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 meters. 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) or mesophotic
(>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.