Ecological implications of removing a concrete gas platform in the North Sea

Abstract Artificial structures such as offshore oil and gas platforms can significantly alter local species communities. It has been argued that this effect should be considered during decisions over their removal during decommissioning. In the North Sea, leaving such structures in place is prohibited but derogations are allowed for large concrete installations. To assess removal options for one such installation, the Halfweg GBS (gravity-based structure) a concrete platform foundation off the Dutch coast, we studied the resident fouling macrofauna community. The faunal structure, biomass and trophic composition of the Halfweg was then compared with those from the surrounding seabed sediments, other local artificial structures and a natural rocky reef. In total, 65 macrofaunal species were observed on the concrete (52 species), steel legs (32) and surrounding rock dump (44) of Halfweg. Mean Simpson diversity per sample was highest on the rock dump (0.71) but concrete (0.54) and steel (0.60) of the GBS were lower than seabed (0.69). Ten of the species observed on the concrete were not reported on other substrates while 10 of the species were also observed in the surrounding seabed. The GBS structure was numerically dominated by Arthropoda which comprised 98% of the total abundance. Mean ash free dry weight (AFDW) was significantly higher (p  Our data show that the Halfweg structures significantly increase local biodiversity and biomass. Removal of the concrete and steel legs of the GBS (leaving the rock dump) will significantly reduce local macrofauna biodiversity. The long-term impact on macrofaunal biomass is low. Leaving the complete Halfweg structure in place will result in an enriched local macrofaunal biodiversity and feeding mode diversity.