Revealing how diversity varies across the depth gradient is key to
understanding the role of mesophotic coral ecosystems in the functioning
of coral reefs. We examined how alpha and beta generic diversity of
scleractinian coral assemblages vary across a wide depth gradient for
coral reefs. We studied generic diversity patterns of scleractinian corals
at sixteen sites in eight islands of three archipelagos in French
Polynesia, as derived from the analysis of photo-quadrats, across the
seafloor from shallow to lower mesophotic depths (6 to 120 m) and on a
wide geographic scale. Our sampling considered quantitative coral cover to
explore the patterns of alpha and beta components of diversity across
depth and horizontal space. We show that in French Polynesia, mesophotic
coral ecosystems host higher alpha and beta generic diversity than shallow
reefs despite decreasing coral cover with depth. The variation of coral
genus richness across the depth gradient is mainly driven by a mid-domain
effect with a peak at 40 m depth. At the same time, we found that the
turnover of coral genera across islands (i.e., spatial beta diversity)
increased steadily along the depth gradient. Our findings report the first
quantitative results of coral cover and diversity from mesophotic coral
ecosystems in French Polynesia and also present one of the few existing
studies to examine the broad breadth of the mesophotic depth gradient. We
demonstrate that mesophotic depths can host unexpectedly high generic
richness of scleractinian coral assemblages. At the same time, we showed
that increasing depth increases the differences in generic diversity
composition across islands, whereas shallow reefs are similar in between.
While a single island could conserve shallow regional biodiversity,
mesophotic depths containing the richest diversity require site-specific
measures, suggesting that considering these mesophotic depths in
conservation is necessary to maintain regional diversity.