Abstract
The biodiversity at the Danish Kriegers Flak wind farm was investigated using multiple methodologies, including environmental DNA (eDNA) analysis of scraping samples from turbine towers (near the surface), eDNA analysis of water samples collected at two water depth near the towers and outside the wind farm, and laboratory-based taxonomic analysis of scraped specimens. Additionally, biological communities on turbine towers, associated erosion protection structures, surrounding sandy seabed, and three nearby natural reef sites were examined through Remote Operated Vehicle (ROV) surveys and subsequent visual assessments. The ROV and scraping for conventional identification were employed as substitutes for a planned diving survey that could not be conducted due to offshore diving regulations, which impose requirements beyond the scope of standard scientific diving protocols.
The study revealed significant differences in species detection across the methods employed. The eDNA analysis detected the highest number of species, particularly small organisms such as crustaceans, polychaetes, and plankton, and proved effective for identifying fish species. However, it failed to detect some dominant organisms, such as Mytilus trossulus (identified only at the genus level as Mytilus), and red algae species were not detected at all, despite two different species being observed during visual assessments.
A comparison between species identified at natural boulder reef sites (through years of diving surveys conducted as part of the national monitoring program) and species detected at Kriegers Flak using ROV surveys and scraping with conventional identification, as well as eDNA analysis on water samples and scrapings, revealed a very little overlap. Macroalgal species and taxa were largely absent from the Kriegers Flak investigation. On the other hand, species were identified at Kriegers Flak, which are not a target group in the national monitoring program and are missing from the two monitoring locations, such as pelagic species and free-living smaller species of crustaceans, mussels and brush worms.
The ROV survey identified the coverage of a limited number of organisms but demonstrated that large blue mussels were overwhelmingly dominant on the turbine towers. Blue mussels were also present on the seabed scour protection structures and at nearby natural reef sites, though they exhibited lower coverage and smaller individual sizes on both of those habitats.
Diver based collection of material and mapping of species would have been a valuable supplement to the investigation at the wind farm and would have allowed for a direct comparison. Current offshore diving legislation, which restricts scientific diving at wind farms, presents a significant obstacle to acquiring robust scientific knowledge on wind farm biodiversity.