Current evidence for offshore wind farms as artificial reefs

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Due in part to the youthful nature of the offshore wind industry, there are still relatively few fully comprehensive studies into the influence of turbine arrays on fish and benthic populations, other than the monitoring requirements set out in the consent conditions. However, where datasets do exist, it is suggested that offshore wind farms are demonstrating benefits for such populations.

The effect on commercial stocks, such as lobster and crab, are an obvious concern to those directly and indirectly involved in the exploitation of such stocks; therefore any impacts are key to the Environmental Impact Assessment (EIA) process. Observations made onboard a commercial potting vessel deploying gear within the operational Barrow Offshore Wind Farm, off the north west coast of England, eighteen months after construction was completed, found that catch rates for lobster were similar inside and outside of the wind farm boundary (Centrica, 2009). In addition to this, the number of undersized crabs taken within the wind farm was greater than the number found outside the boundary, suggesting that the wind farm site is acting as a haven for juvenile crabs. Initial thoughts that this may be due to lack of fishing effort, with the wind farm acting as an unofficial nature reserve, were discounted in the case of Barrow due to anecdotal evidence, which stated that potting had recommenced within the wind farm boundary a matter of weeks after construction was completed (Centrica, 2009).

A recent study by Langhamer and Wilhelmsson (2009) looked into the colonisation of wave power devices off the Swedish coast, with some of the foundations being perforated with holes at different heights and positions around the block foundations, to determine whether this would have a positive influence on colonisation. Surveys on the blocks were carried out by divers. Although fish populations in the area were generally relatively low, it was found that numbers were significantly higher around the foundations than in the control sites (sites of the same area, generally of sandy seabed, near to the foundations). Although the number of lobsters found was low, with individuals inhabiting crevices around the base of the foundation rather than the drilled holes, the foundations were found to have a positive effect on the number of edible crab, which increased around foundations with or without holes (Langhamer and Wilhelmsson, 2009).

At the Horns Rev Offshore Wind Farm, off the Danish coast, Forward (2005), found that in terms of benthic community structure, there was no significant difference between the wind farm site and a reference area. However, there was a substantial increase in the density of sand eels, rising by 300% within the operational wind farm in 2004, compared to a rise of only 20% at the reference site. This increase within the wind farm was mainly due to an increase in the number of juvenile sand eels, with the main reasons behind the increase thought to be reduced mortality through predation, and a reduction in mean particle size as a result of construction. In addition to this, eight new species were recorded within the wind farm site, compared to pre-construction surveys (Forward, 2005).

A number of studies have investigated the potential for offshore wind turbines to act as fish aggregating devices (FADs). FADs are not a modern phenomenon, and have been employed for centuries to concentrate marine fish and ease their capture, proving highly successful (Fayram and de Risi, 2007). In open-water areas, the catch-rates of some tuna species have been found to be 10-100 times greater near FADs, based on mark and recapture studies. This would clearly benefit local fish communities, of both commercial and non-commercial species, and where commercial stocks exist, would have the potential of enhancing such stocks for the local fishing industry. However, there is need for caution to be exercised here. It has been noted that in some situations, juveniles of some species are more associated with FADs than adult fish, thereby potentially resulting in the increased catch-rates of juveniles over adults, should these areas be fished (Fayram and de Risi, 2007).

This element would need further survey work before the true benefits for the fishing industry, if any, could be estimated.

Wilhelmsson et al. (2006) undertook research into the effects on fish populations at five wind farm sites in Sweden, and found that large communities of both demersal and pelagic fish populations developed around the turbines. It was noted that the presence of such populations may in fact lead to further enhancement of benthic communities around the base of the turbines, as a result of the deposition of organic material such as faecal matter, organic litter and dead organisms, all of which provide material for benthic organisms to feed on. In addition, it was reported that mussel beds were starting to develop in the areas adjacent to the wind turbines, possibly as a result of mussels being dislodged from their original attachment locations on the towers. A cyclical effect could develop here, as more benthic organisms means more food for fish, which increases the level of organic waste, thereby allowing further growth of benthic organisms, and so on.

The development of mussel populations on turbines could itself be of interest to the fishing community, as it was noted that previous studies have identified a link between mussel beds and increased fish numbers (Wilhelmsson et al., 2006). Given that mussel growth is present on almost all turbine structures in the correct environmental conditions, this could be of particular interest.

The potential for the advantages of offshore wind farms acting as artificial reefs, and the ever-growing interest in the industry, means that there are frequently new research projects being designed to look into their capacity for colonisation and production. The development of life around turbines is of key interest to the owners of the wind farms, as excessive build up of life can be damaging for the turbine. Surveying around the towers can also be specified as a condition of consent. For the operational Barrow Offshore Wind Farm, in the East Irish Sea, near Barrow-in-Furness, the surveying of colonisation of the monopile foundations and scour protection was required as part of the Food and Environment Protection Act (FEPA) licence granted for the project. The turbines had been installed in 2005, with the surveys being undertaken in 2008 (EMU, 2008a), consisting of video footage, still photography and sample collection by divers.

It was noted that on the four turbines surveyed, colonisation had taken place in a generally similar pattern, with a gradual change in community observed as depth increased. At the intertidal level on the turbines, there was found to be green algae, with barnacles slightly lower, giving way to increasingly dense populations of mussels moving down the tower. As depth increased, anemones increased in number, with mussels decreasing, with crabs and barnacles also being found. Around the base of the monopile was an area of coarse sediment, including shell fragments, pebbles and gravel (EMU, 2008a).

In general, the communities observed were typical of hard-surface communities, and commonly found in waters around the UK and Ireland. It was noted that no species of particular conservation interest or invasive / alien species had been found during the surveys. The results of the 2008 surveys were compared to initial survey work undertaken on six turbines, in 2006, around eight months after construction was completed. It was found that in general, the species found were similar between the two surveys, with abundances and densities increasing in the two years between surveys, as would be expected. Further comparison was also made with surveys undertaken on the North Hoyle Offshore Wind Farm, in Liverpool Bay, completed one year after construction. Again, broadly similar communities were found to be developing on the turbine towers (EMU, 2008a). There was found to be minor variations in community structure; however, this is to be expected given the different locations, and therefore differing environmental influences. Similar survey work has been undertaken on the Kentish Flats Offshore Wind Farm (EMU, 2008b), in the outer Thames Estuary, approximately three years after the installation of the turbines. In this survey, two turbines were assessed, and again, similar patterns of colonisation were found on each tower. Again, a change in community with depth was noted, with barnacles and mussels dominating the intertidal and infralittoral zones of the tower. As depth increased, mussels became scarcer, being replaced by anemones, with hydroids also becoming more prevalent. As with the other developments, at the base of the towers, shell fragments, pebbles and gravel dominated the seabed, with a number of crab species being found, as well as high numbers of starfish, unsurprising given the high densities of mussels, their key prey species (EMU, 2008b).

These studies show the capacity for colonisation within just a few months of the turbines being installed. Although in these cases, there has not been significant variation between turbines, or even wind farms, it is still a useful contribution to the productivity and ecological carrying capacity of the surrounding marine environment, with the potential to attract other species into the area looking for food sources, as the community continues to develop.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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