Benthic community composition of temperate mesophotic ecosystems (TMEs) in New Zealand: sponge domination and contribution to habitat complexity

Temperate mesophotic ecosystems (TMEs) typically occur between 30 and 150 m depth and support rich benthic communities. However, despite their widespread distribution and ecological importance, TMEs are one of the most poorly studied marine ecosystems globally. We measured changes in the benthic community composition of rocky reefs through the infralittoral and mesophotic zone from 5 to 120 m at 6 locations across New Zealand (the Poor Knights Islands, the inner, mid-, and outer regions of the Fiordland Marine Area [FMA], and the North and South Taranaki Bights) which we considered as potential shallow-water TME surrogates due to these sites having environmental conditions and biological communities similar to deeper-water communities. Benthic community data were analysed from videos and photographs collected using SCUBA (< 30 m) and a remotely operated vehicle (ROV) (> 30 m). We found significant changes in community composition with depth at all locations, suggesting that TMEs provide habitats different from those in shallower water. We consistently found that TME benthic communities were dominated by sponges, but their abundance varied significantly with depth at 3 out of 4 locations, while the morphological composition of these assemblages changed with depth at all locations. Given their particularly high abundance and morphological complexity, we suggest that sponge assemblages make an important contribution to habitat complexity in benthic TME communities.