What is Salinity?

The weight of the dissolved materials divided by the weight of the sample seawater is known as salinity. Generally, salinity is defined as the total amount of solid material (present in the dissolved form) in grams contained in one kilogram of seawater and is expressed as part per thousand. 


Salinity of Seawater

The salinity of seawater not only affects marine organisms but also affects the physical properties of the sea, such as temperature, density, pressure, waves and current. Seawater contains a complex solution of several mineral substances in dilute form because it is an active solvent. Since salt is brought from the land every year, the total amount of salt in seawater is steadily rising. Several efforts have been made to estimate the total amount of salts in seawater (salinity of seawater). The salinization of seawater is reported due to the existence of 47 types of salts in seawater. Out of these 47 types of salt, the main cause of salinization is seven important salts. The most significant component of sea salt is sodium chloride, also known as common salt.


The Salinity of the Ocean

The freezing point of ocean water is determined by salinity. More saline water freezes slowly in comparison to less saline water. The boiling point of saline water is higher than the freshwater. Evaporation is also controlled by the salinity so it is lower over more saline water than over less saline water. Salinity also increases the density of seawater. 

Significant salts that contribute to the salinity of ocean water are given below:

  1. Sodium chloride (NaCl)- 77.8%

  2. Magnesium chloride (MgCl2)- 10.9%

  3. Magnesium sulphate (MgSO4)- 4.7%

  4. Calcium sulphate (CaSO4)- 1.26%

Oceanic Salinity Sources- Oceanic salinity is mainly derived from shore. Salts in solution are transported by rivers from continental areas. Surprisingly, the composition of sea salt and riverine salt varies greatly, with calcium sulphate responsible for 60% of river salinity and sodium chloride dominating ocean salinity (77.8 percent of total salinity). River water contains only two per cent of sodium chloride. This is why some scientists do not believe rivers are a significant source of salinity in the oceans and seas, but it should be noted that marine animals absorb a large portion of the calcium carried by rivers into the oceans. Second, in the oceans, the salt borne by the river is slightly changed. Volcanic ash also helps to render the seas salty.


The Average Salinity of Seawater

The average salinity of seawater = 3.5 % (3.5 grams in 100 grams)

Or

The average salinity of ocean water = 35 grams in 1000 grams.

Salinity % = chlorinity % * 1.80


Brackish Water Salinity

The salinity of Brackishwater is between 0.5 and 30 grams of dissolved salt per litre. This is more often expressed as in parts per thousand  (0.5 to 30 parts per thousand (% )).


Soil Salinization

The term Soil salinity refers to the salt content in the soil and the process of increasing the amount of salt content in soil composition is known as the salinization process. Generally, Salts occur naturally in soils and water. When the salt content increases from the normal level it leads to Salinization. It can be caused by natural processes like mineral weathering or by the gradual withdrawal of an ocean. When the level of salts in the soil water is too high (hypersalinity), water can flow from the plant’s root zone back into the soil. This can lead to the dehydration of the plant. Therefore, causing yield decline or even death of the plant. Crop yield can cause losses due to other reasons also. Loss of yield can occur even though the effects of salinity may not be obvious.


Controlling Factors of Salinity

Within the oceans and seas, salinity distribution varies greatly. The factors affecting the amount of salt in different oceans seas are called controlling factors of oceanic salinity. Evaporation, precipitation, the influx of river water, prevailing winds, ocean currents and sea waves are significant controlling factors. 

1. Evaporation- there is a direct positive relationship between the rate of evaporation and salinity. Greater the evaporation rate, the higher the salinity and vice versa. In fact, salt concentration increases with the rapid rate of evaporation. Due to evaporation caused by high temperatures and low humidity, salt concentrations rise, and total salinity rises. Salinity is higher in the tropics than at the equator, for example, and both regions have a high rate of evaporation, but the tropics of Cancer and Capricorn have dry air. According to Wust, the average annual rate of evaporation in the Atlantic ocean is 94 cm to the north of 40 degrees north 149 cm at 20 degrees north and 105 cm near the equator. Salinity is 34.68 parts per thousand at 5 degrees. In general subtropical high-pressure belts and trade winds, belts record a rapid rate of evaporation which increases salinity but a cloudy sky with high humidity lowers down salinity in the equatorial belt. It may be pointed out that salinity also controls evaporation.

2. Precipitation- It is inversely proportional or related to salinity. Higher precipitation equals lower salinity and vice versa. This is why the regions of high rainfall record comparatively lower salinity than the regions of low rainfall. The extra water in the temperate regions supplied by melt-water of ice coming from the polar areas increases the volume of water and therefore reduces salinity. In Simple terms, heavy rainfall raises the amount of water in the oceans, reducing the salt-to-water ratio.

3. The Influx of River Water- While rivers carry salt from the land to the oceans, they often discharge a large amount of water into the ocean, reducing salinity at their mouths. For example, comparatively low salinity is found near the mouths of the Ganga, the Congo, the Nizer, the Amazon etc. the effect of the influx of river water is more pronounced in the example of the enclosed sea the Danube, the Dniester, the Dnieper etc. reduce the salinity in the black sea. Salinity is reduced by five parts per thousand in the gulf of Bothnia due to the influx of immense volume of water brought by the rivers. On the other hand, where evaporation exceeds the influx of fresh river water, there is an increase in salinity. There is a seasonal variation of surface salinity with maximum and minimum runoff from the land. Freshwater salinity decreases with maximum runoff during the rainy season and increases during the dry season.

4. Atmospheric Pressure and Wind Direction- Anticyclonic conditions with stable air and high temperature increases the salinity of the surface water of the oceans. High salinity conditions can be found in subtropical high-pressure belts. Winds also help in the redistribution of salts in the oceans and seas as winds drive away saline water to fewer saline areas resulting in a decrease of salinity in the former and increases in the latter. In other words, in the areas of upwelling of waterless saline water moves up from below whereas, in the areas where the water is piled up, salinity is increased. For example, trade winds drive away saline waters from the western coast of the continents and pile up salinity increases. For example, trade winds drive away saline water from the western coast of the continents and pile them up near the eastern coast causing low salinity in the former area and high salinity in the latter. The salinity of the oceans increases along the western coasts of continents, while it decreases along the eastern coasts. Sometimes, winds minimize the spatial variation in salinity. 

5. Circulation of Oceanic Water- Ocean currents affect the spatial distribution of salinity by mixing seawater. Equatorial warm currents drive away salts from the western coastal areas of the continent and accumulate them along with the eastern coastal areas. This adds to the high salinity of the Mexican Gulf. The North Atlantic drift, the extension of the gulf stream increases salinity along the north-western coasts of Europe. Similarly, salinity is reduced along the north-eastern coast. America due to cold labrador current. Ocean currents have the least impact on salinity in enclosed waters, but currents have a significant impact on salinity in marginal seas with open sea contact through wide openings. For example, the North Atlantic drift raises the salinity of the Norwegian and the North Seas.


Distribution of Salinity

The average salinity in the oceans and the seas is 35 parts per thousand but it spatially and temporally varies in different oceans, seas, and lakes. The variation in salinity is both horizontal and vertical. Salinity varies as well, from sealed to partly closed to open seas. As a result, there are two approaches to investigating the spatial distribution of salinity:

1. Horizontal Distribution

2. Vertical Distribution


Significance of Salinity

The ocean salinity has significant effects on the physical property of seawater and other aspects of the oceans as follows:

  • The freezing and boiling points are greatly affected and controlled by the addition or subtraction of salts in seawater. The saline water freezes slowly in comparison to freshwater. It is known to all that pure water freezes at the temperature of 0 degrees celsius freezing point. If the salinity of seawater becomes thirty-five parts per thousand then it would freeze at the temperature of -1.91degrees celsius. On the other hand, the boiling point of saline water is higher than freshwater.

  • Water salinity and density of seawater have positively correlated. The salinity of seawater increases its density because solutes in water have greater atomic weight than the molecules of freshwater. This is why people are rarely submerged in seawater with a strong salinity.

  • The salinity of the oceans governs evaporation. Solutes (salts) in water actually slow down evaporation in the oceans. As a result, more saline water evaporates less than less saline water. It's worth noting that evaporation regulates the salinity of seawater, causing salt concentrations to rise.

  • Spatial variation in seawater salinity becomes a potent factor in the origin of ocean currents. 

  • The ocean salinity affects the marine organisms and plant community.  


Impacts of Salinity

As a result of increasing salinity, it causes significant impacts on various parameters. Some of them are discussed below:

  1. Agricultural Production- general trend of water movement in the plant is from soil to the plant roots through osmosis. But in the case of high salinity, the reverse process takes place. Water can move from plant root to the soil. Therefore, the plant can be dehydrated. This will lead to the death of the plant and thus affect agricultural production.

  2. Water Quality- the presence of a high amount of dissolved salts in water can affect the taste of water. Such water has the potential of producing a laxative effect.

  3. The Ecological Health of Stream- the high salinity in the river water system can affect the ecological biodiversity of the freshwater system.

  4. Terrestrial Biodiversity- salinity can destroy natural vegetation. This in turn can lead to major landscape changes, habitat fragmentation, and loss of biodiversity.

  5. Soil Erosion- salinity enhances the weathering and erosion process. 

  6. Flood Risk- in shallow water bodies salinity increases the flood risk. Salinity reduces the absorption capacity of the soil due to which the soil of such water bodies does not absorb the precipitated rainfall and lead to flooding.

Did You Know?

  • According to Joly, if all the salts of all the oceans and seas are dried up and are spread over the globe, these will form a 45.72 m thick layer and if these salts are spread only over the land, these will form a 152.4 m thick layer.

  • Besides salts, silver, gold, radium elements also occur in seawater in minute proportion.

FAQs (Frequently Asked Questions)

1. What are the Various Controlling Factors of Salinity?

Answer: The various factors controlling the salinity are given below: 

  • Evaporation

  • Precipitation

  • The influx of river water

  • atmospheric pressure and wind direction

  • Circulation of oceanic water.

2. What are the Impacts of Salinity?

Answer: the various impacts of salinity are given below:

  • Agricultural production

  • Water quality

  • The ecological health of Stream

  • Terrestrial biodiversity

  • Soil erosion

  • Flood risk