Impact of removing a concrete gas platform on benthic communities in the North Sea

Artificial structures such as oil and gas platform or wind turbine foundations in the marine environment can have a significant local impact on species diversity and population connectivity. It has been argued that this should be considered in removal decisions for offshore infrastructure. World-wide, high numbers of oil and gas installations will be decommissioned in the coming years. Most will be removed, but some will be left in place or relocated to be used as artificial reefs. In the North Sea leaving in place is prohibited but derogations are allowed for gravity based concrete installations. One of the few Dutch concrete platform foundations is the Halfweg gravity based structure (GBS). Halfweg stopped production in 2016, which was followed by decommissioning and removal of the topside and legs of the structure. Currently, only the GBS remains. The owner Petrogas is evaluating options of removal of the GBS and is considering removal and leave-in-place scenarios. To facilitate this evaluation, a study on the fouling macrofauna community on the structure was performed. Samples were taken by commercial scientific divers and species identified, counted and weighed. The results were compared to similar data from box corer samples from the surrounding seabed within 30 km from the Halfweg GBS and to samples from other oil and gas and wind farm structures and a natural rocky reef. In total, 65 macrobenthic species were observed in 39 samples from the concrete GBS (52 species), steel legs (32) and surrounding rock dump (44). Simpson diversity index was highest on the rock dump at Halfweg (0.71) but concrete (0.54) as well as steel (0.60) were lower than seabed (0.69). 14 of the species on Halfweg were not found in any of the other investigated substrates. Only 10 species on Halfweg were also observed in the surrounding sandy seabed. 31% of the species found in the Borkum Reef Grounds were also observed on Halfweg. Arthropoda contained the most abundant species on Halfweg, with Monocorophium acherusicum, Jassa herdmani, Phtisica marina and Stenothoe monoculoides composing 98% of the abundance on the GBS. Mean ash free dry weight was significantly higher (p