Recolonization of Marginal Coral Reef Flats in Response to Recent Sea-Level Rise

Abstract

In an era of global change and rising sea levels, the capacity for inshore reefs to survive is increasingly unclear. We report on recent colonization of an inshore reef-flat environment at Sanya Bay, northern South China Sea, in shallow, muddy, eutrophic, and turbid conditions, which are widely viewed as marginal for sustained coral growth. U-Th dating of fossil Acropora substrate indicated that the reef flat has existed in a dormant state since ~5,400 years BP, with no vertical space available to accommodate coral expansion. Our surveys revealed that populations of free-living Porites compressa have recolonized the reef flat through asexual fragmentation, covering 13.9 ± 1.3% of reef-flat substrates. Age-frequency analysis indicated that the majority (86%) of P. compressa colonies were less than 30 years old. Analysis of long-term sea-level data indicated that recent recolonization of the reef flat occurred in response to a sea-level rise of 16.2 ± 0.6 cm over the past 30 years (1987–2016). Modern sea-level rise at Sanya Bay appears to have turned on reef growth which has existed in a senescent turned off state for over five millennia. The asexual life history strategy of P. compressa colonies, which involves forming free-living colonies (coralliths), allows them to overcome turbid environmental conditions that are otherwise adverse to sexual recruitment. Our results provide novel insight into the response of marginal habitats to sea-level rise, and suggest that coral cover on degraded coral reef flats could increase under future sea-level rise, albeit with assemblages dominated by a few well-adapted species.