Tanja Joschko

Colonization of an artificial hard substrate by Mytilus edulis in the German Bight

Abstract The colonization of the underwater construction of an offshore research platform in the German Bight by Mytilus edulis was investigated. Mussel abundance, biomass and percentage coverage of the construction were determined from summer 2003 to summer 2005 from different water depths using digital underwater images and scrape samples of the hard substrate fauna. Growth was estimated from shell length distributions. In 2003 settlement of M. edulis was low at the platform mainly due to a temporal mismatch between platform construction and occurrence of competent larvae. In summer 2004 mussel abundance increased remarkably in the intertidal and upper subtidal. Abundance and biomass increased up to 30,000 individuals m−2 and approximately 40 kg m−2 in summer 2005. At the end of the investigation period, the upper part of the platform foundation was completely covered by M. edulis. Lower parts remained sparsely colonized. Mussel growth rates were high under offshore conditions because of favourable environmental conditions and reduced biological constraints. Cumulative effects from wind farm entities are estimated. Mussel accumulations will be an important component in the estimation of ecological implications of offshore wind farming at least at the local scale.

Benthos in the Vicinity of Piles: FINO 1 (North Sea)

Changes in the macro-zoobenthic communities in the areas of offshore wind farms are the result of the cumulative effects of numerous piles on the marine fauna. The essential processes take place in the vicinity of these piles. An altered hydrographical regime leads to erosion and to changes in sediment composition in the direct surrounding area of the piles. In addition, the underwater structure provides an artificial hard substrate for many organisms which rarely occur in the typical soft bottom communities of the German Bight. These also influence the food supply for the fauna below and attract predators, which in turn may cause changes of the bottom communities by increasing the predatory pressure. Until very recently, the influence of wind farms on the marine environment could only be estimated theoretically on the basis of the results of investigations of other artificial hard substrates, such as wrecks or artificial reefs. These however represent different structures of different dimensions. Initial results from an offshore wind farm at Horns Rev Denmark, have been presented (ELSAM 2004a, 2004b), but are only partially transferable to the planned installations in the German Bight, since both the hydrographic and the sedimentological conditions are considerably different. The possibilities of investigating these processes directly at the planned wind energy plants are somewhat restricted, for technical and logistical reasons. Process-oriented studies are for these reasons not part of the standard monitoring programme during construction and operation of such installations. The research platform FINO 11 therefore provided an ideal possibility to study these processes in depth at the piles, prior to the actual construction of the wind farms. Within the framework of the BeoFINO2 research project, several studies addressed questions of the possible ecological effects of offshore wind farms (see also chapters 9 and 13). It is hoped that the results can help assure the environmentally compatible construction of the planned offshore wind farms.