Beneath the blue: dive into a dazzling ocean under threat – interactive

This is a North Atlantic right whale. They’re near extinction: only about 350 are left. Many are killed by fishing equipment, including abandoned gear or “ghost gear”.

One of the worst culprits: gillnets. These vertical walls of fine-meshed webbing are held down by weights and ropes. They’re nearly invisible, and can be 2km long and stretch down 60m. Whales can easily become entangled.

The ropes cut deep – around the mouth, tail and flippers. These stressful injuries can hinder a whale’s ability to mate, dive and eat, often leading to a slow and painful death. Trapped whales can drown, too. Studies show 85% of right whales have been entangled at some point in their lives.

It’s not just whales. Fish, other marine mammals and sea turtles are also affected, some by crab and lobster traps.

What can be done?
Ghost gear is an easy problem to solve, in theory: ban gillnets, collect the ones that already exist and recycle them. Along with the introduction of ropeless lobster traps, it would go a long way to reducing the threat to animals including our North Atlantic right whale.

That’s bad news for coral reefs, home to a quarter of all ocean species. On a healthy reef, zooxanthellae (a type of algae) live in a symbiotic relationship with the coral.

Marine heatwaves, however, are becoming more severe, devastating the zooxanthellae: one laboratory study found 79.9% less algae after 18 days when water temperatures rose from 28C to 32C.

This can have a disastrous consequence – coral bleaching. This begins when corals lose more than 60% of their zooxanthellae, and has a knock-on effect on other marine life.

The problem is compounded by rising acidification, as the ocean absorbs more carbon dioxide. Acidic water makes corals more vulnerable to bleaching, as it reduces the availability of calcium minerals for skeleton building and repair. Rising ocean temperatures also lead to deoxygenated ‘dead zones’

What can be done?
Fixing coral bleaching is as scientifically straightforward as it is politically complex: cut carbon dioxide emissions, starting immediately. Reducing fertiliser and sewage runoff would also help prevent deoxygenation.

Sound is essential to the survival of the orca. These chatty whales emit clicks, up to 5,000 per second, which bounce off the swim bladders of salmon. This echolocation allows orcas not only to detect the presence of fish but to determine if it’s a favoured prey, such as Chinook salmon.

The highly social orcas also use sound to communicate with family members, to find mates and to hunt collectively. Every clan has its own unique “dialect”, which is taught to each new generation.

But huge ships on the surface are constantly emitting underwater noise, too. Their propellers can drown out the orcas – the equivalent of humans trying to have a conversation under the path of a jetliner taking off.

Seismic airguns used in oil and gas exploration are also deafening. Noise from ships can cause orcas to flee their hunting grounds, and reduce the range of echolocation by as much as 95% – an existential threat to this already endangered species.

What can be done?
Sound pollution can be reduced. Better ship design and ocean speed limits have helped lower the noise. Prohibiting seismic airguns while whales and dolphins are migrating would further help these animals to live, hunt and mate unimpeded.

These are microplastics. They’re particles smaller than 5mm, mainly PET (polyethylene terephthalate, used in bottles and packaging) and polyamide (from clothes). You may have heard of the “islands” of plastic floating on the ocean surface – but it’s here, in the semi-darkness, where plastic is most concentrated.

The tiny bits of plastic are eaten by animals, such as the giant larvacean. These remarkable creatures are 10cm-long filter feeders, which build “mucous houses” up to a metre long to capture food particles.

When a house gets filled with particles too big to eat, the larvacean jettisons it and the structure sinks to the ocean floor … taking all the microplastic with it. The plastic ends up in seabed sediment, where it will be ingested by other organisms.

There’s a lot about microplastics we don’t know. It affects deep sea animals’ ability to forage and reproduce, but the impact on predators such as sharks is less clear – although they would be directly affected if plastic caused their prey to decline.

What can be done?
Given the huge volume and microscopic size of ocean plastic, there’s no easy way to remove it. The best we can do for now is to be proactive: use less plastic, and substitute less damaging materials.

When an oil tanker leaks, or an underwater well head breaks, thousands of barrels of oil can be released. In response, clean-up crew often use something called “dispersant”, which binds to the oil and breaks up the oil slicks. But the oil doesn’t actually vanish.

It’s broken into small droplets, which sink – causing huge plumes of oil, which can spread underwater for miles. For example, the use of dispersants in the Deepwater Horizon disaster in 2010 created a plume that was 35km long, 2km wide and 200m deep … hidden 1km below the surface.

The droplets are tiny, about the diameter of a human hair (not to scale in the illustration). They either remain suspended, are consumed by oil-eating microbes, or attach to heavier particles and sink, potentially harming marine life on the seabed.

Although dispersant itself is not thought to be highly toxic, it magnifies the toxicity of crude oil. Laboratory studies have shown the mixture inhibits the growth and survival of deep sea coral larvae. We just don’t know how harmful to marine life it could be.

What can be done?
Of course, if we consume less oil, then we ship less oil, which means fewer spills. Technological improvements are helping somewhat – the number of large oil spills has fallen, from 77 in the 1990s to 18 in the early 2000s – and new dispersants could be less toxic. But using less oil is best.

Yet, even here, there is a threat. Persistent organic pollutants (POPs) are a group of toxic chemicals that do not break down.

Many of the worst offenders – industrial chemicals such as polychlorinated biphenyls and polybrominated diphenyl ethers (PCBs and PBDEs) – have been discovered in crustaceans in the very deepest part of the ocean: the trenches.

Scientists believe POPs end up in trenches, such as the Mariana Trench in the western Pacific, after being absorbed by microplastics – which sink and are eaten by deep sea animals – or when contaminated dead organisms and particles drift down like snow.

POPs are endocrine disruptors and have been linked to cancer and other diseases. They resist degradation, and bioaccumulate and biomagnify as they move up the food chain.

The trenches could be the worst hit. A 2017 study found that several species of crustaceans there were more contaminated with PCBs and PBDEs than even the most POP-polluted surface waters. Scientists have even named one newly discovered species Eurythenes plasticus.

What can be done?
Obviously, banning these chemicals would help, and PCBs were outlawed in the US in 1976. But the ones that exist aren’t going away. Better landfill controls would at least help to prevent POPs from leaching into waterways, and from there to the ocean.

Mining corporations are poised for what they hope will be a banner year in 2021: the approval of regulations to allow the mining of the planet’s final frontier, the deep sea.

Deep sea mining would be done by gargantuan machines digging into the ocean floor.

Their target is what are known as polymetallic nodules. These potato-sized rocks are rich in nickel, cobalt (used in modern electronics) and other metals. Billions of nodules cover the seabed, particularly in a vast region called the Clarion-Clipperton Zone between Hawaii and Mexico.

Individual nodules are tiny ecosystems – home to anemones and sponges that in turn host corals, tube worms and other bottom-dwelling critters. Ghost octopuses use the nodules as nurseries, laying their eggs on the stalks of dead sponges attached to the rocks.

Deep sea mining machines would suck up the nodules to the surface through a tube. Nodules take millions of years to form, so their loss will be permanent on a human timescale. It will also destroy all marine life on the nodules and in the surrounding soil.

In addition, the process will create plumes of sediment that could settle for miles, suffocating organisms on the seabed and killing other marine life in the water column.

What can be done?
Groups such as Greenpeace and the Blue Planet Society are campaigning against deep sea mining, but many expect the International Seabed Authority to approve it soon, giving the green light to the exploitation of one of the planet’s least-understood ecosystems.