The Salt Waters

Vocabulary/new concepts:

Homogeneous mixture: a homogeneous mixture in which the composition of the elements is the same throughout the mixture. A mixture of tea and sugar is homogeneous because the dissolved sugar is evenly distributed throughout the cup of tea. It is often easy to confuse a homogeneous mixture with a pure substance because both are uniform.

Heterogeneous mixture: a mixture in which the composition of the elements is not uniform throughout the mixture. Vegetable soup is a heterogeneous mixture. Each spoonful of soup contains varying amounts of the different vegetables and other components of the soup.

Halocline – also called salt kiss: occurs when there is a sharp difference in salinity between two layers of water and they cannot mix.

Osmotic stress: a condition in which cells or organisms experience an imbalance in the concentration of solutes (in this case salt) between their internal environment and the external environment.

Brackish habitats: of moderate salinity, intermediate between freshwater and marine habitats. These habitats, such as salt marshes, mangroves and some coastal areas, are characterised by variable salinity levels due to the mixing of fresh and seawater.

What is a mixture?

There are two types of mixtures.

• One type is called a homogeneous mixture. This is like mixing sugar and water (or tea). The sugar dissolves completely in the water and you don’t see it anymore. Everything looks the same, like one thing.
• The other type of mixture is called a heterogeneous mixture. This is like mixing oil and water. They don’t mix completely. You can still see individual oil droplets floating on the water. They don’t mix and become one thing. Another example of a heterogeneous mixture is vegetable soup. You can see the individual vegetables in the soup and easily scoop out a spoonful of just carrots and water.

So when different things are put together but they don’t become something new, but they stand on their own and retain their characteristic qualities, you get a mixture. We can separate them later if we want.

A mixture is a combination of two or more substances that are not chemically related to each other. In a mixture, the individual substances retain their own properties and can be separated from each other by various physical methods.

Salinity of water

About 70% of the Earth is covered with water. Over 97% of this water is found in the oceans. Dissolved salts in ocean water give it a salty taste. River (fresh) water also has dissolved salts, but not as many as ocean water! These dissolved salts can come from land, precipitation or the atmosphere and are particles that have mixed completely with the water.

The amount of dissolved salts in water is called salinity. It is measured in parts per thousand (ppm or ‰). The table below shows the salinity values of different types of water and their uses.

Table 1. Water salinity. Source: Government of Western Australia

Occupancy of water bodies according to their salinity

The salinity of a body of water can have a significant impact on its habitability by various life forms, including plants, animals and microorganisms. The salinity level determines the types of organisms that can survive and thrive in a particular aquatic environment. This is how salinity affects the habitability of a water body:

• Freshwater habitats: Freshwater habitats, such as lakes, rivers, and streams, have low salinity levels. Typically, their salinity is less than 0.5‰. Freshwater habitats provide suitable habitat for a wide range of organisms adapted to living in low salinity conditions. Many freshwater plants, fish species, amphibians and invertebrates need these lower salinity levels to survive.
• Marine habitats: Marine habitats, which include oceans, seas and saltwater estuaries, have higher salinity levels. The average salinity of seawater is about 35‰. Marine organisms such as seaweeds, algae, fish and marine mammals have evolved to tolerate and thrive in high salinity environments.
• Brackish habitats: Brackish habitats are intermediate between freshwater and marine habitats, with a moderate level of salinity. These habitats, such as salt marshes, mangroves, and some coastal areas, are characterized by variable salinity levels due to the mixing of freshwater and seawater. Brackish waters support limited adapted species that can tolerate varying salinity levels, such as saltwater fish, some crustaceans, and halophytic plants.

Examples of brackish habitats can be seen in the presentation:

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Extreme salinity conditions can limit or hinder the ability of organisms to survive. High salinity can cause dehydration and osmotic stress in freshwater organisms that are not adapted to saline environments. Osmotic stress refers to a condition in which cells or organisms experience an imbalance in the concentration of solutes (in this case salt) between their internal environment and the external environment. Similarly, low salinity levels can present a challenge to marine organisms adapted to higher salt concentrations.

However, it is worth noting that some organisms have unique adaptations to extreme salinity conditions. Some salt-tolerant or halophilic organisms, such as some bacteria and archaea, can survive in highly saline environments, such as salt pans, hypersaline lakes, or even the Dead Sea.

In general, the salinity of a body of water is a critical factor affecting the distribution, diversity and adaptation of organisms in aquatic ecosystems. The balance of salinity levels plays a vital role in determining which species can thrive in a particular aquatic environment.