Seawater

Seawater, or salt 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, 599 mM). This means that every kilogram (roughly one litre by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na+) and chloride (Cl−) ions). 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 ever recorded (in a liquid state) was in 2010, in a stream under an Antarctic glacier, and measured −2.6 °C (27.3 °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 g/kg and 38 g/kg, that is 3.1–3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water 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, 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. Seawater pH is limited to the range 7.5 to 8.4. The speed of sound in seawater is about 1,500 m/s (whereas speed of sound is usually around 330 m/s in air at roughly 1000hPa 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. Seawater contains more dissolved ions than all types of freshwater. 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. The most abundant dissolved ions in seawater are sodium, chloride, magnesium, sulfate and calcium. 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. 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.