Salinity
What is salinity?
Salinity is the concentration of all the salts dissolved in water. It is usually measured in parts per thousand (ppt or ‰).
How does salinity vary?
Salinity varies from 5 ppt or less in fresh water (rain, rivers, lakes) to 35 ppt or higher in oceans (also salt lakes or seas). A salinity of 35 ppt is the equivalent of 35 grams of salt in every 1000 grams of water. In areas where fresh water mixes with salt water, such as estuaries, the water is brackish and salinity ranges from 10 to 25 ppt.
Several processes that affect the water cycle also affect the salinity of aquatic ecosystems.
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Source: https://www.slideshare.net/JustMagicMaria/oceans-2014-1
Average Monthly Sea Surface Salinity, 2016 – Departure Bay, BC
Source: http://www.pac.dfo-mpo.gc.ca/science/oceans/data-donnees/lighthouses-phares/data/departut.txt
Why is salinity biologically important?
Salinity is an important abiotic factor because the normal functioning of animals depends on the regulation of the water and ions in their internal environment, which is influenced by the water and ions in their external environment (Moyes & Schulte 2006). The relative concentration of salts inside organisms compared to their external environment affects biophysical processes such as diffusion and osmosis, which impacts chemical reactions including metabolism.
In a hyposmotic environment, such as a lake, the solute concentration in the environment is typically lower than in the cells. Cells will swell up due to the osmotic influx of water.
In a hyperosmotic environment, such as ocean, the solute concentration in the environment is often higher than in the cells. This causes water to move out of the cells.
In a hyperosmotic environment, such as ocean, the solute concentration in the environment is often higher than in the cells. This causes water to move out of the cells.
The balance of intra- and extra-cellular fluids affects the concentration of water and solutes (major anions: Na+, Ca2+, Mg2+, K+; major cations: Cl-, SO42-) that are essential in biochemical processes in living cells and entire organisms. Therefore, variation in salinity can greatly affect the physiology and ecology of organisms.
How do animals respond to variations in salinity?
Variations in salinity could cause osmotic stress to animals. Osmotic stress occurs when organisms are exposed to water and solute concentrations that are different from their normal internal concentrations (Freeman et al. 2014). Animals use two general strategies for coping with variations in salinity — osmoconformation and osmoregulation (Moyes & Schulte 2006).
Osmoconformers, such as most marine invertebrates (e.g. marine sponges, jellyfish, and flatworms), exhibit an internal water and solute concentration that is similar to that of their external environment. They do not expend a lot of energy to move solutes in response to changes in environmental salinities; instead, their internal solute concentration changes to be the same as the salinity of the environment. The resulting increases or decreases in salt concentrations in cells can affect their metabolic rate.
Osmoregulators, such as most marine vertebrates (e.g. bony fish, sea turtles, and sea otters), maintain an internal water and solute concentration that is within a narrow range independent of the external environment. Considerable energy is required to control water and salt balance during the process of osmoregulation. This could reduce the amount of energy available for other normal activities and behaviours.
Some animals (e.g. some species of crabs) use a combination of both osmoregulation and osmoconformation depending on the salinity of the environment.
Osmoconformers, such as most marine invertebrates (e.g. marine sponges, jellyfish, and flatworms), exhibit an internal water and solute concentration that is similar to that of their external environment. They do not expend a lot of energy to move solutes in response to changes in environmental salinities; instead, their internal solute concentration changes to be the same as the salinity of the environment. The resulting increases or decreases in salt concentrations in cells can affect their metabolic rate.
Osmoregulators, such as most marine vertebrates (e.g. bony fish, sea turtles, and sea otters), maintain an internal water and solute concentration that is within a narrow range independent of the external environment. Considerable energy is required to control water and salt balance during the process of osmoregulation. This could reduce the amount of energy available for other normal activities and behaviours.
Some animals (e.g. some species of crabs) use a combination of both osmoregulation and osmoconformation depending on the salinity of the environment.
Key search terms: Salinity · Diffusion · Osmosis · Hyperosmotic · Hypoosmotic · Osmotic
stress · Osmoregulation · Osmoconformation
stress · Osmoregulation · Osmoconformation
References Cited
Freeman S, Harrington M, Sharp J. 2014. Biological science. 2nd Canadian ed. Toronto: Pearson.
Moyes CD, Schulte PM. 2006. Principles of animal physiology. San Francisco: Pearson.
Note: Any edition of the above books or other biology textbook could be useful.
Freeman S, Harrington M, Sharp J. 2014. Biological science. 2nd Canadian ed. Toronto: Pearson.
Moyes CD, Schulte PM. 2006. Principles of animal physiology. San Francisco: Pearson.
Note: Any edition of the above books or other biology textbook could be useful.
To learn more:
Bradley TJ. 2009. Animal osmoregulation. New York: Oxford University Press. (hard copy and e-book available at Woodward Library QU105 B73 2009)
Freeman S, Harrington M, Sharp J. 2014. Biological science. 2nd Canadian ed. Toronto: Pearson.
Gilles R. 1975. Mechanisms of ion and osmoregulation. Marine Ecology. 2:259–347.
Lockwood APM. 1962. The osmoregulation of Crustacea. Biological Reviews. 37(2):257–303. https://doi.org/10.1111/j.1469-185X.1962.tb01613.x
McNamara JC, Faria SC. 2012. Evolution of osmoregulatory patterns and gill ion transport mechanisms in the decapod Crustacea: a review. Journal of Comparative Physiology B. 182(8):997–1014. [accessed 2018 May 17]. https://link.springer.com/journal/360/182/8/page/1.
Pequeux A. 1995. Osmotic regulation in crustaceans. Journal of Crustacean Biology.15(1):1–60.
Rivera-Ingraham G, Lignot J. 2017. Osmoregulation, bioenergetics and oxidative stress in coastal marine invertebrates: raising the questions for future research. Journal of Experimental Biology. 220(10):1749-1760.
Schoffeniels E, Gilles R. 1972. Ionoregulation and osmoregulation in Mollusca. Chemical Zoology. 7:393–420.
Bradley TJ. 2009. Animal osmoregulation. New York: Oxford University Press. (hard copy and e-book available at Woodward Library QU105 B73 2009)
Freeman S, Harrington M, Sharp J. 2014. Biological science. 2nd Canadian ed. Toronto: Pearson.
Gilles R. 1975. Mechanisms of ion and osmoregulation. Marine Ecology. 2:259–347.
Lockwood APM. 1962. The osmoregulation of Crustacea. Biological Reviews. 37(2):257–303. https://doi.org/10.1111/j.1469-185X.1962.tb01613.x
McNamara JC, Faria SC. 2012. Evolution of osmoregulatory patterns and gill ion transport mechanisms in the decapod Crustacea: a review. Journal of Comparative Physiology B. 182(8):997–1014. [accessed 2018 May 17]. https://link.springer.com/journal/360/182/8/page/1.
Pequeux A. 1995. Osmotic regulation in crustaceans. Journal of Crustacean Biology.15(1):1–60.
Rivera-Ingraham G, Lignot J. 2017. Osmoregulation, bioenergetics and oxidative stress in coastal marine invertebrates: raising the questions for future research. Journal of Experimental Biology. 220(10):1749-1760.
Schoffeniels E, Gilles R. 1972. Ionoregulation and osmoregulation in Mollusca. Chemical Zoology. 7:393–420.