Salinity

Salinity is the amount of salt dissolved in water, making it taste salty. We call water with a lot of salt saline water. Scientists measure salinity in special units, like grams of salt per liter of water.

Salinity is very important because it affects many things in nature. It helps decide how water behaves and works with other substances. Together with temperature and pressure, salinity helps control how dense water is and how much heat it can hold. These qualities are key to understanding how oceans move and share heat with the air around us.

When we draw lines on maps that show where salinity stays the same, we call these lines isohalines. They help scientists study how salt moves through oceans and lakes.

Definitions

Salinity is the amount of salt dissolved in water, such as in rivers, lakes, and oceans. Salts are substances like sodium chloride, which break into tiny parts called ions when they dissolve. Seawater usually has about 35 grams of salt in every kilogram of water, but this amount can change, especially near places where rivers flow into the ocean. Rivers and lakes can have much less salt, ranging from very tiny amounts to a few grams per kilogram.

Scientists measure salinity in different ways. In the past, they used a method called titration, but now they often use electrical conductivity, which measures how well water can carry an electric current. This helps estimate the amount of dissolved salts. Salinity is important because it affects how water behaves and moves in the ocean.

Classification of water bodies based upon salinity

Marine waters are found in the ocean and are also called euhaline seas. These waters have a salinity of 30 to 35 ‰. Brackish seas or waters have salinity ranging from 0.5 to 29 ‰, and metahaline seas have salinity from 36 to 40 ‰. All these waters are called thalassic because their salinity comes from the ocean. They are called homoiohaline if their salinity stays about the same over time.

Some environments are poikilohaline, meaning their salinity changes in ways that matter for plants and animals living there. These waters can have salinity from 0.5 to more than 300 ‰ and change over seasons or similar times. Highly saline water, where salts begin to form solid pieces, is called brine.

Environmental considerations

Salinity is very important for nature because it affects what kinds of animals and plants can live in water or on land near water. Some plants, called halophytes, can grow in salty places, and some of these are known as glasswort, saltwort, or barilla. There are also special tiny living things, mostly bacteria, that can survive in very salty conditions; these are called extremophiles or halophiles.

Removing salt from water is costly, so salt levels matter for using water safely to drink or to help plants grow. In the United States, lakes and rivers have become saltier because of road salt and other salts used to melt ice. Salinity in the oceans helps drive the movement of ocean currents. Changes in how salty the ocean is can also affect how much carbon dioxide the ocean can hold.