Temporal trends in benthic community composition differ between shallow and mesophotic temperate rocky reefs

Temperate mesophotic ecosystems (TMEs) generally occur between 30 and 150 m, at the limit of light availability for photosynthesis, and are often characterised by more stable environmental factors than shallower euphotic environments. We propose that this stability in physical factors may result in benthic communities within TMEs being more temporally stable compared to their shallow-water counterparts. We tested this hypothesis by undertaking monitoring at 4 distinct sites around New Zealand, each hosting benthic communities along a vertical depth gradient from shallow to mesophotic depths, over the course of 7 yr. Our results represent the first long-term study of benthic communities inhabiting TMEs. We analysed overall trends in benthic community composition, trends in diversity indices, and changes in the abundance of specific common benthic groups. Overall community composition differed between years at the majority of depths. However, the nature of these changes, along with the patterns in trends of diversity indices, varied between depths at all sites. Two of our 4 sites supported our hypothesis, exhibiting more stable benthic communities at mesophotic depths compared to the shallows; however, 2 sites did not follow expected patterns. At all sites, where communities were found to change through time, these changes tended to be indicative of stochastic variation, with limited evidence for directional or cyclical changes. Our findings highlight the need for monitoring programs that encompass both shallow and mesophotic depths, and management plans which take into account the different temporal trajectories benthic communities from adjacent shallow and mesophotic environments are likely to take.