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

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 5,400years 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 13.91.3% of reef-flat substrates. Age-frequency analysis indicated that the majority (86%) of P.compressa colonies were less than 30years 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 16.20.6cm over the past 30years (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. Inshore coral reefs throughout the world are highly susceptible to anthropogenic disturbance (e.g., increasing pollution and changing land use) and have declined in recent decades. Modern sea-level rises associated with global warming could theoretically increase accommodation space for corals growing on shallow reef flats. Whereas, the potential for degraded inshore coral reefs to respond to increased sea-level rise is as yet unclear. In this study, fringing reefs in Sanya Bay, a typical inshore reef system impacted by severe anthropogenic perturbations, provide a case study of disturbed inshore reefs response modern sea-level rise. With ecological surveys, coral demographic and age-frequency analyses, and high-precision U-Th dating, we provide evidence of a recent switch on of reef growth over the past 50years in response to rising sea levels that is unprecedented since the mid-Holocene. The unique life history strategy of the dominant coral taxa (free-living Porites) driving this partial recovery allows them to overcome highly turbid and eutrophic environmental conditions that have previously been considered adverse to recovery through sexual recruitment. Our results provide novel insight into the response of marginal habitats to sea-level rise, and a glimpse into a potential future condition, or new state, of heavily disturbed Indo-Pacific coral reefs.