The euphotic–mesophotic transition is characterized by dramatic changes in environmental conditions, which can significantly alter the functioning of ecosystem engineers and the structure of their associated communities. However, the drivers of biodiversity change across the euphotic–mesophotic transition remain unclear. Here, we investigated the mechanisms affecting the biodiversity-supporting potential of free-living red coralline algae—globally important habitat creators—towards mesophotic depths. Across a 73 m depth gradient, we observed a general decline in macrofaunal biodiversity (fauna abundance, taxon richness and alpha diversity), but anincrease in beta-diversity (i.e. variation between assemblages) at the deepest site (86 m depth, where light levels were less than 1% surface irradiance). We identified a gradient in abundance decline rather than distinct ecological shifts, driven by a complex interaction between declining light availability, declining size of the coralline algal host individuals and a changing host taxonomy. However, despite abundance declines, high betweenassemblage variability at deeper depths allowed biodiversity-supporting potential to be maintained, highlighting their importance as coastal refugia.

Depth range
13- 86 m

Mesophotic “mentions”
32 x (total of 3507 words)

Classification
* Presents original data
* Focused on 'mesophotic' depth range
* Focused on 'mesophotic coral ecosystem'

Fields
Biodiversity
Ecology

Focusgroups
Algae (Macro, Turf and Crustose Coralline)

Locations
Brazil - Fernando Noronha

Platforms
SCUBA (open-circuit or unspecified)