Abstract
As in 2011, four monopiles of the Princess Amalia Wind Farm were sampled for hard substratum fauna in 2013. At two sides of the monopiles, scrape samples were collected from the intertidal zone, at four different depths and from the scour protection stones. Additional video images were shot in order to assess the percentage cover of different species. While in 2011 the samples were taken at the end of autumn, the 2013 sampling took place in July. A total of 88 species were recorded, four more than in 2011. The speciesaccumulation curves showed that for each depth level, a representative amount of species was found; however, taking samples at more depth levels would have significantly increased the species count. Most species were identified as crustaceans, polychaete worms, bryozoans or cnidarians. As for the 2011 study, large quantities of mussels were also found. This was again the dominant species in terms of biomass, except in the intertidal zone and on the scour protection stones. The most abundant species was the amphipod Jassa herdmani. Total density of fauna ranged between 142 and 1,5 million specimens per square meter (intertidal zone and depth 2m resp.), with up to 1,4 million amphipods. Densities were 3 to 10 times higher than in 2011. The highest biomass value was nearly 3 kg of ash-free dry weight per square meter. Biomass was similar to double the values found in 2011. The fauna and flora could be divided into two distinct zones: the intertidal (here including the splash zone) and subtidal zone. The latter consisted of three different faunal groups: the shallow zone (2-10m), deep zone (17m) and the scour protection stones. No significant differences could be found between monopiles, nor between the different orientations. The patterns in biodiversity, abundance and zonation of fauna are very similar to those found in 2011, and in comparison with other offshore wind farms in the region. The higher values of density and biomass compared to 2011 could indicate that the hard substratum community has matured, six years after the construction of the wind farm. It is clear that the hard substratum, provided by the monopiles, creates new opportunities for a rich and valuable flora and fauna, previously absent from the area. This artificial reef results in a dramatic increase in biodiversity and especially biomass compared to the soft substratum which was present before the construction of the wind farm. The remarkable finding of a European flat oyster, a species on the verge of extinction from the Southern North Sea, illustrates the potential role offshore wind farms could play in the revival of this species.