Depth‐structured sponge assemblages offer limited evidence for mesophotic refuge potential under the deep reef refuge hypothesis

Temperate mesophotic ecosystems (TMEs) have the potential to act as climate refugia for shallower benthic species impacted by environmental change. However, the extent to which mesophotic ecosystems might provide an ecological refuge, particularly for key functional groups like sponges, remains poorly known in temperate systems. Our study investigates sponge assemblage structure across a depth gradient at the Poor Knights Islands Marine Reserve, New Zealand, to assess the potential for mesophotic depths to act as ecological refuges for species inhabiting shallow water in the scenarios where mesophotic depths avoid shallower disturbances. Using ROV and SCUBA‐based image surveys across three sites, sponge abundance data were collected from depths spanning 5 to 65 m. Assemblage composition, species richness, and depth‐dependent abundance patterns were analysed using nMDS, PERMANOVA, and linear regression. Results revealed high beta diversity across depths, with sponge assemblages strongly structured by depth. The majority of sponge operational taxonomic units (OTUs) were restricted to either shallow ( 30 m) zones, with between 26.4 and 32.7% of OTUs shared between both, and a total of 18 sponge OTUs found in multiple depths in each zone across all three sites. Depth related abundance patterns tended to be species specific, meaning the potential for any refuge effect will likely also vary between species depending on their local abundance. Our findings highlight the vertical structuring of sponge assemblages and suggest that, while TMEs may offer some refuge potential to specific species in scenarios where mesophotic habitats avoid disturbances experienced by shallow species, the benefits won't apply to entire sponge assemblages.