Temporal change in the abundance and size distribution patterns of temperate deep reef sessile communities under multiple climate disturbances

Climate change poses significant challenges to sessile benthic shallow reef communities, with increased frequency and intensity of marine heatwaves and storms exacerbating these threats. Marine imagery from autonomous underwater vehicles has recently shown that mesophotic reefs (30–150 m) are also vulnerable to climate-induced changes. However, the absence of historical monitoring in these previously inaccessible systems hinders our ability to detect or interpret changes and their underlying drivers. To better understand the variability, we assessed temporal changes over a six-year period in the abundance and size-frequency distribution of mesophotic sessile benthic reef invertebrates in southern Tasmania, a region that is exposed to some of the world's highest wave energy and accelerated warming rates. Our cutting-edge use of artificial intelligence-assisted methods for size estimation from marine imagery indicates a significant increase in the abundance and size of cup-like sponges and bramble octocorals, which is contrasted by a marked decline in the population of locally abundant soft arborescent octocorals, particularly small individuals. Despite an increase in mean size and apparent stability, the absence of significant recruitment for most morphospecies may mask underlying vulnerabilities. While causation cannot be attributed here, this region frequently experiences large storms capable of affecting the seabed at depths of 140 m. The trends observed align with expectations of how these communities may respond under such conditions. The importance of continued monitoring, alongside robust physical data collection, cannot be overstated if we are to better understand population dynamics and drivers of change in these mesophotic reef communities.