Marine Biome
The marine biome is the saltwater world: the five oceans, the seas, the coastal estuaries where rivers meet the sea, and the coral reefs. It is by far the biggest biome on Earth, covering approx. 71% of the planet's surface. By volume, it contains over 90% of the space where living things can survive. Almost every group of animals has marine members, from blue whales heavier than 20 elephants to coral polyps the size of a grain of rice to microscopic plankton that feed almost all the rest.
- % of Earth's Surfaceapprox. 71%More than all land combined
- % of Earth's Waterapprox. 96.5%Salty ocean water
- Average depth3,688 mApprox. 11 Eiffel Towers stacked
- Deepest point10,935 mMariana Trench, Pacific Ocean
- Largest creatureBlue whaleUp to 30 m, biggest ever on Earth
- Biggest reefGreat Barrier Reef2,300 km long, Australia
Earth's surface: land vs sea
The marine biome dominates the planet.
If you picked a random spot on Earth, you would be twice as likely to land in the ocean as on dry ground.
Zones of the ocean
The marine biome is stacked in layers by depth and light. Conditions change as you go down: less light, colder water, more pressure, fewer (but stranger) animals.
- Sunlight zone (top 200 m): bright enough for plants and most familiar fish. Almost all the photosynthesis in the ocean happens here.
- Twilight zone (200 to 1,000 m): dim blue light, not enough for plants. Many creatures make their own light (bioluminescence) to attract prey or mates.
- Midnight zone (1,000 to 4,000 m): completely dark. Cold, high pressure, food scraps drift down from above.
- Abyss (4,000 to 6,000 m): freezing flat ocean floor. Pressure is hundreds of times that at the surface.
- Trenches (below 6,000 m): the deepest, crushing depths. The Mariana Trench reaches almost 11,000 m down.
Coral reefs: the sea's rainforests
Coral reefs cover less than 1% of the ocean floor but support approx. 25% of all marine species. They are sometimes called the rainforests of the sea. Reefs are built by tiny animals called coral polyps that secrete a hard skeleton of calcium carbonate. Over thousands of years their skeletons stack up to form the reef, a city of interlocking caves and ridges that shelters fish, octopus, turtles, sharks, sea snakes, sea slugs and more.
Healthy reefs are vital nurseries for fish, but they are dying fast. Warming ocean water causes coral bleaching: the coral polyps spit out the colourful algae that live inside them and feed them, and turn bone white. If the water stays warm too long, the polyps starve and die. The Great Barrier Reef has had four mass bleaching events since 2016, the worst in its history.
Where the marine biome is
The ocean is divided into five named basins, but it is really one single connected body of water, the World Ocean, which a current can circulate around in approximately 1,000 years.
- Pacific Ocean: the largest and deepest. Covers more than the entire land area of Earth combined.
- Atlantic Ocean: the second-largest. Separates the Americas from Africa and Europe.
- Indian Ocean: the warmest. Bounded by Africa, Asia and Australia.
- Southern Ocean: rings Antarctica. Home to the Antarctic Circumpolar Current, the strongest ocean current on Earth.
- Arctic Ocean: the smallest and shallowest. Mostly covered by sea ice, which is shrinking fast as the planet warms.
Wildlife: from plankton to whales
Marine life ranges from microscopic to gigantic. The food web starts with phytoplankton: drifting single-celled plants smaller than a grain of sand that, together, produce approximately half of the oxygen we breathe. Zooplankton (tiny drifting animals, including young fish, krill and copepods) eat the phytoplankton; small fish eat the zooplankton; bigger fish, squid, seals, sharks and whales eat the small fish.
At the top sit the giants. The blue whale, at up to 30 m and 180 tonnes, is the biggest animal ever to have lived (yes, bigger than any dinosaur). The great white shark is the largest predatory fish. The colossal squid has eyes the size of footballs to see in the deep. Sea turtles travel thousands of kilometres between feeding and nesting grounds. Reef fish use almost every colour of the rainbow as camouflage, warning or courtship. The deep sea hides creatures we have only just discovered: anglerfish that lure prey with glowing baits, vampire squid that survive with almost no oxygen, and tube worms 2 m long that live around scalding deep-sea vents and feed on bacteria instead of sunlight.
Plants and producers
The marine biome has surprisingly few true plants. Almost all the photosynthesis is done by phytoplankton, the drifting microscopic algae of the surface waters. These tiny producers are the foundation of all open-ocean food webs and the source of half the oxygen on Earth. Without them, life on land would also collapse.
Larger marine plants only grow in shallow coastal waters where sunlight can reach the sea floor. Kelp forms huge underwater forests off temperate coasts (California, Tasmania, South Africa), with fronds up to 50 m tall and dense canopies that shelter sea otters, fish and abalone. Seagrass meadows grow in shallow tropical and temperate bays, providing food for turtles, dugongs and manatees. Mangrove forests grow with their roots in salty water along tropical coastlines, nursing young fish and protecting the land from storms.
People and the ocean
The ocean has shaped human history. It carries trade, food, oil, gas and over 90% of all the cargo moved between continents. Approx. 3 billion people depend on fish from the sea as a major source of protein. Coastal communities everywhere from small Pacific islands to giant ports like Shanghai and Rotterdam depend on the marine biome for their living.
The biggest threats to the ocean today are overfishing (around a third of global fish stocks are over-exploited), plastic pollution (over 8 million tonnes of plastic enter the sea each year and microplastics are now found from the deepest trenches to the Arctic ice), climate change (warming and acidifying the water, melting sea ice, raising sea levels), and habitat damage (bottom trawling, coastal development, oil spills, and coral bleaching). The ocean is so big it seemed for centuries that nothing humans did could affect it. We now know that is not true.
Deeper dive: ocean currents and the global climate
The ocean is one of the most powerful climate engines on Earth. Vast ocean currents circulate water around the globe, moving heat from the equator towards the poles and back again. The most famous example is the Gulf Stream: warm water flowing from the Gulf of Mexico up the east coast of North America and across the Atlantic to north-west Europe. Without it, Britain and Ireland would have a climate more like Labrador in Canada, several degrees colder.
Underneath the surface currents runs an even more powerful system called the thermohaline circulation (or "great ocean conveyor"). At the poles, surface water cools, becomes salty as ice forms, gets dense and sinks to the deep ocean. This cold deep water then flows for hundreds of years through the deep ocean basins, surfacing again in places like the Indian and Pacific Oceans. A single drop of water can take roughly 1,000 years to complete a full lap. This circulation transfers vast amounts of heat, oxygen and nutrients, and helps regulate the climate of the whole planet.
The ocean also acts as a huge carbon sink, absorbing roughly a quarter of all the carbon dioxide humans release into the atmosphere. Some of that CO&sub2; is stored by phytoplankton; some sinks to the deep sea when those organisms die. But all that absorbed CO&sub2; is making seawater more acidic, a process called ocean acidification, which is already damaging shellfish, corals and plankton. Warming water also melts the polar ice that drives the deep-ocean conveyor, and scientists worry that if the conveyor weakens or stops, climate patterns from monsoons to North Atlantic weather could shift dramatically.
For famous individual oceans including the Pacific, Atlantic, Indian, Arctic and Southern, see the Oceans section.