Seawater salinity levels are naturally about the same everywhere. This constant ratio is called Forchhammer's Principle or the Principle of Constant Proportions (it is named after Johan Georg Forchhammer, who discovered it in the 1860s). However, while salinity levels do not vary due to differing amounts of salt minerals in seawater, they can and do vary based on the addition or removal of fresh water in a given area.