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.
In January 2022, Tonga was cut off from the rest of the world after the Hunga Tonga-Hunga Ha’apai volcano exploded and severed a submarine internet cable. Full connection was not restored until February when the cable linking it to Fiji was repaired.
There are other threats, too. Researchers recently discovered underwater “rivers” flowing along sea beds. One running south from Newfoundland cuts across many cables connecting the US to Europe. In 1929, 23 underwater telegraph cables were cut when a rush of sediment roared down the river’s channel. (Read more here.)
Today, undersea cables could be targeted by states wishing to sabotage the economies of their rivals, threats heightened over growing tension with Russia.
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.
In experimental studies performed in aquariums, marine organisms sensitive to magnetic fields have been shown to exhibit behavioural responses to EMF, although at exposure levels far larger than those emitted by power cables. But sharks, rays and chimaeras, for example, are known to have evolved organs that are exquisitely sensitive to electrical fields: the ampullae of Lorenzini. These electroreceptors form a network of mucous-filled pores in the skin of cartilaginous fish – highly specialised organs optimised to detect prey, and that have a threshold sensitivity of less than a single microvolt.
“Future field studies – particularly that represent a collaboration between ocean researchers and cable operators and owners – will help further our understanding,” says Clare. Taormina’s study suggests animals that migrate along the continental shelves might be affected by a cable’s electromagnetic field, moving either inshore or offshore away from their normal path, but he also agrees that more study on EMF is needed.
Submarine cables do not pollute: they are stable, inert structures that can even be recovered and recycled after they’ve served their time
While studies of the deep sea are expensive, time-consuming and resource-heavy, they can help fill that information gap. Almost two decades ago, researchers at the Monterey Bay National Marine Sanctuary, in collaboration with the National Oceanic and Atmospheric Administration (NOAA), conducted a survey of a seamount thermometry cable on the deep seafloor off the coast of central California – a survey considered unique at the time for investigating the biological impact of subsea cables. Remote operated vehicles (ROVs) carried electronic cable-tracking systems into the deep waters of Half Moon Bay, allowing researchers to find parts of the cable that had been buried under sediment (the cable was initially laid down in 1995 as part of an experiment to detect changes in ocean temperature by monitoring the speed of sound waves in the deep sea). As the ROVs scanned the cable’s roughly 95-kilometre (59-mile) length, scientists collected sediment samples, video and still images of animals living on or near the cable.
In silty areas, the most obvious biological effects of the cable were the neat lines of sea anemones that researchers discovered growing on the cable itself. Frequently, these sea anemones were attached directly to parts of the cable that had been buried under mud or silt. Researchers concluded that these anemones likely would not have been able to colonise such soft-bottom areas without the presence of the seafloor cable, which provided a firm footing for the animals. Removal of such cables would therefore affect a small ecosystem of marine creatures who call that cable home.
Beyond localised habitat damage or loss, submarine power and communication cables may temporarily or permanently impact the marine environment through heat, turbidity (during cable burial), risk of entanglement and the introduction of artificial substrates. Still, areas through which cables pass are often designated as protected, meaning anchors, bottom trawls and even fishing can be restricted. The Cook Strait Cable Protection Zone (CPZ) in New Zealand, for example, restricts fishing near cables, effectively creating a reserve and thus improving fish stocks.
And submarine cables do not pollute: they are stable, inert structures that can even be recovered and recycled after they’ve served their time (about 20-40 years, on average). “The carbon footprint is actually relatively low compared to most of the internet’s infrastructure,” says Nicole Starosielski,associate professor at NYU. Her book, The Undersea Network, examines the cultural and environmental dimensions of transoceanic cable systems, and she adds an important social science perspective to the discussion. “We’ve actually advocated for more cables, connecting large onshore data centers on renewable grids, in order to minimise fossil fuel consumption.”
Indeed, small developing island states are crucially tethered to these elaborate cable systems, without which they would struggle to obtain green energy, telecommunications, remote-work technology, e-medicine and other digital services. Ocean life – and its often-complex interaction with human activities – is riddled with unknowns; for ecologists worried about environmental conservation, these subsea cables remain a serpentine question mark.
But, as Clare explains: “There is value in the research, which will help industry leaders, policy-makers, cable companies and other parts of the wider Blue Economy strive to ensure any development of the seafloor is as sustainable as possible.”