Biodiversity on production platforms: investigating the use of an assessment framework for nature-friendly decommissioning decisions

Oil and gas platforms provide habitat for multiple species, which can attach on its underwater parts (foundations and scour protection layers). The attraction of these species increases local biodiversity and changes multiple ecological functions in the areas where the platforms are introduced. As the platforms get old, their operational phase approaches its end and decommissioning should initiate loosing potentially all or some of the additional biodiversity and ecological functions. Therefore, more attention has been given in the ecological effects of decommissioning in the last few years and a draft assessment framework on how to evaluate platforms and their ecological values has been created. In this report, we tested this framework by evaluating its biodiversity measures on different platforms in the Dutch North Sea and compared them with two natural reefs (Cleaver Bank and Borkum Reef Grounds). We further provided recommendations for the improvement of the draft assessment framework and identified knowledge gaps that should be filled with further research. We analysed available data on invertebrate species occurring on the platform foundations, their scour protection layers and the two natural reefs for their species richness, abundance and diversity and identified species of ecological interest with conservation status, and non-indigenous species. Our findings indicated that the highest species richness and diversity can be found in Cleaver Bank, while the largest number of individuals per m2 can be found on platform foundations. Variations among depth zonations and platforms cannot be explained by the data that we currently acquire. In total 11 non-indigenous species were identified on the platforms and the Borkum Reeef Grounds, with some of these species being invasive. Species occurring on the platforms in large densities are considered rare on natural habitats in the southern North Sea, corroborating the statement that the platforms introduce a new type of biodiversity. Only 5 species occurring on platforms with conservation status were identified, but only one of them (Arctica islandica – with one individual occurring on one platform) is included under the OSPAR conservation. Species that can create reefs (Sabellaria spinulosa and Mytilus edulis) also occur on platforms and should be taken into consideration when reporting species of ecological interest. The overall community composition of Cleaver Bank differed remarkably from the communities on the platforms and the Borkum Reef Grounds. Finally, we aligned our results with the assessment framework criteria and indicated knowledge gaps and improvements that could be used in future ecological evaluations of different platforms.