Identifying relevant scales of variability for monitoring epifaunal reef communities at a tidal energy extraction site

•This study presents a new methodology for benthic monitoring in tidal rapids.•A successional model for epifauna on stable subtidal reefs is proposed.•An Ecological Quality Ratio is proposed: The High Energy Hard Substrate Index.•The EQR has the potential to standardise tidal monitoring programs in the future.•The SeaGen turbine is not significantly impacting communities at the impact station. The SeaGen tidal energy turbine is located in the Strangford Narrows, Northern Ireland. The Narrows are designated as a Natura 2000 site, host unique biological assemblages and exhibit very high tidal velocities. This study describes an asymmetrical BACI design monitoring program that was aimed at assessing the potential impact the SeaGen may have on epifaunal boulder reef communities. This study presents a novel methodology for monitoring epifaunal communities within highly variable and poorly understood tidal rapid environments. We identify bare rock as a key measure of disturbance within tidal energy extraction sites and propose a new successional model for epifaunal reef communities on subtidal stable substrates. We also present an Ecological Quality Ratio (EQR); the High Energy Hard Substrate (HEHS) index for use in monitoring programs within tidal energy extraction sites. Seasonality significantly affected epifaunal community structure, bare rock distributions and EQR values at all stations equally over time. SeaGen is not significantly affecting epifaunal community structure, bare rock distributions or EQR values at the impact site. The HEHS index has the potential to standardise benthic monitoring in tidal energy extraction sites.