Offshore Wind Farms and the Environment
BENTHIC COMMUNITIES: IN BOTTOM FLORA AND FAUNA
Wind turbine foundations and scour protections act as sanctuaries for vulnerable species
The main effect of the establishment of the Horns Rev and Nysted wind farms was the introduction of the turbine foundations and the scour protections onto seabeds that previously consisted of relatively uniform sand. These hard bottom structures, covering 0.2% of the total wind farm area, have increased habitat heterogeneity and locally changed the benthic communities from typical fauna communities with most aquatic animals living in the seabed to hard bottom communities with increased abundance and biomass.
The habitats introduced by the wind farms will be suitable for colonisation by a variety of marine animals and algae, and the hard bottom structures may act, both individually and collectively, as an artificial reef and as sanctuary areas for threatened or vulnerable species. Furthermore, it is expected that the introduction of hard bottom communities will increase the availability of food to fish, which again will lead to an increase in the available food to marine mammals and birds.
Different native fauna
The baseline studies of Horns Rev found no vegetation and the native fauna composition was found to be closely associated with the sandy environment. The fauna was very variable and heterogeneous with the bristle worms Goniadella bobretzkii and Ophelia borealis and the thick trough shell amongst the most predominant and characteristic species. Mobile fauna was often found on the seabed in the area. The brown shrimp, which often was observed, is an important prey species for both sea birds and fish.
At Nysted, the native fauna community was characterised as a shallow water Macoma community, named after the Baltic tellin. The bottom fauna was very homogeneous and some species found were typical indicators of brackish water. Common mussels were locally found in large numbers constituting more than 35% of the total biomass.
Massive colonisation by common mussels
After the establishment of the wind turbines at Horns Rev the hard substrates were colonised mainly by species not previously recorded in the sandy seabed community, thereby increasing the species richness in the area.
At Nysted, the colonising communities were mainly composed of species previously recorded in the benthic mussel bed and algae communities. Almost monocultures of common mussels had developed at the turbine structures.
Scour protection with common mussels at Nysted. Photo: Maks Klaustrup
At both wind farm sites, the initial colonisation of the common mussel was massive. In 2003, densities from 90,000 to 200,000 individuals/m² could be found at the uppermost parts of the turbine foundations. Due to a lack of efficient predators at Nysted, the common mussel (Mytilus edulis) was the predominant species controlling the community. Higher salinity and the presence of more efficient predators like the common starfish (Asterias rubens) are interpreted as the main reason for the reduced predominance of common mussel at Horns Rev.
Common starfish at the scour protections of Horns Rev. Starfish is a key predator in the area. Photo: Maks Klaustrup
The whiteweed, a threatened species in the Wadden Sea, overgrown by Jassa marmorata. Photo: Jens Christensen.
Development of algae communities
A succession in the development and distribution of attached algae was found on the new hard bottom structures. In the upper part of the foundations of the Horns Rev wind turbines, a cover of algae was found shifting from an initial colonisation of filamentous green algae to a more diverse and permanent vegetation of green, brown and red algae. Similar species were also found colonising the foundations at Nysted.
The red algae, Polysiphonia fibrillosa, found at foundations at Horns Rev and Nysted. Photo: Maks Klaustrup
Common mussels with growth of hydrozoans at Nysted. Photo: Maks Klaustrup
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METHODS A total of six infauna and vegetation community surveys were performed at Horns Rev and Nysted during the pre-construction and post-construction phases. At Horns Rev, quantitative samples of infauna and sediment were taken by scuba divers and analysed. At Nysted, common mussels were sampled at mussel bed sites by scuba divers, and the seabed character and coverage of benthic communities were mapped and assessed using a photosampler methodology. Sampling of benthic communities of turbine foundations was performed at six turbine sites at Horns Rev and at eight turbine sites at Nysted. The sampling sites were selected according to differences in depth regimes and turbine locations. |
This page forms part of the publication 'Offshore Wind Farms and the Environment' as chapter 8 of 14
Version 1.0. 17-11-2006
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© Danish Energy Authority 2006