Impact hypothesis for offshore wind farms: Explanatory models for species distribution at extremely exposed rocky areas

• Remote sensing data can be used to hypothesise on impacts of wind farms on rocky reefs.
• Changes in rugosity due to wind farm construction may impact sea urchins and sea stars.
• Sea stars may be affected by changes in local elevation or depression.
• Small kelp plants may be affected by turbine positions that shade southerly aspect.

The increasing need for renewable and clean energy production is likely to result in a diversification of locations for the implementation of offshore wind farms which have been so far predominantly sited on soft substrata. In contrast, offshore wind turbines placed on rocky reefs in highly exposed areas are much less common and the impacts on local flora and fauna can only be hypothesized. On the Western coast of Norway, a rocky reef with a highly complex topography has been chosen to be the first full-scale offshore wind farm in the country. Underwater video analyses and multibeam bathymetry data with a generalized linear model were used opportunistically to investigate the influence of geomorphic explanatory variables on the occurrence of selected taxa (algae, sea urchins and sea stars) identified in the study area. Combining video observations and multibeam bathymetry in a generalized linear model revealed that the geomorphic descriptors: aspect, slope, rugosity, and benthic position indexes (BPI), were of significance for algae, sea urchins and sea stars at Havsul and served in showing their habitat preferences. Kelp occurred in areas of high rugosity, on gentle slopes, at elevated areas with a southerly orientation and on the sheltered side of rock or bedrock. Thus, construction disturbance that modify those variables may lead to a change in the area preferred by kelp. Turbines that shade southerly aspects may affect small kelp plants in reducing their available habitat. Sea urchins were more abundant on steep slopes and both sea stars and sea urchins showed a preference for a complex local relief (high rugosity) and heterogeneity in fine and broad elevation (shown by BPI). Thus, foundations and cable route preparation may significantly change the slope, rugosity of BPI broad, which will change the basis for sea urchin populations. It may likewise significantly change the rugosity or BPI (fine or broad), which may change the distribution of sea stars. The combination of video data and models using multibeam bathymetry yields useful information about the potential impacts of the construction of an offshore wind farm on rocky substratum and allows us to generate impact hypotheses that go beyond what could be hypothesized through a mere general knowledge of the area.