People often use the term sea in reference to the ocean. To geographers, a sea is a division of the ocean that is enclosed or partly enclosed by land. For this reason, all seas are saline, or salty. Some seas are called bays (like the Bay of Bengal, between India, Bangladesh, Myanmar, and Indonesia), while some lakes are called seas (like the Caspian Sea, shared by Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan). There are more than 50 seas on Earth.
There are three major types of seas: nearly enclosed seas, partly enclosed seas, and hypersaline lakes.
Nearly enclosed seas reach deeply into continents and are connected with the open ocean by narrow waterways called straits. Seas of this type include the Mediterranean Sea and the Baltic Sea. Because such seas are almost landlocked, they have a small range of tides. Some have no tides at all. Since there is little exchange of water between an enclosed sea and the open ocean, the two may differ from each other physically, chemically, and biologically.
The Red Sea, for instance, is much saltier than the Indian Ocean, to which it is connected by a narrow strait called Bab-el-Mandeb. The organisms, including coral reefs, that live in the Red Sea have adapted to life in salty water. About 10 percent of fish found in the Red Sea do not exist in any other habitat, including the Indian Ocean.
Partly enclosed seas are more like the open ocean, especially in the circulation of their waters.
Some, such as the Weddell Sea of Antarctica, are linked to the ocean by a wide opening. The tides and organisms of the Weddell Sea are virtually the same as the tides and organisms of the Atlantic Ocean.
Other seas, such as the South China Sea, are connected with the ocean by passages between islands. (The islands that dot the South China Sea are controlled by China, Malaysia, the Philippines, and Vietnam.) The organisms of the South China Sea are familiar across the South Pacific Ocean. The large range of tides of the open ocean are slightly reduced in the South China Sea.
Some bodies of salt water that are called seas are really lakes. These bodies of water were part of prehistoric oceans or seas. Tectonic shifts blocked their access to larger bodies of water, and they are now completely surrounded by land. Some of these seas are called hypersaline lakes, due to the extremely high salt content in their waters.
Among these landlocked seas is the Caspian Sea, between Europe and Asia. The Volga River drains into this body of water in the north. As a result, the northern part of the sea, in Russia and Kazakhstan, is almost a freshwater lake. The southern part of the sea, in Iran and Turkmenistan, is much more saline.
Another landlocked sea is the Dead Sea, a hypersaline lake between Jordan, Israel, and the West Bank, an area of land controlled by the Palestinian Authority. The Jordan River flows into the Dead Sea, but no rivers flow out. The Jordan River has been managed for the benefit of agriculture and industry. Because of canals and dams for irrigation and power, less water reaches the Dead Sea. As a result, the Dead Sea is shrinking at a rate of about a meter (3.3 feet) a year. (Due to the complex way evaporation works, the Dead Sea will probably never dry up completely.)
The Dead Sea is a popular tourist destination, known for its spas and historic sites. The governments of Israel, Jordan, and the Palestinian Authority are considering a project called the Two Seas Canal that would divert water from the Red Sea to the Dead Sea to increase economic activity in the area. This canal would replenish water to the Dead Sea and create a series of dams through the desert of Arabah.
Critics of the Two Seas Canal worry about the environmental impact of such a project. The delicate coral reef ecosystem of the Gulf of Aqaba would probably be destroyed by power plants. The aquifer of Arabah would probably be disturbed, limiting the freshwater available to the desert. Finally, the unique, salty ecosystem of the Dead Sea would be altered by exotic species and a different quality of water.