
WATER

Like one huge, interconnected washing machine, Earth's oceans are in constant turmoil.
Heat from the sun warms water at the equator. This warm water is pushed by winds and Earth's rotation to the poles where it sets in motion the churning cycles that oxygenate the deep ocean and bring back vital nutrients to the surface waters where life can make use of them.

Against this backdrop, life has exploited the myriad marine habitats and all but one of the major animal groups alive today have made their homes there.
Water is essential for life on Earth.
It maintains cell shape, takes part in chemical reactions, and transports nutrients around the body.
Earth's oceans were probably condensed from water vapour about 150 million years after the formation of the planet. The first, single-celled, lifeforms on Earth probably evolved deep underwater, in hydrothermal vents approximately 750 million years after the formation of the Earth.
Today, the deepest part of Earth's oceans is the Mariana Trench, in the Pacific Ocean southeast of Japan, which reaches 11km below sea level.

Animal cells are composed of around 50-95% water.
Animal cells are composed of around 50-95% water.

It is estimated that only around 20% of the oceans have been explored and documented by humans.
It is estimated that only around 20% of the oceans have been explored and documented by humans.

UNDERWATER RAINFORESTS
From towers of green across the ocean surface to carpets lining the sea floor ...
...kelp beds fringe coastlines across the world.
These brown algae create ecosystems that are home to over 200 species of animals and, like their tiny phytoplankton cousins, play a part in absorbing carbon from the atmosphere.
Kelps are known for their remarkably high growth rate. Some species grow half a metre a day and reach heights of 80 metres.

Kelps are found in mild, temperate, nutrient-rich waters.
Kelp forests are home to dozens of species.
They graze, hunt, and live as parasites on one another in a living network called a food web.

Intricate kelp forest food webs show us the connectivity of life in the oceans:
Studying food webs can also help us understand how energy is transferred between organisms.
Food webs are balanced as energy is transferred between all of the member species, starting with the primary producers (e.g. kelp) at the base of the web. Primary consumers (e.g. sea anemones) eat primary producers, meaning energy flows from producer to consumer. Secondary consumers (e.g. otters) eat primary consumers, and energy is transferred once again. Food webs also contain decomposers , which eat decaying organic matter such as faeces and the remains of dead organisms.
The connectivity of life means that even small changes in ecosystems can have dramatic effects.
If the sensitive balance of a food web is disrupted, it can cause an event called a trophic cascade. These events affect every level of the food web, sometimes with devastating consequences for entire ecosystems.
A bottom-up trophic cascade in a kelp forest.
A bottom-up trophic cascade in a kelp forest.
Bottom-up cascades in the kelp forest
These are caused by events at the base of the food web — in this case, the kelp itself. Bottom-up cascades are often triggered by changes in the environment which affect the growth of primary producers.
If sea temperatures rise too high. nutrient levels drop and the kelp dies off. First to feel the impact are creatures that feed on kelp, followed by the creatures that feed on them. A bottom-up cascade quickly spreads throughout the entire trophic web.
A top-down trophic cascade in a kelp forest.
A top-down trophic cascade in a kelp forest.
Top-down cascades in the kelp forest
These are caused by events at the middle or top of the food web, usually as a result of a predator population crashing.
In the 1990s, researchers at the University of California noticed a decline in sea otter populations around the Aleutian Island in the North Pacific Ocean. There was an increase in the presence of orcas in the surrounding waters, acting as a new predator in the food web. They preyed on sea otters, meaning urchin numbers rose, and the kelp was overgrazed.
The mystery was solved when researchers found that orcas were adapting to the loss of their natural prey: large whales. Whale populations in the North Pacific had plummeted due to commercial whaling after the Second World War.

SMALL BUT MIGHTY
Life on Earth relies on an abundant source of carbon.
Carbon is probably the most useful element that we know of; used to build structures from tree trunks to coral reefs to bird feathers. But it is also a major greenhouse gas, trapping heat in the atmosphere and warming climates.

The global availability of this element relies, in part, on some of the smallest organisms in the ocean: phytoplankton.
Phytoplankton drift with the ocean currents, sometimes protected by a minuscule silica or calcium shell.
Phytoplankton include a range of tiny but beautifully bizarre photosynthesising organisms, such as:
algae
cyanobacteria
diatoms
dinoflagellates





As phytoplankton photosynthesise, they generate energy from sunlight and atmospheric carbon dioxide. Simultaneously, they lock carbon away in their shells and soft tissues and release oxygen into the water.
Phytoplankton are responsible for around half the oxygen production on Earth, despite being only 1% of global biomass.
Phytoplankton 'blooms', like the one shown here off the southwest coast of the UK, can cover hundreds of square kilometres.
Blooms draw tonnes of carbon from the atmosphere down into the oceans as the tiny phytoplankton are eaten, or die and float down into the depths as marine snow.
Phytoplankton make a meal of sunlight, using it to grow.
Where photosynthesising organisms – even tiny ones – grow, there are animals to eat them.
Krill, jellyfish, segmented worms and thousands more animal species make nightly journeys from the depths to the surface waters in order to feed on plankton. Migrating upwards during the cover of darkness allows animals to avoid the predators that patrol the sunlit surface waters.
When they return to the deep each morning, these organisms carry with them the carbon they have ingested from the phytoplankton.

Explore the interactive map below to learn more about diurnal vertical migration...
Areas of the ocean high in nutrients can support explosive rates of phytoplankton growth. These areas are termed 'productive' as they kickstart a food chain that can support huge numbers of animals, as well as lock away carbon from the atmosphere.
Few places on Earth are more productive than the polar oceans in summer where around-the-clock sunlight allows for strong phytoplankton growth.
Despite comprising only around 10% of total ocean surface area, the Antarctic Ocean is responsible for around a third of the carbon dioxide absorbed by all oceans.

Phytoplankton production at the North Pole
Phytoplankton production at the North Pole