Description: Over the last years, the development of offshore renewable energy installations such as offshore wind farms led to an increasing number of man-made structures in marine environments. Since 2009, benthic impact monitoring programs were carried out in wind farms installed in the southern North Sea. We collated and analyzed data sets from three major monitoring programs. Our analysis considered a total of 2849 sampling points converted to a set of biodiversity response metrics. We analyzed biodiversity changes related to the implementation of offshore wind farms and generalized the correlation of these changes with spatial and temporal patterns. Our results demonstrate that depth, season and distance to structure (soft-bottom community) consistently determined diversity indicators and abundance parameters, whereas the age and the country affiliation were significantly related to some but not all indices. The water depth was the most important structuring factor for fouling communities while seasonal effects were driving most of the observed changes in soft-sediment communities. We demonstrate that a meta-analysis can provide an improved level of understanding of ecological patterns on largescale effects of anthropogenic structures on marine biodiversity, which were not visible in single monitoring studies. We believe that meta-analyses should become an indispensable tool for management of offshore wind farm effects in the future, particularly in the view of the foreseen development of offshore renewable energies. This might lead to a better picture and more comprehensive view on potential alterations. However, this requires a modern open-source data policy and data management, across institutions and across national borders.

Description: The establishment of artificial hard substrates (i.e. offshore wind farms and oil and gas platforms) on marine soft sediments increases the available habitat for invertebrate communities that would otherwise be restricted to natural hard bottoms. Suspension feeding invertebrates clear a significant amount of particles from the water column and release organic matter in the form of feces, influencing the basis of marine food webs and affecting surrounding environments. Artificial structures in the southern North Sea are dominated by a suspension-feeding crustacean in terms of abundance and sometimes even biomass: the amphipod Jassa herdmani. Animal densities of this tiny biofouler are known to exceed 1 million individuals per m2. Despite their small body sizes and their simple filter apparatus, we hypothesized that J. herdmani is a highly effective suspension feeder with a significant impact on neighboring communities due to its high abundances. In a feeding experiment, individuals of J. herdmani were provided with either an algal or an animal diet under two different temperature regimes. Clearance rates and fecal-pellet carbon (FPC) were measured. The results revealed high clearance rates and subsequent FPC, which were more pronounced at the higher temperature. Furthermore, clearance rates and FPC varied insignificantly with different food items. We further used the current findings for upscaling calculations to the total number of offshore windfarms and oil and gas platforms in the southern North Sea. Our calculations indicated that J. herdmani alone clears 0.33–4.71 km3 water per year in the southern North Sea. At the same time, these amphipods release 255–547 tons of carbon per year by means of defecation, thus enriching the surrounding soft sediments with organic matter. Our study highlights that tiny amphipods can mediate indirect effects of man-made structures in the North Sea, which could have a profound impact on pelagic and benthic habitats.