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You might think only fresh or salt water is on our planet.

Did you know there is a third kind called brackish water? In this lesson, we will learn its characteristics and discover what kind of wildlife inhabits it!

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What is Brackish Water?

Go to a sea or ocean and what kind of water do you find? Salt water. Head to a lake or pond and what will it be? Fresh water. What do you think happens when you visit an estuary connecting a fresh water stream with seawater from the ocean? You now have the seawater mixing with your fresh water and the result is called brackish water! The specific definition of brackish water is, ”water that is saltier than fresh water, but not as salty as seawater.”

Brackish water can be found in estuaries and watersheds
brackish water

Salinity of Brackish Water

Do you know how to tell if water is brackish water instead of seawater? There is no universal scale stating how much salt brackish water has in it, but generally, there is an accepted salinity range; sort of like measuring how much sugar to put into water for hummingbird food. Salinity tells you how much salt is dissolved in a given liquid and is usually measured in parts per thousand (ppt) or parts per million (ppm).

1 ppt means there is one ounce of salt for every 1000 ounces of water. Sugar water, where you have 20 percent sugar and 80 percent water equates to 200 ppt of dissolved sugar. This range was mentioned by Daniel Hillel when he wrote for Salinity Management for Sustainable Irrigation: Integrating Science, Environment, and Economics. According to this scale, brackish water has between 0.5 and 2 parts per thousand (ppt) of total dissolved salts, fresh water has less than 0.5 ppt of dissolved salts and seawater has over 2 ppt of dissolved salts.

Water Type Salinity
Fresh water <0.5 ppt
Brackish water 0.5 to 2 ppt
Salt water >2 ppt

Density of Brackish Water

In addition to the salinity, another measurement used to differentiate between brackish, salt and fresh water is its density. Density tells you how thick your substance is by dividing its mass by its volume.

  • Density = Mass / Volume

When you take the density of something and divide by the density of water, then you have what is called the specific gravity of that liquid.

  • Specific gravity = Density / Density of water

An interesting point here, is that both your specific gravity and your density are affected by temperature. The warmer the temperature, the less dense your substance is with a corresponding lower specific gravity.For water with a temperature of 77 degrees Fahrenheit, brackish water has a density between 997.453 kg/m3 and 998.584 kg/m3.

Fresh water with a salinity of 0 at 77 degrees has a density of 997.075 kg/m3. Dividing the density of the brackish water by the density of fresh water, you get a specific gravity range for brackish water between 997.453 / 997.075 = 1.0004 and 998.584 / 997.

075 = 1.0015.

Density Specific gravity
Fresh water 997.075 kg/m3 1
Brackish water 997.

453 kg/m3 to 998.584 kg/m3

1.0004 to 1.0015

Wildlife in Brackish Water

Salinity, density and specific gravity measurements are very important, not just from a scientific point-of-view, but also from nature’s stance. As part of the wildlife, some fish require a certain level of salinity and specific gravity in brackish water to survive.

For example, the Green Spotted Puffer is a fish that requires brackish water with a specific gravity between 1.005 and 1.200. By looking at this specific gravity range, you can see that this particular fish can tolerate seawater with a low specific gravity. This brackish water fish is not a real seawater fish; just as we know salt water fish cannot live in fresh water, true brackish water fish cannot live in fresh water either.

They may stay alive for a little while, but will eventually get sick and die because it’s not their optimal habitat.The majority of plants and trees cannot tolerate salt in their water or surroundings, but there are several that can. One example is the mangrove tree; you can find them in tropical and subtropical conditions that are flooded with brackish water for part of the year. Other trees and plants that can tolerate the salt in brackish water include the acacia tree, the oak tree, some species of eucalyptus, cacti, succulents, ice plants, sunflowers, gardenias and some palm trees.

Lesson Summary

Brackish water is water that is saltier than fresh water. but not as salty as seawater.

Salinity tells you how much salt is dissolved in a specific liquid; usually measured in parts per thousand (ppt) or parts per million (ppm).Density is how heavy your substance is, solved by dividing the mass by its volume. Specific gravity is the density divided by the density of water.Brackish water has a salinity between 0.5 and 2 ppt of dissolved salts with a specific gravity between 1.

0004 and 1.0015 for water at 77 degrees Fahrenheit.Some fish, like the Green Spotted Puffer require brackish water to survive. Various trees and plants also require brackish water to thrive in their environments.

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