The assessment of potential environmental impacts is of particular importance in the development of offshore wind energy. The protected goods to be considered include organisms living in and on the seabed (benthos), fish, migratory and resting birds, marine mammals and bats in the Baltic Sea. In order to be able to assess possible impacts on the marine environment on a broad basis of knowledge, these objects of protection are examined according to standardised methods.
The "Preliminary Investigation" unit commissions and accompanies the systematic investigation of these natural resources within the marine spatial planning, examines and evaluates the newly acquired data (§10 WindSeeG). Based on these investigations and with informations on the subsoil, wind conditions, oceanography and shipping traffic, the suitability of a marine area for the construction of wind turbines is finally assessed (§9 WindSeeG). The result of this so-called suitability test, together with the data and analyses, forms the basis for the tendering of sea areas on which wind farm projects are to be realised.
Protected property: Benthos
The umbrella term "benthos" covers very different groups of organisms which live on (epifauna) or in (infauna) the seabed and which mainly spread via larvae floating in the water. The benthic community includes such different life forms as sponges, corals, mussels, snails, worms, crabs, starfish and much more. More than 1,200 species are considered established in the German Exclusive Economic Zone (EEZ) of the North Sea and the Baltic Sea. As a result of this amazing diversity of forms, functions and lifestyles, very different threats exist. Many species are dependent on solid subsoil, which is why Helgoland and artificial structures are home to a different species spectrum and often a higher species diversity.
The foundations of wind turbines provide a suitable substrate for benthic organisms that depend on solid ground. To date, no long-term, large-scale changes in benthic fauna have been detected as a result of the construction and operation of a wind farm. However, biomass and the number of species growing on the foundation structures have increased since the construction of the turbines. This so-called reef effect is probably also the cause of the increased occurrence of planktonic larvae, including e. g. echinoderms, on the downstream side of two wind farms investigated. Since fishing in wind farms is largely excluded, one of the most important hazards for benthic organisms in these areas is eliminated.
In order to determine the sampling stations for the assessment of the benthos, the sediment and biotope structure has to be recorded with a side-scan-sonar. In case of a heterogeneous biotope structure, the structure, the epifauna and the possible occurrence of aquatic plants must be recorded by underwater video. The infauna is sampled with a Van–Veen-grab and the epifauna with a 2m wide bottom trawl, a so-called beam trawl once a year in autumn, in the baseline survey also in spring. This takes place within two weeks in the project area and the reference area.
Growth on the foundation of an offshore wind turbineSource: S. Fuhrmann
The grab sample is sieved, the grain size distribution of the sediment is determined and the organisms are conserved for later determination of the species, the number of individuals and the particular wet weight in the laboratory. The organisms from the beam trawls are determined, counted and weighed as far as possible on board. The number of captured animals is related to the area covered in order to calculate the number of individuals and biomass per unit area. The dominance by number and weight and other community analyses such as diversity and the occurrence of Red List species are described, with particular attention to the comparison between the project area and the reference area.
Protected property: Fish
As the most species-rich group of all vertebrates living today, fish are equally important as predators and as prey in marine ecosystems. Although more than 200 fish species have been identified in the North Sea and Baltic Sea to date, less than half of them reproduce regularly in the German EEZ or are found as larvae, juveniles or adult specimens. According to these criteria, only 94 species are considered established. Most species live either predominantly on the seabed (demersal fish) or in open water (pelagic fish). However, demersal fish also rise into the water column and pelagic fish occasionally stay close to the bottom. Bottom living fish feed mainly on invertebrates living in and on the bottom, while pelagic fish feed almost exclusively on zooplankton. Offshore wind turbines can have positive and negative effects on fish. During the construction phase, fish density may temporarily decrease as a result of construction activities. All studies to date have shown that benthic organisms colonise the foundations of wind turbines. This increase in local biomass and species diversity can lead to an expansion of the food spectrum and food availability for individual fish species. Indirectly, a wind farm at sea can also have positive effects for the marine fish community, as fishing is largely excluded within the areas. Offshore wind farm areas can thus become a retreat for fish, as long as relevant species are not deterred by operating noises. However, these forecasts have not yet been confirmed during operation.
The so-called beam trawl is the standard fishing gear for fishing in the North Sea. In the Baltic Sea, a trawl net known as a "wind park trawl" is used. These two bottom trawls specifically catch demersal fish. Pelagic fish, on the other hand, are caught randomly, for example when lifting or lowering gear.
Dimension of a clod (Pleuronectes platessa)Source: A. Dänhardt
The aim of the fish surveys is to describe and assess the status quo of the fish fauna on the survey area and on a sufficiently similar reference area as a basis for the assessment of possible impacts of wind turbines. In the first year, sampling is carried out in spring and autumn to assess seasonal changes on the study area and reference area, in the second year only in autumn. Depending on the size of the area and the fishing gear used, up to 40 hauls are carried out at randomly selected stations. The state and changes of the fish community is described separately for the study area and the reference area. Metrics recorded include the species-specific number of individuals, biomass and the size structure of dominant species. Before evaluation, statistical procedures are used to test relationships and differences formally.
Protected property: Marine mammals
In the German North Sea and Baltic Sea EEZ, the harbour porpoise is the only native whale species. Furthermore, grey seals and harbour seals occur in the coastal sea and near the islands, which also use the area of the EEZ in search of food. Bottlenose dolphins, white-sided dolphins and minke whales are among the species that occasionally cross the outer area of the German EEZ in the North Sea.
The North Sea population of the smallest European whale species, the harbour porpoise, is currently estimated at 345,000 animals. Harbour porpoises occur clustered near “Doggerbank”, “Borkum Riffgrund” and “Sylter Aussenriff”. The latter is considered the main distribution area, where the calves are born and raised in spring and summer. The spatial distribution of harbour porpoises is mainly determined by the distribution of their prey, which includes gobies, herring and sand eels.
Harbour porpoises are protected under several international conservation agreements. They fall under the protection mandate of the European Flora-Fauna-Habitats-Directive for the Conservation of Natural Habitats and of Wild Fauna and Flora (FFH-Directive), according to which special areas must be designated for the protection of the species. In Germany, according to the Red List of Mammals, the harbour porpoise is considered highly endangered (BINOT et al., 1998).
The grey seal and harbour seal are also protected under the FFH-Directive. According to the Red List, the grey seal is also highly endangered and the seal is classified as endangered. In the central Baltic Sea, a separate subpopulation can be defined. It has been classified as highly endangered in the International Red List (Cetacean update of the 2008 IUCN Red List of Threatened Species).
The occurrence and spatial distribution of harbour seals, grey seals and harbour porpoises are determined using ship- and airplane-based transect counts. The habitat use of harbour porpoises is continuously recorded with the aid of click detectors attached to buoys, the so-called PODs (porpoise detectors).
Spar buoy with click detector (POD) for harbour porpoise detection Source: Bioconsult-SH
Resting birds and marine mammals are counted together on monthly boat trips and 8-10 digital aircraft counts per year. The surveys take place for at least 2 successive, complete annual cycles before the start of construction. This results in sighting rates, individual densities, group size and a rough characterisation of behaviour (swimming direction, behaviour patterns, associations). A central aspect in the later evaluation are the changes between this baseline survey and the subsequent construction and operation phase, which can be used to identify possible effects of the wind turbines. The results of the study on habitat use and hydro-acoustic measurements are included in this study.
Protected property: Migratory and resting birds
Hardly any other natural phenomenon is as admired as the migration of birds. The North Sea and Baltic Sea also serve as a resting and a growth area for species that breed at German coasts. Some Nordic breeding birds such as divers and sea ducks spend the winter in the North Sea and Baltic Sea, whereby parts of the German coastal sea and the EEZ are of great international importance. In the EEZ of the German North Sea, 19 seabird species rest regularly and in larger populations, with 38 species in the Baltic Sea twice as many species occur regularly, among them auks, seagulls, terns, divers and geese. In the North Sea, nothern gannets and fulmars are added, both species breed on Helgoland.
The German Bight lies on the migratory path of numerous bird species. More than 200 species are regularly found on Helgoland, an important stepping stone. Songbirds (in particular thrushes), whose majority cross the North Sea at night and that can be determined largely by their migration calls, represent the biggest portion. The homeward migration into the breeding-areas in the spring takes place almost predominantly from southwest to northeast, the migration into the hibernation-areas in the autumn takes place into the opposite direction. The Baltic Sea is also on the migratory route of numerous bird species, 95% of which are small terrestrial birds. Since direct and indirect effects of wind turbines on the distribution and migration of birds cannot be excluded, a variety of studies on avifauna have been a central component of the environmental impact assessment of offshore wind farms for many years.
When recording migratory and resting birds, special attention is paid to species which, for example, need special protection in accordance with national and EU regulations. It is therefore essential to recogniseand describe the individual species.
The observations of resting birds in a two-year baseline survey before construction begins serve to determine the status quo of the spatial distribution, abundance and behaviour of the birds in order to assess the significance of the study area as a resting, feeding and/or moulting area. Monthly counts are carried out from a moving ship. Depending on the seasonal occurrence of certain species, such as divers, the frequency of surveys can be increased. The ship-based resting bird counts are supplemented by 8-10 counting flights with an aeroplane, in which georeferenced aerial photographs are taken and the birds and marine mammals recorded are later determined as far as possible down to the species. Possible effects of the construction and operation of a wind farm will later be assessed on the basis of investigations during construction and operation.
Bird watching with vertical radar on board a shipSource: IBL Umweltplanung GmbH
The migration flows of birds are recorded with the aid of a vertical radar, which not only records bird migration, but also shorter flights, for example for the purpose of feeding or between feeding and resting areas. In addition to the intensity of the migration, the flight height is also measured, enabling the potential collision risk of migratory species with wind turbines to be assessed. Accordingly, flight directions and avoidance movements are recorded with a horizontal radar. A species connection is established by supplementary visual observations during the day or by migration call recordings at night. The investigations on bird migration are continued within the framework of the monitoring of offshore wind farms during construction and operation in order to determine possible effects such as barrier effects, small-scale evasive movements or attraction.