What Determines Salinity in Physics?
Salinity is determined by dividing the weight of the dissolved materials by the weight of the sample seawater. A kilogram of seawater is weighted by the amount of solid matter (present as dissolved solid matter) contained in it, expressed as a part per thousand.
The Salinity of the Seawater
In addition to affecting aquatic organisms, seawater salinity also affects a number of physical properties, such as temperature, density, pressure, waves, and current. Especially in seawater, there are a large number of mineral substances dispersed in a diluted solution due to its properties as an active solvent. Seawater contains a constant amount of salt since salt from the land is added every year. Seawater salinity has been estimated several times through different efforts. The presence of 47 different types of salts in seawater results in the salinization of seawater. Among these 47 salt types, seven important salts are the main cause of salinization. In addition to sodium chloride, which is also known as common salt, sea salt has other components as well.
What is the Ocean's Salinity?
Saline water freezes at a certain point based on its salinity. The freezing of more saline water is slower than the freezing of less saline water. A saline solution has a higher boiling point than fresh water. Over more saline water, evaporation is also less than over less saline water due to its effect on salinity. Seawater density is also greater when saline.
Some Salts that make up Ocean Water and contribute to its Salinity
Sodium chloride (NaCl)- 77.8%
Magnesium chloride (MgCl2)- 10.9%
Magnesium sulfate (MgSO4)- 4.7%
Calcium sulfate (CaSO4)- 1.26%
Ocean Salinity Sources
Coastal waters provide the vast majority of oceanic salinity. From continental areas, rivers transport salts into solution. Interestingly, the composition of ocean salt and freshwater salt differs greatly, with sodium chloride making up 77.8% of the total ocean salinity and calcium sulfate accounting for 60 percent of river salt. Sodium chloride is present in river water only in small amounts. The reason some scientists believe rivers do not contribute significantly to ocean salinity is that marine animals take up much of the calcium transported by rivers. The salt carried by rivers is slightly altered when it reaches the oceans. Volcanic ash also contributes to seawater's saltiness.
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 liter. 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.
Factors that Control Salinity
It is not uncommon for the distribution of salinity to be quite different within oceans and seas. Controlling factors of oceanic salinity refer to the factors affecting the amount of salt in various oceans. There are many factors controlling the weather, including evaporation, precipitation, rivers, winds, ocean currents, and sea waves.
Evaporation.
Precipitation.
The Influx of River Water.
Atmospheric Pressure and Wind Direction.
Circulation of Oceanic Water.
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 salinity in the equatorial belt. It may be pointed out that salinity also controls evaporation.
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.
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.
Atmospheric Pressure and Wind Direction
Anticyclonic conditions with stable air and high temperature increase 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.
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.
Salinity Distribution
There are 35 parts per thousand of salinity on average in the oceans and seas, but this varies with spatial and temporal variations among the oceans, seas, and lakes. Salinity varies horizontally as well as vertically. There are also variations in salinity, from partially closed to open. Therefore, there are two ways of investigating the spatial distribution of salinity:
Horizontal Distribution
Vertical Distribution
The Importance of Salinity
Ocean salinity affects many aspects of the oceans, such as the physical properties of seawater:
The addition or subtraction of salts in seawater greatly affects and controls the freezing and boiling points. Salinity affects freezing rates more than freshwater. All people are aware that pure water freezes at 0 degrees Celsius. The water will freeze at -1.91 degrees Celsius if the salinity of the water reaches thirty-five parts per thousand. Fresh water's boiling point is higher than that of saline water.
Saline water evaporates faster than freshwater. Water (salt) slows evaporation in oceans due to its solute (salt). Thus, saline water evaporates at a slower rate than less saline water does. Seawater's salinity is regulated by evaporation, increasing salt concentrations.
In the emergence of ocean currents, variations in seawater salinity become a significant factor.
Seawater salinity affects the plants and the organisms that live in the ocean.
FAQs on Salinity Explained: Key Concepts & Impacts
1. What exactly is salinity, and how is it commonly expressed?
Salinity is a fundamental property of water that measures the total concentration of all dissolved salts. It is typically expressed in Parts Per Thousand (ppt) or ‰. For instance, an average ocean salinity of 35 ppt means that there are 35 grams of salt dissolved in every 1,000 grams of seawater. Scientists also use the Practical Salinity Unit (PSU), which is a dimensionless value roughly equivalent to ppt.
2. What are the main factors that increase or decrease the salinity of ocean water?
The salinity of ocean water is controlled by a balance of processes that add or remove freshwater. Key factors include:
- Evaporation: Increases salinity by removing freshwater and leaving the salts behind, which is common in subtropical regions.
- Precipitation: Decreases salinity by adding freshwater in the form of rain or snow.
- River Inflow: The discharge of freshwater from rivers significantly lowers salinity in coastal areas.
- Freezing and Melting of Ice: When seawater freezes, the resulting sea ice contains very little salt, which increases the salinity of the remaining water. Conversely, the melting of ice caps and glaciers adds freshwater, decreasing salinity.
3. How does salinity affect the physical properties of water, such as its density?
Salinity has a direct impact on the density of water. The more dissolved salt water contains, the denser and heavier it becomes. This principle is crucial for ocean circulation. Colder, saltier water is denser and sinks, while warmer, less salty water is less dense and stays at the surface. This density difference is a primary driving force behind large-scale global ocean currents, a process known as thermohaline circulation.
4. Which ocean has the highest average salinity and why?
The Atlantic Ocean has the highest average salinity. This is primarily due to high rates of evaporation in its subtropical regions, combined with receiving less freshwater input from major rivers compared to other oceans like the Pacific. Ocean currents also play a role by transporting salty water from the Mediterranean Sea into the Atlantic.
5. What is the difference between ocean salinity and soil salinity?
While both refer to salt concentration, their context and impact are different. Ocean salinity refers to dissolved salts (like sodium chloride) in seawater and is a natural, vital component of marine ecosystems. Soil salinity refers to the accumulation of salts in the upper layers of soil, often due to poor irrigation practices or natural processes in arid regions. High soil salinity is generally harmful as it hinders the ability of plants to absorb water, negatively impacting agriculture.
6. Why do some bodies of water, like the Dead Sea, have extremely high salinity?
The exceptionally high salinity of bodies like the Dead Sea (over 300 ppt) is due to specific geographical and climatic conditions. It is a landlocked lake located in a hot, arid region with a very high rate of evaporation and no outlets for the water. Rivers flow into it, bringing dissolved minerals, but the water only escapes through evaporation, leaving a highly concentrated salt solution behind over thousands of years.
7. How is salinity measured in a practical scientific setting?
Scientists measure salinity using several methods. The most common modern instrument is a salinometer or a CTD (Conductivity, Temperature, and Depth) sensor. This device measures the water's electrical conductivity—the higher the salt content, the better the water conducts electricity. This reading is then precisely converted into a salinity value in Practical Salinity Units (PSU).
8. Why is maintaining a stable salinity level so important for marine organisms?
Marine organisms are adapted to live within specific salinity ranges. Their cells rely on a process called osmosis to maintain a stable internal water balance with their environment. A sudden change in external salinity can be fatal. For example, if the water becomes too fresh, their cells could absorb too much water and burst. If it becomes too salty, they could lose water and dehydrate. Therefore, stable salinity is critical for the survival and health of entire marine ecosystems, from plankton to fish.
