Offshore windfarms are sited in marine locations and have greater potential for generating electricity than their landbased counter parts. Greater windspeeds and fewer obstacles to reduce windspeeds result in offshore wind farms being more efficient. However, being sited far away from land greatly increases their installation costs.
Northern Europe currently holds the greatest density of offshore wind farms with the UK and Denmark being the leading countries in their development. The largest offshore windfarm is currently London Array (in the outer Thames Estuary, UK) which has a capacity of 630MW. This is planned to increase to 1000MW. Considerably larger windfarms are planned, notably on the Dogger Bank in the North Sea, where a 9000MW development has been proposed.
The impacts of offshore windfarm developments on marine wildlife and habitats is not fully understood and research is on-going to improve knowledge in this area. However it is known that offshore windfarm developments can affect wildlife through:
- Noise pollution
- Electromagnetic fields generated by power cables
- Disturbance and loss of sea floor communities
- Disruption of marine mammal habitats
- Obstruction of bird migration routes and seabird fishing grounds.
It is also known that man made structures can provide a surface on which a variety of marine life can live. Many plants and animals require a hard surface to attach to and submerged turbine bases can provide the perfect place for them. The varying depths on the submerged base of a turbine creates a number of different habitats, each with a variety of species which are adapted to life there.
Renewable energy sources are now a widely preferred alternative to traditional fossil fuels for producing electricity. There are many reasons for this, but the main argument is that they are cleaner and cause less harm to the environment. There are many types of renewable energy sources available but recently wind power has emerged as the most popular due to decreasing costs and efficiency. As a result windfarms are now becoming a common sight and after first appearing on land, developments are increasingly common in offshore areas.
Despite their higher installation costs, OWFs are often preferable to terrestrial sites. This is because there is a reduction in visual and noise pollution and they are space efficient, as no land is lost to their developments which could be used in other ways. However, throughout their lifecycle of construction, operation and decommissioning, OWFs do have an impact on their surrounding environment. These impacts can have both positive and negative consequences for biodiversity.
The first OWF was installed in Denmark in 1991 and by the end of 2012, 55 OWFs were operational in 10 European countries. Despite considerable work being done, more research is required to fully understand the long term impacts of the OWFs on biodiversity. Work is being done to map, assess and quantify the impacts of the installation and operation on the environment and biodiversity.
Local environmental conditions at each OWF also influence the impacts. These include:
- Sea floor geomorphology
- Sea floor topography
- Wave conditions
- Proximity to land
- Sitting on migration routes.
As a result, each OWF can affect different species in different ways.
During construction and installation of OWFs, marine habitats are altered and the sediment is disturbed. Consequently, there is a negative impact on the species living on the sea floor where wind turbines are installed. However, this affects a relatively small proportion of the total windfarm area as the bases of turbines are relatively narrow. Therefore, only 1% of the total windfarm area results in permanent habitat alteration. Sediment disturbed during the installation process can create problems for fish, sea mammals and seabirds. However this affect is not permanent as the sediment settles again.
Electromagnetic pollution is caused by electrical currents which flow through the cables in the seafloor. Migratory fish, sea mammals, sea turtles, sharks and some crustaceans all use the earth’s natural magnetic field to navigate and this makes them susceptible to electromagnetic pollution. To mitigate these effects cables are sheathed and buried deep in the sea floor. Research has shown that these measures are an effective way of controlling electromagnetic radiation.
Noise pollution is caused during all stages of a OWF’s life. During the construction and decommissioning phases, noise levels are high due to pile driving and increased ship and helicopter traffic. During operation there are constant but low levels of noise pollution due to rotation of the blades. The effects of noise pollution can be divided into four main effects: masking, behavioural disturbance, hearing loss and in severe cases, injury. Some marine species are more sensitive to noise pollution than others with marine mammals and fish most sensitive.
Additional impacts, particularly during the operational phase, include disturbances and alterations to the water circulation patterns and shadowing (caused by the rotating blades). Each stage of the OWF may affect species in various ways. The most vulnerable species to the environmental effects of the OWFs are seabirds (avifauna), migratory birds, marine mammals (dolphins, whales, porpoises and seals), sea turtles, fish populations and the seabed (benthic) community.
It has been observed that collisions between birds (both sea and migratory) and wind turbines do occur, especially in low light conditions. OWFs can also act as a barrier on migration routes or between populations of the same species. Collisions between birds and the wind turbines can therefore have direct effects on the bird population. Since currents naturally remove carcasses from the area, the impact of collisions cannot be easily quantified. However observations have revealed that birds often fly beneath the blades of wind turbines or avoid flying-passing over OWFs.
Marine mammals communicate, navigate and hunt using sound. Interference from external noise can cause them to become disorientated and become ineffective in finding food and maintaining social structure. This could lead to them abandoning the areas surrounding OWFs. Presently, there is a lack of research concerning the impacts of noise disturbance of the OWFs on all species of marine mammals. However, in Denmark research studying the behaviour of seals during the construction and operation of offshore windfarms has taken place. According to the results, seal behaviour was affected only during the installation of offshore windfarms but not during their operation. Further research is required to fully understand the impacts of noise pollution.
Fish populations are also affected by noise pollution in the OWF area. Experiments have shown that noise levels greater than 150 dB can cause fish to become confused or panicked. During the construction of wind turbines noise levels can reach 260 dB. As a result, fish may be forced to temporarily abandon the area. Noise pollution is reduced once construction work is complete and fish populations may re-populate the area. During the operation of OWFs, noise pollution can continue to affect fish behaviour. However, this is dependent on the size and number of wind turbines, background noise level, wind speed, depth and sea floor features. It is estimated that fish acoustic behaviour is affected within a 4m radius of the source of the noise at wind speeds of 13m/s. Research is on-going to determine these effects on fish behaviour.
The bases of wind turbines can impact on the sea floor by altering its geochemical and geomorphological state. This consequently affects benthic communities. It is not clear yet if changes in the benthic environment will have negative, neutral or positive impacts. However it is expected that new habitats will be created which benthic communities will colonise. Wind turbine bases often behave like artificial reefs which new flora and fauna colonise. New species can enrich the biodiversity of an area but they can also disrupt the delicate ecological equilibrium. An increase in competition in the food chain can have effects on all species in the area.
Scientific research on the exact impacts of offshore wind farms is on-going and future research must be based on integrated programmes of observing their impacts and understanding how to mitigate them. However existing knowledge and experience can still be used in order to take precautionary measures in advance.
One such practice concerns the location of new OWFs. It is suggested that OWFs should not be sited in biologically sensitive areas, such as reefs and ledges, or in areas used by marine species at crucial stages of their life cycle (such as reproduction). The reduction in fishing activities and sea traffic in and around OWFs can have the effect of protecting biodiversity in the area. In this way, OWFs behave similarly to Marine Protected Areas (MPAs) by protecting biodiversity from human activities. The design and cumulative impacts of OWFs must also be considered, as should the effect of climate change.
As technology develops, the potential for OWFs in deep waters may become possible, which will reduce the environmental impacts on flora and fauna in coastal regions. Deep water OWFs will also reduce the visual impact of OWFs and facilitate the use of near shore areas for other economic and social activities (i.e. fishing, transport, etc).
Finally, we must ensure that all stakeholders participate fully in public consultation processes, which play an important role in future design applications.