What Is Salinity? - Definition & Effects

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  • 0:05 Definition of Salinity
  • 1:24 Salinity Regulation
  • 3:25 Effects of Salinity
  • 4:42 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The ocean is salty, but how does it differ from a lake or an estuary? Salinity is the 'saltiness' of a body of water, and describing this factor helps us understand the environmental conditions and organisms that can be found there.

Definition of Salinity

If you had a choice of drinking water from your sink versus from the ocean, you'd likely pick the tap. Why? Because ocean water is so salty of course. The concentration of dissolved salt in a given volume of water is called salinity.

Salinity is either expressed in grams of salt per kilogram of water, or in parts per thousand (ppt, or ‰). For example, if you have 1 gram of salt and 1,000 grams of water, your salinity is 1 g/kg, or 1 ppt.

Freshwater has very little salt, usually less than 0.5 ppt. Water with a salinity of 0.5 - 17 ppt is called brackish water, which is found in estuaries and coastal salt marshes. Depending on their location and source of fresh water, some estuaries can have salinities as high as 30 ppt.

Seawater is on average 35 ppt, but it can range between 30 - 40 ppt. This variation occurs because of differences in evaporation, precipitation, freezing, and freshwater runoff from land at different latitudes and locations. Seawater salinity also varies by water depth because water density and pressure increase with depth. Water with salinity above 50 ppt is brine water, though not many organisms can survive in such a high salt concentration.


Salinity Regulation

Have you ever noticed that when you eat something salty you get very thirsty? This is due to an important process in your body called osmosis. Osmosis is the movement of water through a semipermeable membrane (like skin and cell membranes) from an area with a low concentration of solute to an area of high concentration. Solute means a dissolved substance.

Osmosis occurs in your cells because they need to maintain a specific balance of water and solutes. If a cell loses too much water it will become dehydrated and die, but if it takes on too much water it will swell and burst.

When you eat something salty your body tries to get rid of the extra salts you just took in. Through osmosis, water will move to where there are lots of salts, flushing them out. However, if you are not taking in new water, your body is simply losing water trying to push the extra salts out and you become dehydrated. Being thirsty is your body's way of signaling that you need to drink more water to replenish those lost fluids.

Maintaining the correct amount of salt and water is not too difficult for you because you can simply drink more water. This is much more difficult for marine organisms because they live in water that has a very high concentration of salt and other solutes.

Many marine animals simply go with the flow. Marine animals with bodies that are permeable to both salts and water, so the concentration of solutes in their body fluids changes with the surrounding water, are called osmoconformers. Jellyfish and sea anemones are examples of osmoconformers.

However, there are other marine animals that are able to actively control the salt concentration in their body. These are osmoregulators, and there are a variety of ways that they regulate the amount of salt in their body fluids. For example, some osmoregulators such as some crabs and fish are able to maintain the appropriate salt concentration in their bodies by actively absorbing salt through their gills. Other osmoregulators have adapted to have a less permeable skin, which decreases the amount of water exchange.

Effects of Salinity

While many marine organisms are able to withstand changing salinity by either regulating or conforming, they are still bound by tolerable ranges. If you were to put a marine crab in fresh water, it would not survive because it needs a minimum amount of salinity to provide salt to its body fluids. The same goes for a freshwater fish - put it in the ocean and it would quickly die because it can only regulate so much.

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