Contrasting patterns of decadal stability for shallow water sponge boulder assemblages and subtidal rocky cliffs at Lough Hyne, Ireland

Lough Hyne (LH) Marine Nature Reserve in Ireland is a globally recognised biodiversity hotspot that hosts mesophotic-like communities in shallow water, however, major changes have occurred to most of the rocky cliff (>6 m) communities in one or more events between 2010 and 2015. To provide insights into these changes, we compared the sponge assemblage composition on the undersides of different sized, shallow (<1 m) subtidal boulders between 2000 and 2022 at two sites in LH. We also measured sponge species richness at seven sites in 2018. We found that unlike earlier reports from the deeper subtidal reef sponge assemblages, there was no evidence for changes in sponge assemblage composition on the undersides of boulders at either site. We also found high levels of sponge species richness at all seven sites sampled in 2018. We did find differences in sponge assemblages between sites and for different boulder sizes, which we propose is a result of site-specific environmental conditions and disturbance and size–area relationships. Since we found no changes in the shallow subtidal sponge assemblages between 2000 and 2022, our results support the hypothesis that changes to the deeper subtidal sponge assemblages at LH are driven by local processes associated with deeper water in LH, potentially related to the seasonal oxythermocline that forms within LH. Given the national and global importance of LH, understanding the drivers of change is critical to determine if management actions can prevent any future alterations to the LH sponge assemblages and support wider mesophotic community management.