Beyond the Surface: Mesophotic Reefs as Potential Refuges for Shallow Fish Assemblages

The deep reef refugia hypothesis suggests that the effects of disturbance decrease as depth increases; thus, reefs in the mesophotic zone potentially serve as refuges for communities in shallower zones. This study challenged this hypothesis by evaluating fish diversity in shallow and mesophotic reefs in a marine protected area in the Gulf of California. During 2021-2022, we conducted 189 5-min video transects using remotely operated vehicles to document species richness and abundance. We evaluated six biological traits for each species (length, mobility, position, gregariousness, diet, and activity period) to estimate four functional indices (number of entities, richness, originality, and divergence), one phylogenetic index (Δ*), and Hill's numbers for taxonomic and functional indices. Benthic organisms were analyzed to explore relationships with ichthyofauna, while monthly water turbidity satellite data products were transformed into a light attenuation coefficient to identify the mesophotic zone (area between 10% and 0.1% of the incident light at the surface). At the study site, the mesophotic zone was identified to extend to 21 m under optimal conditions, which is shallower than what is typically observed in oligotrophic regions. Generalized linear models revealed significant variations in reef fish composition across spatial (site and zone) and temporal (season and year) dimensions. Additionally, generalized linear mixed models of functional richness and taxonomic Hill's numbers exhibited significantly higher values in the shallow zone. However, functional and phylogenetic indices showed similarities in fish assemblages. Despite differences in fish taxonomic diversity among zones that could be related to less environmental variation and resource availability in deep strata, mesophotic reef fish assemblages presented similar functions. Functions were maintained in mesophotic reefs, which suggests that the two zones are connected and that mesophotic reefs have the potential to act as partial refugia in the face of current and near-future climate change-related disturbances that could affect shallow zones.