Adrienne Bernhard/ BBC:

Tens of thousands of miles of cables crisscross our deep seas, ferrying data between continents and carrying renewable power from offshore energy platforms to the land. These snaking, artificial structures can serve as shelter to a vast array of bottom-dwelling sea life: anemones, sponges, corals, sea stars, urchins, worms, bivalves, crabs and other invertebrates have been found to take up residence on or near undersea cables.
But marine scientists believe we need a greater understanding of how electromagnetic fields (EMF) generated by submarine power cables might affect some of these delicate creatures, many of which rely on their own internal sense of magnetic north to navigate or use electric fields to help them hunt. Given that the number of submarine cables will only multiply as the marine renewable energy sector grows, what threats do they pose to life underwater, one of the last spots on Earth largely untouched by humans?
Undersea cables can be divided into two broad categories: telecommunication cables and high-voltage power cables. Telecommunications cables are laid on the surface of the seabed where they cross deep seas, while power cables, which tend to be found closer to shore, are typically buried under sediment for protection. Today, around 380 underwater telecommunications cables are in operation around the world, spanning a length of over 1.2 million kilometres (745,000 miles). This map shows all active subsea fibre-optic telecommunications cables – many of them featuring whimsical names like Apricot, Concerto, Topaz, Polar Express or Meltingpot.
Telecommunications cables provide the information pathways for more than 95% of international data. And offshore wind and hydrokinetic power plants also rely on submarine cables. Over the past few decades, as renewable energy projects proliferate, researchers have begun studying their environmental effects.
For most of its journey along the ocean floor, a telecommunications cable is about as wide as a garden hose, its digital data-carrying filaments no larger in diameter than a human hair. Power cables are generally larger in size (between 7-30 cm/2.75-12in) and are sheathed in a few layers of metal for enhanced protection. Subsea cables are carefully routed to avoid hazards that could damage them, such as earthquakes and underwater landslides. To minimise any accidental damage that may occur in shallower waters (for example, damage caused by human activities such as fishing, ocean trawling and anchoring), cables must be buried below the seafloor.
“During subsea installation, companies will try to bury a [power] cable beneath the sediment to protect it,” says Bastien Taormina, a researcher at the Norwegian Institute of Marine Research in Bergen. “This has a much bigger impact on the surrounding habitat.” Taormina is the lead author of an oft-cited study on the effects of artificial structures on marine ecosystems, published in the Journal of Environmental Management. Over a span of five years, he and his team studied the submarine power cable of a tidal energy test, taking pictures of species that colonised the cable and associated structures.
Installation of a cable disturbs the surrounding seabed. Somewhat paradoxically, that can lead to greater initial biodiversity, says Taormina. “Opportunistic species will survive, but that doesn’t mean it’s a good ecosystem, because these species, while diverse, won’t stick around.” This phenomenon is what’s known as ecological succession: the process by which communities gradually replace one another until a “climax community” – such as a mature coral reef – is reached, or until a disturbance, like a fire (or in this case an electrified submarine cable), occurs.
With nearly all of the world’s internet and banking transactions conducted over underwater cables, there is growing concern about their vulnerability.
Another possible consequence of undersea power cables is their generation of electromagnetic fields (EMF). The intensity of EMF is a direct function of the current passing through a cable and the depth at which it is buried, as well as the distance between cables (if multiple cables are running in close proximity, for example). EMF can distort the natural geomagnetic field that marine organisms rely on to navigate, particularly if they swim or drift 10 metres near the cables.
“There is a need to further study electro-magnetically susceptible species,” says Michael Clare, leader of Marine Geosystems at the National Oceanography Centre. “What’s the threshold at which EMF presents a problem for these sea creatures?” Most institutions and scientists (including Clare) are hesitant to make any causal link between subsea cables and the behaviour of marine organisms.
“It has been suggested that behavioural movements in organisms such as skates and lobsters can be affected by EMFs, but whether they are affected by the EMF intensities generated by power cables remains unclear and the subject of ongoing research,” Clare adds.
After completing several impact studies, the US Department of the Interior noted that “brief lingering activity near undersea cables have been observed, the data do not currently support a finding that overall navigational capabilities in fish are impaired”. Much of the available peer-reviewed field studies performed to date also support this statement.