Monitoring of the effects of offshore wind farms on the endobenthos of soft substrates: Year-0 Bligh Bank and Year-1 Thorntonbank

The consortia C-Power NV and Belwind NV obtained an environmental permit to build and exploit a wind farm in the Belgian part of the North Sea. The wind turbines for C-Power will be/are placed on the Thorntonbank (60 wind turbines, 300 MW). The ones for Belwind will be placed on the Bligh Bank (110 wind turbines, 330 MW). A research design has been drafted to monitor the environmental effects of the construction and exploitation of these wind farms in time. Stations on the Goote Bank, Bligh Bank and Thorntonbank were selected as a reference sites (no impact site). To scientifically evaluate the ecological effects of a human disturbance on the environment, a BACI (Before After Control Impact) strategy is used. The baseline study (Year-0) of the Thorntonbank has been completed in 2005. In 2008, six wind turbines have been placed on the Thorntonbank. This monitoring project aims at evaluating the Year-0 situation of the Bligh Bank and the Year-1 situation of the Thorntonbank on the benthic environment and demersal fish during the construction and exploitation phase of the wind farms. Most samples at the Bligh Bank and Goote Bank are characterized by medium sand (350-500 μm), with low mud content (max. mean of 4.3 %) and low percentages of organic material (max. mean of 0.3 %), both in spring and autumn 2008. The macrobenthos densities are higher in autumn (max. 3500 ind./m²) in comparison with spring (max. 900 ind./m²). Species richness is rather low at all sampling locations (max. 26 species/0.1 m²) and a broad range in biomass (26 to 6000 mg/m²) is present both in spring and autumn. Productivity is low (less than 10 mg/day.m² in most samples). Nephtys cirrosa is the dominant species over all the stations on the Bligh Bank and Goote Bank and variation in community composition is of the same order of magnitude within and between sample locations. The macrobenthos of the concession area, the border zone and the reference site can be characterized as the transitional community between the Nephtys cirrosa and Ophelia limacina – Glycera lapidum community. Seasonal variations in density, diversity, biomass, productivity and community composition seem to be important. Most samples at the Thorntonbank and Goote Bank are characterized by medium sand (350-500 μm), low mud content (max. mean of 5.9 %) and low percentages of organic material (max. mean of 0.23 %). These sediment characteristics are comparable to the ones found in the baseline study performed in 2005. A broad range in densities (50-3500 ind.m²) and biomass (6-6000 mg/m²) is present for the macrobenthos, while species richness is rather low (max 26 species/0.1 m²). Productivity is low (less than 10 mg/day.m² in most samples). The dominant species over all the stations on the Thorntonbank and Goote bank are Nephtys cirrosa and Spiophanes bombyx. A clear distinction in community composition can be made between the samples of 2005 and the ones of 2008. Within each year, no further ecologically relevant distinction can be made between the sample locations. The macrobenthos samples of the concession areas WTA and WTB, the border zone WTC and the reference sites WTR and BGR show a gradual transition from the N. cirrosa community to the O.limacina - G. lapidum community. The results indicate that the impact of the first six windmills on the endobenthos of soft sediments in the first year after implementation is rather low or could not be demonstrated yet. Seasonal and annual variations in densities, species richness, biomass, productivity and community composition seem to be more important at this moment. However, it has to be mentioned that the close vicinity of the windmills was not sampled properly. Therefore, slight modifications of the sample locations shall be made for future monitoring programs.