Oceans become salty when the rivers and their tributaries and streams that flow off mountains and out of all higher to lower places pick up biological materials and small rocks and dump these into the ocean. Their salinity then develops from the washing out of the contents of these liquid dumpsters. Looked at from this angle the sea then can be said to be one large watery trash heap.
Where is the ocean saltier? This question is possible because there is only one large ocean. Geographically, it is sectioned and named according to its location on the earth’s surface. Consequently, different areas will be saltier than others. The Red Sea and the areas around the Persian Gulf are the saltiest waters; the least salty are in the colder Polar Regions. The reasons are obvious: In the lesser areas fresh water from melting ice diluting the salinity; in dense areas there are less fresh water and lesser rain fall that would serve to dilute the heavy salt content.
Why then are some lakes salty while others are not? This is possible only because there is no run off. The Great Salt Lake in Utah, as an example, is salty because it a contained unit of water. There is no overflow to dilute the salt content. It is also in a desert like condition similar to the saltier ocean areas in the Red Sea and in the gulf areas. In surrounding areas of California and in other dry humid areas there are other salt lakes. Similarly, the Dead Sea in Israel is actually a salt water lake that is neither dead nor a sea. It is the saltiest lake in the world and has its own “indigenous” salt water fish and other living things that have adapted to living in salty water.
Why do oceans exist? There are many theories here and probably none are absolutely correct and none are absolutely false. The one theory that makes sense to me is that once the whole earth was covered with water. The activity underneath with its resultant heating and cooling properties forced debris upward and formed land and mountains. These in turn, as we continue to see, are not always calm and peaceful. Earthquakes do rearrange the topography from time to time, and lakes and rivers dry up, form and cut new pathways out of the resultant earth. The oceans as well as the dry land are continually changing. It is, simply put, a matter of high and low. The ocean flows to the point of least resistant. Yet with its consistent ebbing and flowing and splashing, it creates, over time, change.
Lakes are formed at lower levels in the earth. This is understandable since water flows from high to low. Just as it falls from out of the sky when the rain clouds become supersaturated, landed water runs from higher levels to lower levels, puddling into the ditches and indentations in the earth. Thus, fresh water lakes are those that are receiving fresh run off water and are, over spilling contents at the same time. There is no time for this water to gather the incidental chemicals from rocks and the earth as do salt water lakes.