Ocean covers 71 percent of the Earth’s surface and teems with life. From microscopic algae to whales and octopuses, the ocean’s five life zones are filled with organisms that have evolved for their specific marine ecosystems. The ocean’s depths also contain dramatic physical features including mountains, canyons and cliffs.
The ocean has been a source of food, a route for trade and exploration and a home to many species over the millennia. Plate tectonics and other natural forces continue to shape the ocean’s coastline and structure.
Ocean currents transport heat from the tropics to northern latitudes, moderating the climate of Europe and other temperate regions. They also transport oxygen and nutrients, providing food to living things throughout the ocean and bringing waste products back down to the seafloor.
During the planet’s early formation, molten rocks inside the Earth cooled and released water vapor. This water vapor covered the surface of the young Earth and created a primitive ocean. The ocean is blue in color because, unlike other colors of light, water molecules and particles preferentially absorb red light and reflect blue light. This allows sunlight to reach deep into the ocean, where it reaches up to 200 meters (660 feet).
The sunlit upper layer of the ocean is called the epipelagic zone or photic zone. This region is rich in plant life, containing the large algal plants known as kelp and the tiny green splotches of freshwater and saltwater algae. It is also the location of photosynthesis, a process by which some types of plants and algae convert solar energy into food for themselves and other living organisms.
Below the epipelagic zone, the ocean floor drops off into a steep slope called the continental rise. This slope then descends further into the mesopelagic zone, where fish and other animals live. Beyond the mesopelagic zone is the bathypelagic zone, which contains krill and other small zooplankton, followed by the abyssal plain where hydrothermal vents may occur.
Humans have added to the ocean’s biodiversity, but have also caused harm. Marine pollution includes chemicals, debris and other substances that enter the ocean from land sources. These can include sewage, agricultural and residential waste, excess carbon dioxide, noise and invasive species.
There are more than 3,400 species of fish and other aquatic organisms that live in the ocean. These animals range from the smallest, plankton-like creatures to the largest mammal on Earth, the blue whale. In addition, the ocean is a vital source of minerals, metals and fossil fuels.
An influx of nutrients, such as silicates and nitrates, into the ocean is also harmful. These nutrients increase the growth rate of algae and other marine plants, which reduces dissolved oxygen levels. This decrease in dissolved oxygen causes hypoxia, or low oxygen levels in the water. In turn, this can lead to disease and death in marine animals. Humans can help to restore the health of the ocean by reducing the amount of nutrients that enter the ocean.