Autonomous reef monitoring structures as a tool for assessing UK marine benthic biodiversity

In benthic marine ecosystems, small organisms that dwell within the habitat matrix comprise the majority of species richness but are inherently difficult to sample. This limits our ability to document the biodiversity of these ecosystems. Autonomous Reef Monitoring Structures (ARMS), artificial reefs that mimic the three-dimensional complexity of benthic habitats, can alleviate this challenge. However, ARMS have been applied infrequently in many temperate locations, including the United Kingdom (UK). To showcase the applicability of ARMS to the UK, this paper applies standardized Smithsonian ARMS protocols for image analysis as well as DNA barcoding and metabarcoding of the cytochrome c oxidase subunit I (COI) gene to 3 ARMS units deployed in the Sussex Inshore Fisheries and Conservation Authority (IFCA) Nearshore Trawling Exclusion zone, a conservation area created in 2021 to facilitate the recovery of seabed habitats. We document 176 genera across 22 eukaryotic phyla as well as 1,920 > 2 mm motile specimens with a biomass of 312 g. We identify 15 notable species, including the 4th UK record of Cephalothrix simula , a non-native poisonous nemertean with the potential to enter the food supply. We also assess the complementarity of image analysis and DNA metabarcoding in describing sessile communities, finding that the two methods produce meaningfully different estimates of relative abundance for some phyla, particularly Cnidaria (77x difference) and Bryozoa (72x difference). As our oceans come increasingly under threat from global change, it is imperative that we can accurately describe their biodiversity. We advocate for ARMS as a critical tool for measuring UK marine benthic biodiversity and discuss the use of ARMS in closing gaps in reference databases as well as in assessing ecosystem function and environmental disturbance in benthic habitats.