THE FIGHT FOR DOMINANCE ON THE GREAT BARRIER REEF: A MARRIAGE OF FOSSILS AND MOLECULES

Phase shifts and the declining ability for ecological communities to recover from human-induced disturbance are now commonplace in marine ecosystems. For coral reefs, phase shifts range from changes in the dominance of reef-building coral species to their replacement by their major competitors for space, the fleshy macro-algae. On inshore sites along the Great Barrier Reef (GBR), coral dominance has shifted from the fast-growing Acropora toward slower-growing or weedier species with reduced capacity for the maintenance of reef growth and ecosystem function. Phase shifts have also resulted in the replacement of these corals by macro-algae.

We conducted an extensive coring program of inner shelf reef sediments over 14 degrees of latitude along the GBR. Through study of the timing and occurrence of coral skeletons preserved within the cores, we found recent unprecedented disruption of millennia-scale dominance of Acropora. The timing of these shifts is consistent with degrading water quality brought about through land use changes associated with European colonization of the adjacent coastline. Using ancient DNA (aDNA), here we test the hypothesis that shifts in coral community composition were associated with changes in the abundance of macro-algae.

In our pilot study of 2 cores from Pandora Reef (PAN1 and PAN3), all of the sediment samples yielded aDNA spanning the past 750 years. Multivariate dispersion analysis of 12 functional groups (planktonic and benthic Dinophycea, Bryozoa, Tunicata, Hexacorallia, Octocorallia, Porifera, Rhizaria, Corallinacea, Chlorophyta, Bacillariophyta, and Phaeophyta) showed temporal variation in the aDNA community structure of PAN3 (Distlim r² = 0.44, p = 0.001), but not PAN1 (Distlim r² = 0.12, p = 0.099). The abundance of brown macro-algae from aDNA was negatively related to the abundance of the most dominant corals for both cores.

Our results demonstrate the ability for reef sediments to record environmental aDNA, providing a valuable archive for understanding the past distribution and relative abundance of important reef dwelling organisms that do not leave a skeletonized record. Our ultimate goal is to utilize this information to detect temporal trends in the frequency and magnitude of past coral to macro-algae phase shifts over decadal to millennial time scales.