Cables and pipelines & Marine protection and restoration

The cable route preparation and the cable installation itself are critical steps during which disturbance can occur to marine habitats. According to a 2018 study led by IFREMER and France Energies Marine studying the potential impacts of submarine power cables on the marine environment, “these methods of reworking the seabed may lead to direct destruction of benthic habitats, flora and fauna” [5]. Benthic communities can indeed be deeply affected by such processes, notably by displacement, damage or crushing of organisms.

Additionally, this disturbance to the seabed from cable installation can also cause sediment resuspension, which can decrease water transparency, thus limiting light and impacting feeding ability of fish that detect their prey visually. Turbidity can also temporarily obstruct the filtration mechanisms of some benthic and pelagic organisms. Eventual settlement of resuspended sediments can also smother sensitive benthic habitats and associated species.

Such impacts can cumulate over a wide area as each installation and maintenance operation are consecutive and follow their own timeline. However, it is important to bring some nuance when it comes to such impacts. A 2009 study led by OSPAR on the assessment of the environmental impacts of cables recalls that “these effects are mainly restricted to the installation, repair works and/or removal phase and are generally temporary. In addition, their spatial extent is limited to the cable corridor (in the order of 10 m width if the cable has been ploughed into the seabed).” [6].

Submarine power cables can have a localised influence on water temperature that can impact sediment-dwelling species and the egg development of species that bury their eggs in the sediment.

The transport of electricity generates a certain amount of heat, leading to an increase in temperature of the cable’s immediate surrounding environment. Studies conducted on this issue showed a maximal temperature increase of about 2.5 ºC at 50 cm directly below the cable [7]. When cables are buried, thermal radiation can significantly warm the surrounding sediment in direct contact with the cable, especially in the case of cohesive sediment [8] .

The increase in temperature around the cable can theoretically lead to the modification of the chemical and physical properties of the water such as oxygen concentration and therefore indirectly impact the development of microorganisms. Additionally, temperature increase can cause small spatial changes in migratory behavior of species avoiding areas with warmer waters [9].

This specific impact from the increase of water temperature caused by underwater cables on surrounding ecosystems must however be nuanced by the fact that in-situ investigations and field measurement are significantly lacking. The lack of field data calls for more research work in this domain in order to fully understand the real impact on species. 

Submarine cables can emit electromagnetic fields (EMF), which may disrupt the behavior of certain species that rely on them to navigate or hunt. The emission of artificial EMF can alter their ability to detect and respond to natural electromagnetic signals.

Some species are known to be particularly sensitive to EMF such as elasmobranchs (rays and sharks), fish, mammals, turtles, molluscs and crustaceans, as they utilize the Earth’s geomagnetic field for orientation and migration. EMF can then negatively interact with these sensitive marine species, notably through modified predator/prey interactions, avoidance of certain areas, effects on navigation/orientation capabilities, as well as physiological and developmental effects [10].

The EMF characteristics are highly dependent on the type of cable, the type of current it uses and whether it is buried or not, as the sediment layer reduces EMF [11]. 

Consideration of this impact also needs to be balanced alongside several studies that demonstrate a relatively low level of impact of EMF on species' behavior. A 2016 study on “Assessment of Potential Impact of Electromagnetic Fields from Undersea Cables on Migratory Fish Behavior” concluded that “cable activation did not appear to impact the success of either migration type in this species.” [12]

When insufficiently buried, the submarine cables can create artificial reefs that act as solid substrate and lead to a process of bio-colonization by some hard-substrate benthic species.

This “reef effect” is usually considered to be a positive anthropogenic impact, as artificial reefs provide grounds for species to develop, thus increasing biomass. 

However, reef effect may potentially result in long-term negative effects as the structure can facilitate the introduction of non-indigenous species. This introduction of non-local fauna can lead to an alteration to the natural benthic community. Indeed, the non-indigenous species can outcompete the native species as their numbers on new hard artificial substrate can be 2.5 times higher than on natural substrate [13].

In the event that a pipeline is damaged by an anchor or fishing gear, this can lead to the loss of potentially harmful elements into the environment. Such spills can occur over a relatively large period of time as the leak can sometimes only be detected several weeks or even months after being damaged by an anchor [14]. However, this kind of incident remains very uncommon. 

The principal risk posed by cables and pipelines is the potential impact on sensitive habitats such as habitats hosting slower growing, vulnerable or fragile species. Avoidance of areas such as MPA’s for cable placement is a basic prerequisite and an immediate and appropriate mitigation measure is to avoid habitats of conservation interest (seagrass meadows for example).

As mentioned above, the main challenge is related to the environmental impact cables can have when crossing sensitive habitats. To avoid such impacts, any new cable settlement should comply with an Environmental Impact Assessment covering the mapping, installation, operation and removal of such cables. As mentioned in the OSPAR study on the Assessment of the environmental impacts of cables, such assessments “should take into consideration the site specific biotic and abiotic features of the cable route” and “appropriate mitigation measures should be identified and applied” [15].

Some technical solutions can be considered in order to mitigate the negative impacts of submarine cables. Here are a few: as explained above, the EMF characteristics are highly dependent on the type of cable, the type of current it uses and whether it is buried or not, as the sediment layer reduces EMF. Therefore, it is recommended by the OSPAR Commision [16] to use adequate shielding to minimize the emission of EMF, as well as to properly bury the cables to reduce exposure of sensitive species to EMF by increasing the physical distance of the species to the cable. Additionally, OSPAR encourages the use of cables generating reduced EMF such as three-phase AC-cables or bipolar HVDC transmission systems. 

When it comes to the disturbance of the seabed, OSPAR also recommends “an appropriate trenching depth to limit the increase in sediment temperature to prevent macrozoobenthic fauna from harm and benthic communities and processes from changes”.