What Is the Ocean?

An ocean is a large body of salt water that covers the Earth’s surface. It is one of the major components of Earth’s hydrosphere, and it plays a critical role in the climate system by absorbing, distributing, and regulating heat from the Sun. The ocean also produces half of the oxygen in the world’s atmosphere and provides food to countless species.

Oceans are formed by the movement of tectonic plates, post-glacial rebound, and sea level rise. They are the world’s largest ecosystems and contain an astounding array of life. They provide vital environmental services to humankind, including regulating the planet’s temperature, providing raw materials for food and fuel, and acting as a natural flood control mechanism. In addition, they serve as a means of transportation and trade, and provide a platform for exploration and discovery.

The world’s oceanic waters are divided into several principal areas, usually recognized as the Pacific Ocean, Atlantic Ocean, Indian Ocean, Arctic Ocean, and Southern Ocean; the latter two being sometimes consolidated into the first three. The ocean’s surface waters are divided into various zones based on the properties of the water, such as temperature (thermocline), salinity (halocline), and chemical composition (chemocline).

Within each zone, distinct boundaries can be drawn that mark changes in the characteristics of the water: the thermocline is found in tropical waters, while a mesopelagic zone lies directly below it, and the bathypelagic zone is the deepest part of the ocean. The mesopelagic zone is a highly biodiverse area that supports many marine organisms. It is the only place on Earth where sunlight can penetrate to depths of about 1,000 meters (3,280 feet).

In contrast, the aphotic zone, below the mesopelagic zone, is very dark. It is a relatively stable habitat that contains fewer animals but is also the source of many nutrients, such as nitrogen and carbon dioxide. The aphotic zone is also a repository of organic material, such as sand, silt, and shells.

The vast and varied ecosystems of the oceans are threatened by human activity, including pollution, overfishing, and climate change. The first step in making a difference is learning about the ocean and how your everyday actions affect it. You can make a difference by supporting nonprofits that work to protect and restore the ocean, by becoming a member or making a one-time donation. You can also help by taking steps to reduce your personal waste, by shopping for sustainable seafood, and by visiting a beach to enjoy its beauty—and to pick up any trash you see.

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