrations daily, often ascending to feed at night and descending to safety by day.[100] Ships can introduce or spreadinvasive species through the discharge ofballast water or the transport of organisms that have accumulated as part of thefouling community on the hulls of vessels.[101]
The demersal zone supports many animals that feed on benthic organisms or seek protection from predators and the seabed provides a range of habitats on or under the surface of thesubstrate which are used by creatures adapted to these conditions. The tidal zone with its periodic exposure to the dehydrating air is home tobarnacles,molluscs andcrustaceans. Theneritic zone has many organisms that need light to flourish. Here, among algal-encrusted rocks livesponges,echinoderms,polychaete worms,sea anemones and other invertebrates. Corals often contain photosyntheticsymbionts and live in shallow waters where light penetrates. The extensive calcareous skeletons they extrude build up into coral reefs which are an important feature of the seabed. These provide abiodiverse habitat for reef-dwelling organisms. There is less sea life on the floor of deeper seas but marine life also flourishes aroundseamounts that rise from the depths, where fish and other animals congregate to spawn and feed. Close to the seabed livedemersal fish that feed largely on pelagic organisms orbenthic invertebrates.[102] Exploration of the deep sea by submersibles revealed a new world of creatures living on the seabed that scientists had not previously known to exist. Some like thedetrivores rely on organic material falling to the ocean floor. Others cluster round deep seahydrothermal vents where mineral-rich flows of water emerge from the seabed, supporting communities whose primary producers are sulphide-oxidisingchemoautotrophic bacteria, and whose consumers include specialised bivalves, sea anemones, barnacles, crabs, worms and fish, often found nowhere else.[3]: 212 A dead whale sinking to the bottom of the ocean provides food for an assembly of organisms which similarly rely largely on the actions of sulphur-reducing bacteria. Such places support unique biomes where many new microbes and other lifeforms have been discovered.[103]
TheAncient Egyptians andPhoenicians explored theMediterranean and Red Sea with the EgyptianHannu reaching theArabian Peninsula and the African Coast around 2750 BC.[109] In the first millennium BC, Phoenicians and Greeks established colonies throughout the Mediterranean and theBlack Sea.[110] Around 500 BC, theCarthaginian navigatorHanno left a detailedperiplus of an Atlantic journey that reached at leastSenegal and possiblyMount Cameroon.[111][112] In theearly Medieval period, theVikings crossed the North Atlantic and even reached the northeastern fringes of North America.[113]Novgorodians had also been sailing theWhite Sea since the 13th century or before.[114] Meanwhile, the seas along the eastern and southern Asian coast were used by Arab and Chinese traders.[115] The ChineseMing Dynasty had a fleet of 317 ships with 37,000 men underZheng 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 theCape of Good Hope in 1487 andVasco da Gama reached India via the Cape in 1498.Christopher Columbus sailed fromCadiz 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 theCaribbean Sea and a few years later, the Venetian navigatorJohn Cabot reachedNewfoundland. The ItalianAmerigo Vespucci, after whom America was named, explored the South American coastline in voyages made between 1497 and 1502, discovering the mouth of theAmazon River.[3]: 12–13 In 1519 thePortuguese navigatorFerdinand Magellan led the SpanishMagellan-Elcano expedition which would be the first to sail around the world.[3]: 12–13
Gerardus Mercator's 1569 world map. The coastline of the old world is quite accurately depicted, unlike that of the Americas. Regions in high latitudes (Arctic, Antarctic) are greatly enlarged on thisprojection.
As for the history ofnavigational instrument, acompass 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 anastrolabe,Jacob's staff orsextant. Thelongitude (a line on the globe joining the two poles) could only be calculated with an accuratechronometer to show the exact time difference between the ship and a fixed point such as theGreenwich Meridian. In 1759,John Harrison, a clockmaker, designed such an instrument and James Cook used it in his voyages of exploration.[116] Nowadays, theGlobal Positioning System (GPS) using over thirty satellites enables accurate navigation worldwide.[116]
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 orCanary Islands, eastward to theGulf of Thailand. This map was used in 1492 when Christopher Columbus set out on his voyages of discovery.[117] Subsequently,Gerardus Mercator made a practical map of the world in 1538, his map projection conveniently makingrhumb lines straight.[3]: 12–13 By the eighteenth century better maps had been made and part of the objective ofJames Cook on his voyages was to further map the ocean. Scientific study has continued with the depth recordings of theTuscarora, the oceanic research of theChallenger voyages (1872–1876), the work of the Scandinavian seamenRoald Amundsen andFridtjof Nansen, theMichael Sars expedition in 1910, theGerman Meteor expedition of 1925, the Antarctic survey work ofDiscovery II in 1932, and others since.[19] Furthermore, in 1921, theInternational Hydrographic Organization (IHO) was set up, and it constitutes the world authority onhydrographic surveying and nautical charting.[118] A fourth edition draft was published in 1986 but so far several naming disputes (such as the one over theSea of Japan) have prevented its ratification.
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 OnHMSBeagle, which providedCharles Darwin with ideas and materials for his 1859 bookOn 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 biologistsW. B. Carpenter andC. 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.[119] In the southern Atlantic in 1898/1899,Carl Chun on theValdivia 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 byWilliam Beebe andOtis Barton who descended to 434 metres (1,424 ft) in the spherical steelBathysphere.[120] This was lowered by cable but by 1960 a self-powered submersible,Trieste developed byJacques Piccard, took Piccard andDon Walsh to the deepest part of theEarth's oceans, theMariana Trench in the Pacific, reaching a record depth of about 10,915 metres (35,810 ft),[121] a feat not repeated until 2012 whenJames Cameron piloted theDeepsea Challenger to similar depths.[122] Anatmospheric 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.[123]
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, eitherremotely operated underwater vehicles with lights and cameras or crewedsubmersibles. The battery-operatedMir 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.[124]
Bathymetry is the mapping and study of thetopography of the ocean floor. Methods used for measuring the depth of the sea include single or multibeamechosounders,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.[125]
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.[126] 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.[127] Researchers make use of satellite-basedremote sensing for surface waters, with research ships, moored observatories and autonomous underwater vehicles to study and monitor all parts of the sea.[128]
"Freedom of the seas" is a principle ininternational law dating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought ininternational waters.[129] 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 allstates, whether coastal orland-locked." Article 87(1) (a) to (f) gives a non-exhaustive list of freedoms including navigation, overflight, the laying ofsubmarine cables, building artificial islands, fishing and scientific research.[129] The safety of shipping is regulated by theInternational 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.[130]
UNCLOS defines various areas of water. "Internal waters" are on the landward side of abaseline 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 forhot 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 thenatural prolongation of the land territory to thecontinental 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.[129]
Control of the sea is important to the security of a maritime nation, and the navalblockade 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 theHittites, defeated and burned a fleet fromAlashiya (modernCyprus).[131] In the decisive 480 B.C.Battle of Salamis, the Greek generalThemistocles trapped the far larger fleet of the Persian kingXerxes in a narrow channel and attacked vigorously, destroying 200 Persian ships for the loss of 40 Greek vessels.[132] At the end of theAge of Sail, the British Royal Navy, led byHoratio Nelson, broke the power of the combined French and Spanish fleets at the 1805Battle of Trafalgar.[133]
Submarines became important in naval warfare in World War I, when German submarines, known asU-boats, sank nearly 5,000 Allied merchant ships,[139] including theRMS Lusitania, which helped to bring the United States into the war.[140] In World War II, almost 3,000 Allied ships were sunk by U-boats attempting to block the flow of supplies to Britain,[141] but the Allies broke the blockade in theBattle of the Atlantic, which lasted the whole length of the war, sinking 783 U-boats.[142] Since 1960, several nations have maintained fleets of nuclear-poweredballistic missile submarines, vessels equipped to launchballistic missiles withnuclear warheads from under the sea. Some of these are kept permanently on patrol.[143][144]
Sailing ships orpackets carried mail overseas, one of the earliest being the Dutch service toBatavia in the 1670s.[145] These added passenger accommodation, but in cramped conditions. Later, scheduled services were offered but the time journeys took depended much on the weather. When steamships replaced sailing vessels,ocean-going liners took over the task of carrying people. By the beginning of the twentieth century, crossing the Atlantic took about five days and shipping companies competed to own the largest and fastest vessels. TheBlue Riband was an unofficial accolade given to the fastest liner crossing the Atlantic in regular service. TheMauretania held the title with 26.06 knots (48.26 km/h) for twenty years from 1909.[146] TheHales Trophy, another award for the fastest commercial crossing of the Atlantic, was won by theUnited States in 1952 for a crossing that took three days, ten hours and forty minutes.[147]
The great liners were comfortable but expensive in fuel and staff. The age of the trans-Atlantic liners waned as cheap intercontinental flights became available. In 1958, a regular scheduled air service between New York and Paris taking seven hours doomed the Atlantic ferry service to oblivion. One by one the vessels were laid up, some were scrapped, others became cruise ships for theleisure industry and still others floating hotels.[148]
Shipping routes, showing relative density of commercial shipping around the world
Maritime trade has existed for millennia. ThePtolemaic 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.[149] 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.[150] 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 theAtlantic slave trade.[151][152]: 194
Large quantities of goods are transported by sea, especially across the Atlantic and around the Pacific Rim. A major trade route passes through thePillars of Hercules, across the Mediterranean and theSuez Canal to the Indian Ocean and through theStraits of Malacca; much trade also passes through theEnglish Channel.[153]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.[154] Increased melting of Arctic ice since 2007 enables ships to travel theNorthwest Passage for some weeks in summertime, avoiding the longer routes via the Suez Canal or thePanama Canal.[155]
Fish and other fishery products are among the most widely consumed sources of protein and other essential nutrients.[160] In 2009, 16.6% of the world's intake of animal protein and 6.5% of all protein consumed came from fish.[160] In order to fulfill this need, coastal countries have exploited marine resources in theirexclusive economic zone, although fishing vessels are increasingly venturing further afield to exploit stocks in international waters.[161] In 2011, the total world production of fish, includingaquaculture, was estimated to be 154 million tonnes, of which most was for human consumption.[160] The harvesting of wild fish accounted for 90.4 million tonnes, while annually increasing aquaculture contributes the rest.[160] 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.[160] 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.[160]
Modern fishing vessels includefishing trawlers with a small crew, stern trawlers, purse seiners, long-line factory vessels and largefactory 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 bepurse seines, other seines,trawls, dredges,gillnets andlong-lines and the fish species most frequently targeted areherring,cod,anchovy,tuna,flounder,mullet, squid andsalmon.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.[162] It has been estimated that "industrialized fisheries typically reduced community biomass by 80% within 15 years of exploitation."[162] In order to avoid overexploitation, many countries have introducedquotas in their own waters.[163] However, recovery efforts often entail substantial costs to local economies or food provision.
Fishing boat in Sri Lanka
Artisan fishing methods 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. TheFood and Agriculture Organization is encouraging the development of local fisheries to provide food security to coastal communities and help alleviate poverty.[164]
About 79 million tonnes (78M long tons; 87M short tons) of food and non-food products were produced by aquaculture in 2010, an all-time high. About six hundred species of plants and animals were cultured, some for use in seeding wild populations. The animals raised includedfinfish, aquaticreptiles, crustaceans, molluscs,sea cucumbers,sea urchins, sea squirts and jellyfish.[160] Integratedmariculture has the advantage that there is a readily available supply of planktonic food in the ocean, and waste is removed naturally.[165] Various methods are employed. Mesh enclosures for finfish can be suspended in the open seas, cages can be used in more sheltered waters or ponds can be refreshed with water at each high tide.Shrimps can be reared in shallow ponds connected to the open sea.[166] Ropes can be hung in water to grow algae, oysters and mussels. Oysters can be reared on trays or in mesh tubes. Sea cucumbers can be ranched on the seabed.[167] Captive breeding programmes have raisedlobsterlarvae for release of juveniles into the wild resulting in an increased lobster harvest inMaine.[168] At least 145 species of seaweed – red, green, and brown algae – are eaten worldwide, and some have long been farmed in Japan and other Asian countries; there is great potential for additionalalgaculture.[169] Few maritime flowering plants are widely used for food but one example ismarsh samphire which is eaten both raw and cooked.[170] A major difficulty for aquaculture is the tendency towards monoculture and the associated risk of widespreaddisease. Aquaculture is also associated with environmental risks; for instance,shrimp farming has caused the destruction of importantmangrove forests throughoutsoutheast Asia.[171]
Beneath the surface,freediving is necessarily restricted to shallow descents.Pearl divers can dive to 40 feet (12 m) with baskets to collectoysters.[181] Human eyes are not adapted for use underwater but vision can be improved by wearing adiving mask. Other useful equipment includesfins andsnorkels, andscuba equipment allows underwater breathing and hence a longer time can be spent beneath the surface.[182] 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 preventdecompression sickness as they return to the surface. Recreational divers restrict themselves to depths of 100 feet (30 m) beyond which the danger ofnitrogen narcosis increases.Deeper dives can be made with specialised equipment and training.[182]
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, nearSt Malo inBrittany 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 bywind 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.[186] The first offshore wind farm was installed in Denmark in 1991,[187] and the installed capacity of worldwide offshore wind farms reached 34 GW in 2020, mainly situated in Europe.[188]
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 specialisedshipyards andinfrastructure costs are lower. Disadvantages include problems caused by waves and tides, the tendency for excavations to silt up and the washing away ofspoil heaps. There is a risk of coastal erosion and environmental damage.[189]
Minerals precipitated near a hydrothermal vent
Seafloor massive sulphide deposits are potential sources ofsilver,gold,copper,lead andzinc and trace metals since their discovery in the 1960s. They form whengeothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers". The ores are of high quality but prohibitively costly to extract.[190]
There are large deposits ofpetroleum andnatural gas, in rocks beneath the seabed.Offshore platforms anddrilling rigsextract 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.[191] Drilling for oil in the sea has environmental impacts. Animals may be disorientated byseismic waves used to locate deposits, and there is debate as to whether this causes thebeaching of whales.[192] Toxic substances such asmercury, lead andarsenic may be released. The infrastructure may cause damage, and oil may be spilt.[193]
Large quantities ofmethane clathrate exist on the seabed and inocean sediment, of interest as a potential energy source.[194] Also on the seabed aremanganese nodules formed of layers ofiron,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 fornickel was investigated in the 1970s but abandoned in favour of more convenient sources.[195] 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.[196]
The sea holds large quantities of valuable dissolved minerals.[197] 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).[198]
Desalination is the technique of removing salts from seawater to leavefresh water suitable for drinking or irrigation. The two main processing methods,vacuum distillation andreverse 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.[199]
Severalnomadic indigenous groups inMaritime Southeast Asia live in boats and derive nearly all they need from the sea. TheMoken people live on the coasts ofThailand andBurma and islands in theAndaman Sea.[200] Some Sea Gypsies are accomplishedfree-divers, able to descend to depths of 30 metres (98 ft), though many are adopting a more settled, land-based way of life.[201][202]
The indigenous peoples of the Arctic such as theChukchi,Inuit,Inuvialuit andYup'iit hunt marine mammals including seals and whales,[203] and theTorres 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 mainlandAboriginal Australians.[204]
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.[205] TheAncients personified it, believing it to be under the control of abeing who needed to be appeased, and symbolically, it has been perceived as a hostile environment populated by fantastic creatures; theLeviathan of theBible,[206]Scylla inGreek mythology,[207]Isonade inJapanese mythology,[208] and thekraken of lateNorse mythology.[209]
InIslamic tradition, the sea symbolizes both the mercy and might ofGod, serving as a powerful sign (ayah) of divine creation and control over nature. TheQur'an frequently references the sea to illustrate God's benevolence, as in:"It is He who subjected the sea so that you may eat fresh meat from it and extract ornaments which you wear" (Qur'an 16:14), while also portraying it as a place of trial and humility, as seen in verses that describe the helplessness of humans during storms at sea (Qur'an 10:22). The sea's dual nature, as a source of sustenance and a manifestation of divine power, reflects a broader Islamic perspective that views natural elements as signs pointing to the oneness and majesty of God.[212] This contrasts with certain other religious traditions; for instance, the sea in biblical literature is often associated with chaos and danger,[213] while inHindu mythology, it plays a role in cosmic cycles such as theSamudra Manthan, the churning of the ocean.[214] Islamic scholarship, including classicaltafsir (Qur'anic exegesis), emphasizes that the sea is not merely a physical reality but a theological symbol that calls believers to reflect on God's greatness and their own dependence on Him.[215]
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.[216]: 4–8
As a symbol, the sea has for centuries played a role inliterature,poetry anddreams. 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.[216]: 45 In hisepic poem theOdyssey, written in the eighth century BC,[217]Homer describes the ten-year voyage of the Greek heroOdysseus who struggles to return home across the sea's many hazards after the war described in theIliad.[218] The sea is a recurring theme in theHaiku poems of the JapaneseEdo period poetMatsuo Bashō (松尾 芭蕉) (1644–1694).[219] In the works of psychiatristCarl Jung, the sea symbolizes the personal and thecollective unconscious indream interpretation, the depths of the sea symbolizing the depths of theunconscious mind.[220]
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 impactmarine ecosystems andfood webs and may result in consequences as yet unrecognised for thebiodiversity and continuation of marine life forms.[221] An overview of environmental issues is shown below:
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 andsewage contributeheavy metals,pesticides,PCBs,disinfectants, household cleaning products and othersynthetic 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.[224] The heavy metals of greatest concern are copper, lead, mercury,cadmium and zinc which may bebio-accumulated by marine organisms and are passed up the food chain.[225]
Much floating plastic rubbish does notbiodegrade, instead disintegrating over time and eventually breaking down to the molecular level. Rigid plastics may float for years.[226] In the centre of the Pacific gyre there is the permanentGreat Pacific Garbage Patch, a floating accumulation of mostlyplastic waste.[227] There is a similargarbage patch in the Atlantic.[228] Foraging sea birds such as thealbatross andpetrel 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.[229]
Most oil pollution in the sea comes from cities and industry.[230] 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.[231] The marine environment has self-cleansing properties and naturally occurring bacteria will act over time to remove oil from the sea. In theGulf of Mexico, where oil-eating bacteria are already present, they take only a few days to consume spilt oil.[232]
Run-off offertilisers from agricultural land is a major source of pollution in some areas and the discharge of rawsewage has a similar effect. The extra nutrients provided by these sources can causeexcessive plant growth. Nitrogen is often the limiting factor in marine systems, and with added nitrogen, algal blooms andred tides can lower the oxygen level of the water and kill marine animals. Such events have created dead zones in theBaltic Sea and the Gulf of Mexico.[230] Somealgal blooms are caused bycyanobacteria that makeshellfish thatfilter feed on them toxic, harming animals likesea otters.[233] Nuclear facilities too can pollute. The Irish Sea was contaminated by radioactivecaesium-137 from the formerSellafield nuclear fuel processing plant[234] and nuclear accidents may also cause radioactive material to seep into the sea, as did the disaster at theFukushima Daiichi Nuclear Power Plant in 2011.[235]
The dumping of waste (including oil, noxious liquids, sewage and garbage) at sea is governed by international law. TheLondon Convention (1972) is aUnited Nations agreement to control ocean dumping which had been ratified by 89 countries by 8 June 2012.[236]MARPOL 73/78 is a convention to minimize pollution of the seas by ships. By May 2013, 152 maritime nations had ratified MARPOL.[237]
^There is no accepted technical definition ofsea amongst oceanographers. One definition is that a sea is a sub-division of an ocean, which means that it must haveoceanic basin crust on its floor. This definition accepts theCaspian as a sea because it was once part of an ancient ocean.[5] TheIntroduction to Marine Biology defines a sea as a "land-locked" body of water, adding that the term "sea" is only one of convenience.[6]The Glossary of Mapping Sciences similarly states that the boundaries between seas and other bodies of water are arbitrary.[7]
^According to this definition, the Caspian would be excluded as it is legally an "international lake".[10]
^Hydrousringwoodite recovered fromvolcanic eruptions suggests that thetransition zone between thelower andupper mantle holds between one[13] and three[14] times as much water as all of the world's surface oceans combined. Experiments to recreate the conditions of the lower mantle suggest it may contain still more water as well, as much as five times the mass of water present in the world's oceans.[15][16]
^"As the waves leave the region where they were generated, the longer ones outpace the shorter because their velocity is greater. Gradually, they fall in with other waves travelling at similar speed – where different waves are in phase they reinforce each other, and where out of phase they are reduced. Eventually, a regular pattern of high and low waves (or swell) is developed that remains constant as it travels out across the ocean."[3]: 83–84
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