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The largest ocean-based source of plastic pollution is discarded fishing gear (including traps and nets), estimated to be up to 90% of plastic debris in some areas. [56] [57] Continental plastic litter enters the ocean largely through storm-water runoff, flowing into watercourses or directly discharged into coastal waters. [58]
The ocean can be described as the world's largest ecosystem and it is home for many species of marine life. Different activities carried out and caused by human beings such as global warming, ocean acidification, and pollution affect marine life and its habitats.
Organisms that live freely at the ocean surface, termed neuston, include keystone organisms like the golden seaweed Sargassum that makes up the Sargasso Sea, floating barnacles, marine snails, nudibranchs, and cnidarians. Many ecologically and economically important fish species live as or rely upon neuston.
A marine coastal ecosystem is a marine ecosystem which occurs where the land meets the ocean. Worldwide there is about 620,000 kilometres (390,000 mi) of coastline. Coastal habitats extend to the margins of the continental shelves, occupying about 7 percent of the ocean surface area.
Ocean acidification can impact marine ecosystems that provide food and livelihoods for many people. About one billion people are wholly or partially dependent on the fishing, tourism, and coastal management services provided by coral reefs. Ongoing acidification of the oceans may therefore threaten food chains linked with the oceans. [70] [71]
Worldwide, continental shelves occupy a total area of about 24 million km 2 (9 million sq mi), 8% of the ocean's total area and nearly 5% of the world's total area. [ 29 ] [ 30 ] Since the continental shelf is usually less than 200 metres deep, it follows that coastal habitats are generally photic , situated in the sunlit epipelagic zone .
The world's northernmost individual coral reef is located in the Finlayson Channel, in the inside passage of British Columbia, Canada. [72] The world's southernmost coral reef is at Lord Howe Island, in the Pacific Ocean off the east coast of Australia.
At the other end of the spectrum, benthos of the deep ocean includes the bottom levels of the oceanic abyssal zone. [7] For information on animals that live in the deeper areas of the oceans see aphotic zone. Generally, these include life forms that tolerate cool temperatures and low oxygen levels, but this depends on the depth of the water. [8]