Salinity
Salinity map showing areas of high salinity (36 o/oo) in green, medium salinity in blue (35 o/oo), and low salinity (34 o/oo) in purple. Salinity is rather stable but areas in the North Atlantic, South Atlantic, South Pacific, Indian Ocean, Arabian Sea, Red Sea, and Mediterranean Sea tend to be a little high (green). Areas near Antarctica, the Arctic Ocean, Southeast Asia, and the West Coast of North and Central America tend to be a little low (purple). (NOAA image edited by GA)
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The salinity of seawater is usually 35 parts per thousand (also written as o/oo) in most marine areas. This salinity measurement is a total of all the salts that are dissolved in the water. Although 35 parts per thousand is not very concentrated (the same as 3.5 parts per hundred, o/o, or percent) the water in the oceans tastes very salty. The interesting thing about this dissolved salt is that it is always made up of the same types of salts and they are always in the same proportion to each other (even if the salinity is different than average). The majority of the salt is the same as table salt (sodium chloride) but there are other salts as well. The table below shows these proportions:
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Chemical Ion Contributing to Seawater Salinity |
Concentration in o/oo (parts per thousand) in average seawater |
Proportion of Total Salinity (no matter what the salinity) |
Other |
less than 0.001 |
less than 0.001 |
The measurements listed in the table above from Castro and Huber's, Marine Biology textbook.
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Variations occur in ocean salinity due to several factors. The most common factor is the relative amount of evaporation or precipitation in an area. If there is more evaporation than precipitation then the salinity increases (since salt is not evaporated into the atmosphere). If there is more precipitation (rain) than evaporation then the salinity decreases. Another factor that can change the salinity in the ocean is due to a very large river emptying into the ocean. The runoff from most small streams and rivers is quickly mixed with ocean water by the currents and has little effect on salinity. But large rivers (like the Amazon River in South America) may make the ocean have little or no salt content for over a mile or more out to sea. The freezing and thawing of ice also affects salinity. The thawing of large icebergs (made of frozen fresh water and lacking any salt) will decrease the salinity while the actual freezing of seawater will increase the salinity temporarily. This temporary increase happens in the first stages of the freezing of seawater when small ice crystals form at about minus 2 degrees Centigrade. These tiny, needle-like ice crystals are frozen freshwater and the salts are not part of them so the liquid between these crystals becomes increasingly salty to the point of it being a brine. Eventually though, as seawater freezes, the ice crystals trap areas with brine and the entire large piece of frozen seawater (ice floe) is salty.
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Many marine organisms are highly affected by changes in salinity. This is because of a process called osmosis which is the ability of water to move in and out of living cells, in response to a concentration of a dissolved material, until an equilibrium is reached. In general the dissolved material does not easily cross the cell membrane so the water flows by osmosis to form an equilibrium. Marine organisms respond to this as either being osmotic conformers (also called poikilosmotic) or osmotic regulators (or homeosmotic).
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Marine algae (left) and marine feather duster worms (right) are osmotic conformers. (GA images)
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Osmotic conformers have no mechanism to control osmosis and their cells are the same salt content as the liquid environment in which they are found (in the ocean this would be 35 o/oo salt). If a marine osmotic conformer were put in fresh water (no salt), osmosis would cause water to enter its cells (to form an equilibrium), eventually causing the cells to pop (lysis). If a marine osmotic conformer were put in super salty water (greater than 35 o/oo salt) then osmosis would cause the water inside the cells to move out, eventually causing the cells to dehydrate (plasmolyze). These marine osmotic conformers include the marine plants and invertebrate animals which do not do well in areas without a normal salinity of 35 o/oo.
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Arctic charr fish (left) and humpback whales (right) are osmotic regulators. (GA images)
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Osmotic regulators have a variety of mechanisms to control osmosis and the salt content of their cells varies. It does not matter what the salt content is of the water surrounding a marine osmotic regulator, their mechanisms will prevent any drastic changes to the living cells. Marine osmotic regulators include most of the fish, reptiles, birds and mammals. These are the organisms that are most likely to migrate long distances where they may encounter changes in salinity. An excellent example of this is the salmon fish. The fish is about 18 o/oo salt so in seawater it tends to dehydrate and constantly drinks the seawater. Special cells on the gills (called chloride cells) excrete the salt so the fish can replace its lost water. When a salmon migrates to fresh water its cells start to take on water so the salmon stops drinking and its kidneys start working to produce large amounts of urine to expell the water.
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