What causes sea water to have higher or lower salinity?
Salinity is measures as the amount of salt content in water in ppt (parts per thousand). The amount of salinity depends on the source of water that feeds water into the sea. At the sea bed there is sand, rocks and sediments which contribute to the salt content in the sea. If there are many fresh water rivers opening into the sea with continuous supply the salinity will be low. Also if there is continuous supply of melting snow (this happens near the poles), content of salt will be low. However, if there is not much of fresh water flowing into the sea, the salinity will be high. Also, if the rate of evaporation is high, salinity will be high (this happens in tropical regions) because water evaporates leaving behind salts. The Dead Sea has the highest salinity in the world. In fact the density of water is so high due to salt content that humans can actually float on the water. This is because Dead Sea is completely land-locked and has no source to supply fresh water.