The Deep Lagoon Areas of Oceanic Atolls May Serve as Environmental Refugia for Acroporid Coral Communities

Abstract

Alison Reef is an oceanic atoll located in the southern part of the Spratly Islands in the South China Sea. A total of 46 genera of scleractinians, the octocorals Heliopora spp. and Tubipora sp., and two hydrocoral species, Millepora platyphylla and M. cf. dichotoma, were recorded during the coral survey conducted in July 2022 in this atoll. Moreover, a diverse and abundant coral community (mean cover 51.4 ± SD 14.3%) dominated by acroporids was revealed in the deep zone of the lagoon (20–25 m depth). In contrast, the shallow zone of the lagoon (2–5 m) revealed two times lower coral cover, whereas the mean proportion of coral rubble was five times greater than in the deep zone, demonstrating the general coral reef decline in the shallow zone. Of the 34 coral genera observed in the deep zone of the lagoon, 14 species from 6 genera were found to be most abundant in this coral community. Ten of these species belonged to Acroporidae (Acropora, Montipora, Isopora and Anacropora). The dominance of acroporids distinguished this community from that in the same depth zone on the forereef. A thriving coral community dominated mostly by thermally susceptible coral taxa in the deep zone of the lagoon seemed to have been maintained for at least the last three decades. This phenomenon suggests the deep lagoons of open-type atolls with pronounced water renewal and negligible anthropogenic impact may serve as environmental refugia for a number of reef-building corals in light of global climate change.