Using environmental DNA to inform decommissioning decisions for offshore oil and gas infrastructure

Abstract Artificial reefs are being utilised globally to aid in natural resource management, conservation, habitat restoration or the creation of unique marine habitats. There is much discussion around the optimal construction materials and designs for artificial reefs, the influences these have on biological communities, and the resulting ecological and social benefits. This discussion also includes the ecological value of repurposed marine infrastructure, such as decommissioned oil and gas platforms. Platforms often have an operational life spanning multiple decades, over which time they can develop extensive and unique community assemblages. The creation of artificial reefs by repurposing oil and gas platforms can have ecological, economic and sociological merit. However, with more than 12,000 platforms requiring decommissioning globally, there is the need for holistic assessment of biological communities associated with these platforms to inform the potential outcomes of different decommissioning options. We use environmental DNA metabarcoding (eDNA) of water, bio-foul and sediment samples to census broad eukaryotic diversity at eight platforms in the Gulf of Thailand (GoT) and five nearby soft sediment habitat locations. From the three depths targeted, we detected 430 taxa at platforms, with higher diversity in shallow (near-surface) samples (313 taxa), compared to mid (30 m collection depth; 261 taxa) and deep (50 m; 273 taxa). Only three percent of taxa were shared among all depths at platforms. Indicator taxa driving differences at platform depths were of the phyla Chordata, Arthropoda and Echinodermata for shallow samples and Cnidaria, Chlorophyta and Porifera for deep. No known introduced species were detected in platform samples. The frameworks supporting eDNA methodologies, such as taxonomy, assay availability and local reference material, particularly in diverse tropical habitats, require improvement for robust taxonomic analysis. This study highlights the versatility and scalability of eDNA metabarcoding in being able to holistically census marine infrastructure and inform the management and potential conservation of extant platform communities