Salinity or salt content in water depends on several influencing factors. That is why there are types of water based on the level of salinity, namely fresh water with a salt content of less than 0.05%, brackish water with a salt content of 0.05 to 3%, saline or salt water with a salt content of 3 to 5%, and brine with a salt content exceeding 5%.
Seawater is included in the saline water category with an average salinity of 3.5%. Although specifically explained that the salinity concentration of each sea is different, the ratio between the different ions will always remain. This condition is evidenced by the study of the voyage of H.S. Challenger who studied scientifically for four years and ended in 1876. This is known as the Law of Constant Proportion.
Law of Constant Proportion: “Sea salinity is in a stable state because the whole process is in steady-state equilibrium. The amount of salt added to the sea is approximately the same as the amount released from it. The main mechanism for salt transfer occurs in the sub-production or subduction area of the continental shelf where seawater is attracted to the Earth’s mantle to eventually be recycled.”
The salt content in each ocean is different, depending on the following points.
- Evaporation
Water has organic minerals and one of them is salt. If the water evaporates, the salt and mineral content will be left behind and settle on the seabed. Therefore, the more water evaporates; the seawater will become increasingly salty due to the high level of salt.
- Freshwater Import
As with too salty solutions, one way to neutralize it is to add fresh water into it, so over time, the salt content will decrease. In the case of salt content in the sea, fresh water can come from rain, river or swamp water, and also from ice that melts in the polar regions.
- Mixing Water
This happened to the Dead Sea and the Black Sea. The Dead Sea has very high salinity because basically, the Dead Sea is a salty lake. The lake is not connected to the ocean but because the volume of water in the Dead Sea has fallen far enough so that its surface appears, then an estuary that connects the Dead Sea with the Black Sea is made. This is also to neutralize the salt levels that are too high in the Dead Sea.
- River Water
The more rivers that empty into the sea, the salinity level of seawater will decrease because it is mixed with lower levels of seawater.
- Location and Size of the Sea
The sea that is isolated or not connected to the high seas will have high salinity. As in the case of the salt lake, the Dead Sea, seven million tons of water evaporates every day in the lake and increases the amount of salt in the bottom.
- Sea Wave
The sea which is influenced by heat flow, the salinity will rise (high). This applies also the opposite, where the sea is affected by cold currents; the salinity will decrease (low).
- Humidity
The more evaporation occurs; the air around it becomes moist then the higher the salinity of seawater.
- Mineral content
The highest concentration of minerals in seawater is the magnesium content. Freshwater and sea water both have magnesium. However, the amount contained in seawater is greater, so this proves that the more minerals contained by water, the more salty water is. This is also evidenced by the condition of the Dead Sea.
Salinity on the surface is unique and varied. Salinity values on the surface are influenced by physical processes that occur in the waters. Salinity will decrease due to the intensity of rain, river flow, and melting of ice. The difference between evaporation and rainfall at latitude causes some of these differences. A decrease in surface salinity near the equator is caused by greater or higher rainfall. (Millerro and Sohn, 1992)
The salt content of seawater changes due to the addition and reduction of water molecules through the process of evaporation and rain. Salinity increases when the rate of evaporation in an area is greater than rain. Conditions depend on the latitude of an area and the change of seasons. This pattern can be seen in areas with latitude between 20˚ and 30˚ north and south of the equator. The area will have waters with higher salinity than the surroundings because the rate of evaporation in the area is greater than the amount of water received during rain.
These places have the same characteristics as the desert because of the same latitude. In addition, the salt content of seawater is also influenced by local conditions. Large outflows from large river systems can reduce salt water levels. The salinity of a flooded river will decrease temporarily.
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