Written by Owen Harris
Increasing awareness about anthropogenic climate change and mounting public pressure has led to many countries committing to reduce their use of fossil fuels and increase their development of renewable energy sources. Although the switch to renewable energy will have an overall positive effect on the global climate and natural world, the construction of renewable energy installations can have both positive and negative effects on local ecosystems. One such example is offshore wind farms (OWFs) which can have some significant effects on the marine environment. This article focuses on some of these effects and the individual marine species that can be impacted by them, which need to be properly considered before new OWFs are installed.
On the surface OWFs seem like a positive environmental solution, but look beneath the waves and there is more to consider than you might think
Environmental damage from construction
One of the first effects OWFs have on their nearby surroundings is physical damage caused by their construction. The main effects of this are seafloor habitat destruction and sediment suspension in the water column, caused by the disruption of sand and silt from the seafloor. Sediment suspension is likely to have a negative impact on fauna by increasing turbidity, mobilising contaminants and smothering sessile suspension-feeding animals, such as corals, sponges and anemones. A reduction in visibility from sediment suspension can also affect photosynthesis in algae and disrupt key behaviours in visual animals.
Different types of OWF foundations will have different levels of effect on the seabed. Monopile foundations consist of a single tube which is hammered into the seabed. This type of foundation can only be used in shallower waters up to a depth of 30 m due to hydrodynamic forces. Tripod (three-legged) and jacket (four-legged) foundations are more stable and therefore can be used at greater depths, but their construction will have a greater environmental impact due to the amount of material that is driven into the seabed. However there are alternative solutions, such as floating structures which can be anchored to the seabed and reduces the need for pile-driving.
One of the main issues caused by the construction and operation of OWFs is that they emit a lot of noise into the marine environment. Known as marine noise pollution, this can affect the behaviors of marine animals as well as potentially causing serious injury. Pile-driving during the construction of OWFs can generate noise up to 200 dB, while the operation generates up to 120 dB. This noise is mainly generated above the water but transmits through the tower and is then radiated into the surrounding water, adding to pre-existing noise from other sources. This can affect animal behaviour, particularly those that are more sensitive to sound, that rely on their use of vocalization for communication and those that use echolocation for navigation, such as cetaceans.
In salmon, bass and harbour porpoises, it has been found that 90 dB is the level of noise that causes avoidance behaviours. This amount of noise is not instantly harmful to these animals, but prolonged exposure for around 8 hours could equal exposure to 130 dB which can cause either temporary or permanent hearing damage. An example of an affected species is harbour porpoises who have been found to initiate avoidance responses within 20 km of pile-driving activity, whilst pile-driving noise has also been shown to disrupt their vocalizations which can take up to 72 hours to return to normal.
Noise mitigation systems such as bubble curtains (bubbles produced by hoses on the sea floor around the base of the turbine) have been shown to reduce the level of disturbance in harbour porpoises by 90%. However the hindrance that OWF construction poses to their communication could have a negative impact on their social interactions and migrations. This could also be the same for other cetaceans, but there is a lack of research in other species.
Other affected species include cod and herring, who can detect pile-driving noise from up to 80 km away. Additionally, cod and sole have been found to have behavioural responses to pile-driving noise, which included initial avoidance, higher swimming speed and habituation after time.
When exposed to the sound produced by an air gun (222.6 dB which is 10% louder than the noise produced by pile-driving) pink snappers have also been found to have sustained ear damage, meaning they and many other fish species are likely to be impacted as well.
Meanwhile seismic surveys (used by OWF construction vessels to map the seafloor) have also been shown to cause avoidance behaviors in humpback whales. Although the noise output from seismic surveys may differ from that of OWF construction/operation, it is still worth noting the effects that noise pollution can have on animal behaviour, biodiversity and ecosystem functioning.
Another potential issue with OWFs is electromagnetic fields (EMFs), which are generated by the transportation of the acquired energy through electric cables that are built into the seabed. They could have an effect on the behaviour or physiology of fauna which use electroreception for detecting prey or conspecifics such as sharks and rays. A study on the small-spotted catshark found that they showed avoidance behaviours when exposed to electric fields which were equal to the maximum output of undersea cables, despite being attracted to much smaller electric currents which mimicked their prey. Thornback rays and spurdogs have also shown avoidance behaviour in the presence of EMFs, but more research is needed.
One of the more overlooked issues associated with OWFs is the introduction of non-indigenous and invasive species, which presents a threat to biodiversity. Artificial structures (including OWFs, oil rigs, breakwaters and ports), are known to promote the spread of non-indigenous species, which can disrupt trophic webs and cause shifts in the populations of native species, normally with a negative impact on the overall ecosystem.
Between 2008 and 2011, nine non-indigenous species were found on monopiles in the Egmond aan Zee OWEZ wind farm off the coast of Holland, eight of which have been described as invasive. One of the species, the Pacific oyster, even increased in abundance during the survey. On the Thorntonbank wind farm in the Belgian part of the North Sea (BPNS), ten non-indigenous species were found, seven of which are potentially invasive. The same invasive species were found in the C-Power and Belwind wind farms on Bligh bank, also in the BPNS. At the Horns Rev OWF the species dominance in a benthic community was found to have changed after its construction, in favour of the invasive amphipod Jassa marmorata. These are all examples of how OWFs can encourage invasive species and with more sites being constructed throughout the world’s oceans this is a problem that is likely to only get worse.
Known invasive species in OWFs include…
- Barnacles (Balanus perforatus, Elminius modestus & Megabalanus coccopoma)
- Japanese skeleton shrimp (Caprella mutica)
- Pacific Oyster (Crassostrea gigas)
- Slipper limpit (Crepidula fornicata)
- Japanese shore crab (Hemigrapsus sanguineus)
- Amphipods (Jassa marmorata)
- Midges (Telmatogeton japonicus)
On the flipside of the invasive species problem, OWF construction also introduces new habitats for indigenous species as well. It does so by introducing three-dimensional, hard substrate structures which act as artificial reefs in what would usually be a vast and flat seabed. It has also been argued that OWF construction only temporarily causes damage (through things like sediment suspension) and in the long-term can actually create up to 2.5 times more habitat space, which in turn has more potential for colonization than the original sediment based habitats. Not only does the foundation pile have the potential for harbouring intertidal, sessile organisms, but the scour protection at the foundation can also provide habitat for fish as well (see below). As more research is done in this area future OWFs may even be designed to maximise their ability to create new habitats and provide an even bigger positive benefit.
Another potential benefit of OWFs is an increase in biodiversity, which is one of the main indicators of ecosystem health. This is believed to be the result of a reduction in sediment grain size and an increase in organic matter in the vicinity. As well as the introduction of ‘ecosystem engineers’, such as the polychaete worm Lanice conchilega, which have been shown to increase biodiversity in their ecosystems.
On the Nysted OWF in the Danish part of the Baltic Sea, more biological activity was recorded further up a turbine where blue mussels were most abundant. This species modifies habitats by filtering organic matter from the surrounding water and acts as a secondary hard substratum which promotes biodiversity. The Egmond aan Zee OWEZ wind farm has also increased the diversity and abundance of benthic organisms and attracted higher abundances of certain fish, mammals (including harbour porpoises) and in some cases even birds. Biodiversity at the Horns Rev OWF in the Danish North Sea was found to be higher and also had a 7% increase in biomass resulting in 50 times more food available for local fish populations.
Refuges for commercially exploited species
It has been suggested that OWFs can also act as a safe space for many commercially targeted species due to the increased abundance of food available and protection from fishermen, who tend to avoid OWFs for fear of entanglement. Research has shown that cod and pouting have benefitted from the construction of OWFs, as well as some species of crab. This is an area of research that again requires further work, but it shows another potential benefit that OWFs can provide. In the future it may even be possible to combine OWFs and Marine Protected Areas (MPAs) as a way of further protecting commercial species whilst revitalizing threatened ecosystems.
For now, evidence from the reviewed literature shows that OWFs have an overall negative impact on local ecosystems, mainly due to the damaging effects of marine noise pollution from construction and operation of turbines. However, it is impossible to determine to what extent these negative effects can be counteracted by the positive effects. For example, OWF construction temporarily destroys habitat but in the long-term creates more habitat once the structure has been colonised. Additionally, OWFs have been found to increase local biodiversity but at the cost of facilitating the colonisation of invasive species. More research is needed on this topic so that it can be determined whether the expansion of renewable energy industries should focus on alternative sources of renewable energy such as solar energy to mitigate the ongoing climate crisis, especially if we are to protect the marine environment at the same time.
Owen is a BSc (Hons) Marine Biology student at Swansea University. He is currently interning with Swansea University’s Coastal Ecology Research Group, assessing megafauna biodiversity and conservation priorities in anthropogenically disturbed habitats with hopes to enhance marine renewable energy structures. You can follow him on Instagram @oceanmaster93 or get in contact with him on LinkedIn here.