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The benthos includes the organisms that live on or in the sediment at the bottom of a sea, lake, or deep river. The benthic community is complex and is composed of a wide range of plants, animals and bacteria from all levels of the food chain. It can be differentiated by their habitat: infauna are animals and bacteria of any size that live in bottom sediments, such as worms and clams. They form their own community structures within bottom sediments, connected to the water by tubes and tunnels. Epifauna are animals that live either attached to a hard surface (for example, on rocks or pilings) or move on the surface of bottom sediments. Epifauna include oysters, mussels, barnacles, snails, starfish, sponges and sea squirts.
These communities are very dependant of some abiotic factors such as depth of water, temperature, turbidity and salt content. Fluctuations of any of those parameters result in changes on species composition and the numbers of individuals.
The introduction of hard bottom structures such as turbine foundations provides a new artificial substrate which helps to develop a new habitat for marine epifaunal organisms. Theses structures can attract specific benthos species generating changes in the previous benthic associations by the colonization of these new substrates. The most susceptible groups are non mobile (for example mussels, barnacles and sponges) or hardly mobile species (snail, starfish) or sand-filtering species (oysters). Small fish species depredating over benthic animals and plants may also appear in the new area. Furthermore, larger benthic or pelagic fish as well as sea birds may be attracted from the surroundings areas. Therefore, the construction of offshore wind farms will modify the relationships of benthic communities, changing the existing biodiversity in the area and creating a new local ecosystem (Köller et al., 2006).
The knowledge gained from Horns Rev monitoring shows that indigenous infauna habitats have been replaced by the epifauna community associated with hard bottom habitats with an estimated 60-fold increase in availability of food for fish and other organisms in the wind farm area compared with the native infauna biomass. An increase of general biodiversity in the wind farm area and progress succession in the benthic community has been verified. The new hard bottom substrates have provided habitats as nursery grounds for larger and more mobile species like the edible crab Cancer parugus. The most noticeable news is the introduction of two new species: the ross worm Sabellaria spinulosa and the white weed Sertularia cupressina in the Horns Rev wind farm area, both considered as threatened or included on the Red List in the Wadden Sea area. The epifauna community in artificial underwater structures differs from the natural marine fauna in the vicinity of wind turbines not only in its species composition but also in the dynamics of its faunal succession (Boesen and Kjaer, 2005).
The installation of steel structures in the western Baltic marine waters has also increased the diversity and abundance of benthic communities (Koeller et al, 2006).
The construction work phase increased temporarily the water turbidity. This effect may have had a negative impact on vegetation, because of a decrease in the possible received light. However, this impact was transient so the habitat loss caused is expected to be negligible (Boesen and Kjaer, 2005).
The potential effects from offshore wind energy installations may be divided into:
The construction phase probably disturbs many of the fish species. However, the underwater movements, noise and increased turbidity of the water associated with the works period disappear at the end of this stage.
The response from fish species to the introduction of wind turbine foundations is comparable with artificial reefs. Fish attraction behaviour to artificial reefs has been demonstrated in several European studies (Koeller et al, 2006). It is expected that fish abundance and species diversity will be increased around the turbine foundations as the new habitat becomes more integrated with the marine environment.
The new artificial habitats created by the construction of Horns Rev and Nysted wind farms have had insignificant effects on fish. The species composition was similar inside and outside of the wind farm areas. Only sand eels show a different pattern, with the population increasing by about 300 per cent in Horns Rev wind farm and decreasing by 20 per cent outside of it. More clear and definitive results will be obtained in the coming years, when the colonisation process becomes more mature (DEA, 2006).
Positive impacts from offshore wind energy are foreseen with the ban of fishing, especially demersal trawling, in the wind farm area resulting in more local fish. The increase of biomass in benthos communities as a result of the construction of new foundations would support this supposition (Greenpeace, 2005).
The low frequency noise may be audible to many fish species. The frequency, intensity and duration of the noise will determine the grade of disturbance. Studies on goldfish, cod and Atlantic salmon have indicated that they can detect offshore turbines from 0.4 to 25 km at wind speeds of 8 to 13 m s-1. The detection distance depends on the size and numbers of wind turbines, the hearing organs of the fish, the water depth and bottom substrate. The fish produce a variety of sound for communication that may be interfered with by the noise from turbines. This could decrease the effective range of communication by fish. However, the extent of this interference and its influence on the behaviour and fitness of fish is not known and additional studies are needed. There is no evidence that turbines damage the hearing of fish, even at low distances of a few metres. The avoidance distance is about 4m, but only at high wind speeds of 13 m/s. The noise impact mainly masks communication and orientation signals, whereas it does not produce serious damage to hearing organs or strong avoidance reactions are produced (Wahlberg and Westerberg, 2005; Greenpeace, 2005).
Overall, the environmental monitoring in Horns Rev and Nysted shows that the effects of noise and vibrations from the wind farms on fish are negligible. However, the current knowledge about wind energy impacts on fish presents large uncertainties. Knowledge of the behavioural response of fish to noise and vibrations from offshore wind developments is still limited. (Boesen and Kjaer, 2005; Thomsen et al., 2006). Future studies must gather better data on the nature of the acoustic field around wind turbines and the physiological and behavioural impacts on fish (Wahlberg and Westerberg, 2005; Thomsen et al., 2006).
Maintenance of wind farms need more or less daily activity, with ships moving into the wind farm area. This associated noise should create more impacts than the operating turbines (Greenpeace, 2005).
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