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Water from a sea or an ocean

Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximately of dissolved salts (predominantly sodium (Na+) and chloride (Cl ) ions). The average density at the surface is 1.025 kg/L. Seawater is denser than both fresh water and pure water (density 1.0 kg/L at 4 °C (39 °F)) because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At typical salinity, it freezes at about 2 °C (28 °F). The coldest seawater still in the liquid state ever recorded was found in 2010, in a stream under an Antarctic glacier: the measured temperature was 2.6 °C (27 °F). Seawater pH is typically limited to a range between 7.5 and 8.4. However, there is no universally accepted reference pH scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units.

Although the vast majority of seawater has a salinity of between 31 and 38 g/kg, that is 3.1–3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with freshwater runoff from river mouths, near melting glaciers or vast amounts of precipitation (e. g. monsoon), seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and low river run off, and confined circulation result in unusually salty water. The salinity in isolated bodies of water can be considerably greater still about ten times higher in the case of the Dead Sea. Historically, several salinity scales were used to approximate the absolute salinity of seawater. A popular scale was the "Practical Salinity Scale" where salinity was measured in "practical salinity units (PSU)". The current standard for salinity is the "Reference Salinity" scale with the salinity expressed in units of "g/kg".

The density of surface seawater ranges from about 1020 to 1029 kg/m3, depending on the temperature and salinity. At a temperature of 25 °C, the salinity of 35 g/kg and 1 atm pressure, the density of seawater is 1023.6 kg/m3. Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg/m3 or higher. The density of seawater also changes with salinity. Brines generated by seawater desalination plants can have salinities up to 120 g/kg. The density of typical seawater brine of 120 g/kg salinity at 25 °C and atmospheric pressure is 1088 kg/m3. Since then, it has been decreasing due to a human caused process called ocean acidification that is related to carbon dioxide emissions: Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. The pH value of seawater is naturally as low as 7.8 in deep ocean waters as a result of degradation of organic matter in these waters. It can be as high as 8.4 in surface waters in areas of high biological productivity. Measurement of pH is complicated by the chemical properties of seawater, and several distinct pH scales exist in chemical oceanography. There is no universally accepted reference pH scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units. However, the ratios of solutes differ dramatically. For instance, although seawater contains about 2.8 times more bicarbonate than river water, the percentage of bicarbonate in seawater as a ratio of all dissolved ions is far lower than in river water. Bicarbonate ions constitute 48% of river water solutes but only 0.14% for seawater. Differences like these are due to the varying residence times of seawater solutes; sodium and chloride have very long residence times, while calcium (vital for carbonate formation) tends to precipitate much more quickly. Its osmolarity is about 1000 mOsm/L. Small amounts of other substances are found, including amino acids at concentrations of up to 2 micrograms of nitrogen atoms per liter, which are thought to have played a key role in the origin of life. + Seawater elemental composition(salinity = 3.5%) ElementPercent by massOxygen85.84Hydrogen10.82Chlorine1.94Sodium1.08Magnesium0.1292Sulfur0.091Calcium0.04Potassium0.04Bromine0.0067Carbon0.0028

+ Total molar composition of seawater (salinity = 35) Component Concentration (mol/kg) H2O 53.6 0.546 0.469 0.0528 SO2 4 0.0282 0.0103 0.0102 CT 0.00206 0.000844 BT 0.000416 0.000091 0.000068

Research in 1957 by the Scripps Institution of Oceanography sampled water in both pelagic and neritic locations in the Pacific Ocean. Direct microscopic counts and cultures were used, the direct counts in some cases showing up to 10 000 times that obtained from cultures. These differences were attributed to the occurrence of bacteria in aggregates, selective effects of the culture media, and the presence of inactive cells. A marked reduction in bacterial culture numbers was noted below the thermocline, but not by direct microscopic observation. Large numbers of spirilli like forms were seen by microscope but not under cultivation. The disparity in numbers obtained by the two methods is well known in this and other fields. In the 1990s, improved techniques of detection and identification of microbes by probing just small snippets of DNA, enabled researchers taking part in the Census of Marine Life to identify thousands of previously unknown microbes usually present only in small numbers. This revealed a far greater diversity than previously suspected, so that a litre of seawater may hold more than 20,000 species. Mitchell Sogin from the Marine Biological Laboratory feels that "the number of different kinds of bacteria in the oceans could eclipse five to 10 million." Bacteria are found at all depths in the water column, as well as in the sediments, some being aerobic, others anaerobic. Most are free swimming, but some exist as symbionts within other organisms – examples of these being bioluminescent bacteria. Cyanobacteria played an important role in the evolution of ocean processes, enabling the development of stromatolites and oxygen in the atmosphere. Some bacteria interact with diatoms, and form a critical link in the cycling of silicon in the ocean. One anaerobic species, Thiomargarita namibiensis, plays an important part in the breakdown of hydrogen sulfide eruptions from diatomaceous sediments off the Namibian coast, and generated by high rates of phytoplankton growth in the Benguela Current upwelling zone, eventually falling to the seafloor. Bacteria like Archaea surprised marine microbiologists by their survival and thriving in extreme environments, such as the hydrothermal vents on the ocean floor. Alkalotolerant marine bacteria such as Pseudomonas and Vibrio spp. survive in a pH range of 7.3 to 10.6, while some species will grow only at pH 10 to 10.6. Archaea also exist in pelagic waters and may constitute as much as half the ocean's biomass, clearly playing an important part in oceanic processes. In 2000 sediments from the ocean floor revealed a species of Archaea that breaks down methane, an important greenhouse gas and a major contributor to atmospheric warming. Some bacteria break down the rocks of the sea floor, influencing seawater chemistry. Oil spills, and runoff containing human sewage and chemical pollutants have a marked effect on microbial life in the vicinity, as well as harbouring pathogens and toxins affecting all forms of marine life. The protist dinoflagellates may at certain times undergo population explosions called blooms or red tides, often after human caused pollution. The process may produce metabolites known as biotoxins, which move along the ocean food chain, tainting higher order animal consumers. Pandoravirus salinus, a species of very large virus, with a genome much larger than that of any other virus species, was discovered in 2013. Like the other very large viruses Mimivirus and Megavirus, Pandoravirus infects amoebas, but its genome, containing 1.9 to 2.5 megabases of DNA, is twice as large as that of Megavirus, and it differs greatly from the other large viruses in appearance and in genome structure. In 2013 researchers from Aberdeen University announced that they were starting a hunt for undiscovered chemicals in organisms that have evolved in deep sea trenches, hoping to find "the next generation" of antibiotics, anticipating an "antibiotic apocalypse" with a dearth of new infection fighting drugs. The EU funded research will start in the Atacama Trench and then move on to search trenches off New Zealand and Antarctica. The ocean has a long history of human waste disposal on the assumption that its vast size makes it capable of absorbing and diluting all noxious material. While this may be true on a small scale, the large amounts of sewage routinely dumped has damaged many coastal ecosystems, and rendered them life threatening. Pathogenic viruses and bacteria occur in such waters, such as Escherichia coli, Vibrio cholerae the cause of cholera, hepatitis A, hepatitis E and polio, along with protozoans causing giardiasis and cryptosporidiosis. These pathogens are routinely present in the ballast water of large vessels, and are widely spread when the ballast is discharged.

The speed of sound in seawater is about 1,500 m/s (whereas the speed of sound is usually around 330 m/s in air at roughly 101.3 kPa pressure, 1 atmosphere), and varies with water temperature, salinity, and pressure. The thermal conductivity of seawater is 0.6 W/mK at 25 °C and a salinity of 35 g/kg. The thermal conductivity decreases with increasing salinity and increases with increasing temperature.

Minerals have been extracted from seawater since ancient times. Currently the four most concentrated metals – Na, Mg, Ca and K – are commercially extracted from seawater. During 2015 in the US 63% of magnesium production came from seawater and brines. Bromine is also produced from seawater in China and Japan. Lithium extraction from seawater was tried in the 1970s, but the tests were soon abandoned. The idea of extracting uranium from seawater has been considered at least from the 1960s, but only a few grams of uranium were extracted in Japan in the late 1990s. The main issue is not one of technological feasibility but that current prices on the uranium market for uranium from other sources are about three to five times lower than the lowest price achieved by seawater extraction. Similar issues hamper the use of reprocessed uranium and are often brought forth against nuclear reprocessing and the manufacturing of MOX fuel as economically unviable.

In order for seawater mineral and element extractions to take place while taking close consideration of sustainable practices, it is necessary for monitored management systems to be put in place. This requires management of ocean areas and their conditions, environmental planning, structured guidelines to ensure that extractions are controlled, regular assessments of the condition of the sea post extraction, and constant monitoring. The use of technology, such as underwater drones, can facilitate sustainable extractions. The use of low carbon infrastructure would also allow for more sustainable extraction processes while reducing the carbon footprint from mineral extractions. Although seawater extractions can benefit society greatly, it is crucial to consider the environmental impact and to ensure that all extractions are conducted in a way that acknowledges and considers the associated risks to the sustainability of seawater ecosystems.

ASTM International has an international standard for artificial seawater: ASTM D1141 98 (Original Standard ASTM D1141 52). It is used in many research testing labs as a reproducible solution for seawater such as tests on corrosion, oil contamination, and detergency evaluation.

The minerals found in seawater can also play an important role in the ocean and its ecosystem's food cycle. For example, the Southern Ocean contributes greatly to the environmental carbon cycle. Given that this body of water does not contain high levels of iron, the deficiency impacts the marine life living in its waters. As a result, this ocean is not able to produce as much phytoplankton which hinders the first source of the marine food chain. One of the main types of phytoplankton are diatoms which is the primary food source of Antarctic krill. As the cycle continues, various larger sea animals feed off of Antarctic krill, but since there is a shortage of iron from the initial phytoplankton/diatoms, then these larger species also lack iron. The larger sea animals include Baleen Whales such as the Blue Whale and Fin Whale. An environmentally friendly approach to mining that allows for more sustainability would be to extract these metals from the seafloor. Lithium mining from the seafloor at mass quantities could provide a substantial amount of renewable metals to promote more environmentally friendly practices in society to reduce humans' carbon footprint. Lithium mining from the seafloor could be successful, but its success would be dependent on more productive recycling practices above ground. There are also risks that come with extracting from the seafloor. Many biodiverse species have long lifespans on the seafloor, which means that their reproduction takes more time. Similarly to fish harvesting from the seafloor, the extraction of minerals in large amounts, too quickly, without proper protocols, can result in a disruption of the underwater ecosystems. Contrarily, this would have the opposite effect and prevent mineral extractions from being a long term sustainable practice, and would result in a shortage of required metals. Any seawater mineral extractions also risk disrupting the habitat of the underwater life that is dependent on the uninterrupted ecosystem within their environment as disturbances can have significant disturbances on animal communities.