Slow recovery in temperate mesophotic communities following disturbance: An example from Lough Hyne (Ireland).

Despite the ecological importance of temperate mesophotic ecosystems (TMEs), our understanding of the ecological processes within these systems and their potential to recover from disturbance remains limited. Here, we investigated the temporal dynamics of mesophotic sponge-dominated communities at Lough Hyne Marine Nature Reserve (Ireland) following a significant sponge mortality event in the early 2010s. Through the analysis of permanent quadrat data from 1994 to 1995 (pre-disturbance) and 2018-2021 (post-disturbance), we quantified the temporal variation in recovery rates of benthic communities and sponge populations across an environmental gradient. Although communities changed as a result of the disturbance, multivariate analyses did not detect any temporal changes in communities or sponge assemblages in the periods preceding or immediately after the disturbance. However, we did find a small increase in the abundance of some habitat-forming sponges and anemones at some sites, suggesting that a slow recovery may be underway. In contrast, other sites showed no signs of recovery. The heterogeneity of temporal dynamics and recovery trajectories among sites highlights the potential influence of environmental conditions on recovery. At current rates, some sponge populations may take decades to recover assuming there is no further disturbance. In general, we found that sponges and cnidarians showed very small temporal fluctuations both before and after the disturbance, suggesting high temporal constancy of these organisms in TMEs. These small temporal fluctuations likely derive from the slow growth, long lifespan, and low fecundity of many temperate mesophotic sponges and cnidarians. These results provide rare empirical evidence for long recovery times (several to many decades) of TME benthic communities following disturbance, as little recovery was observed at Lough Hyne after at least six years following the mortality events. As many TMEs worldwide remain largely unprotected, urgent global action is needed to conserve these ecosystems since any human impact may have long-lasting effects on TME biodiversity and ecosystem services.