ISMOS9: Impact of decommissioning on benthic microbial communities in the North Sea

There are currently >2100 offshore installations in the North Sea, which require full removal at the end of their service. Information on impacts of decommissioning on benthic microbial communities is limited, however, especially those microorganisms involved in macronutrient cycling, such as ammonia-oxidizing Archaea (AOA) and bacteria (AOB) which are involved in nitrification- a key process of the nitrogen (N) cycle. Two North Sea oil and gas platforms: Northwest Hutton (NWH) and Miller, (decommissioned in 2009 and 2018 respectively), were investigated across an increasing distance
gradient away from the platforms, from 50 to 3200m, to understand the impacts on sediment microorganisms. Bacterial 16S rRNA gene abundances increased 10-fold at Miller and significantly decreased 10-fold at NWH with increasing distances from the platform. Archaeal 16S rRNA gene abundances remained stable at Miller but decreased 100-fold with increasing distance at NWH. Both
AOA and AOB ammonia monooxygenase (amoA) genes decreased in abundance, whilst ammonium and nitrate concentrations increased (~8- and 16-fold respectively) with increasing distance from NWH. However, at Miller, AOA amoA gene abundancesremained similar, but AOB increased ten-fold, whilst
ammonium and nitrate concentrations remained unchanged. The legacy of operations and decommissioning of platforms can still be detected in sediment microbial communities nearly 15 years after decommissioning, including functionally important microorganisms involved in N-cycling, suggesting potential impacts on ecosystem function. Further work will assess heavy metals and hydrocarbon concentrations, and microeukaryotic communities. Information on decommissioning impacts on benthic ecology is crucial to better manage these structures and their legacy in marine environments.