Coral reef ecosystems have experienced a decline in health over the last decades, yet historical trends and potential factors contributing to such decline have been hardly investigated. The Great Barrier Reef (GBR) has faced the cumulative impacts of human activities, and recent accelerated climate change. Since European settlement in the mid-19th century, catchments adjacent to the GBR have undergone extensive modification potentially contributing to coral reef degradation. While recent studies have found links between human development since European settlement and coral decline in the northern and central GBR, the southern region remains overlooked from a historical perspective. This research focused on the reconstruction of the environmental context in the southern GBR and linked coral responses and mortality processes to disturbance events. A cross-disciplinary approach, using geochemical proxies (e.g. Sr/Ca, Ba/Ca), coral luminescence, dating of corals, field surveys and secondary sources of information (instrumental records and climate indices), was applied.
The reconstruction of seawater surface temperatures (SSTs) derived from coral proxy Sr/Ca revealed a significant warming trend (p < 0.001, R2 = 0.026) from 1933 to 2010 in the Keppel Islands. Significant correlations between sediment runoff proxies (Ba/Ca and Y/Ca) and instrumental records of river discharge and rainfall confirmed the influence of the Fitzroy River catchment on Keppel reefs. Extreme spikes (2-14-fold higher than baseline values) of Ba/Ca after 1950 suggested an increased influence of continental runoff, which coincided with the intensification of land clearing activities in the Fitzroy catchment. All geochemical proxies were further correlated with the Pacific Decadal Oscillation (PDO) and Southern Oscillation Index (SOI). Overall, our findings provided evidence that past environmental conditions of inshore reefs in the southern GBR have been influenced by the climatic regime of the Pacific region along with land-use change in adjacent catchment areas. Coral luminescence, using spectral luminescence scanning (SLS), distinctively reproduced temporal changes of river discharge on coral reefs. Luminescence signals were clearly related to La Niña events and the negative phase of PDO. These results suggested that the disturbance regimens on Keppel reefs might be modulated by climate-related factors.
Comparisons of living and dead assemblages suggested no evident shifts in coral community structure during the last decades and showed dominance of Acropora. Most U-series ages of death assemblages (Acropora) showed a recent history of mortality matching the timing of significant disturbance events over the last three decades (1980-2010). While growth parameters of massive Porites did not show a long-term trend of decline (1945-2009), inter-annual decreases in calcification and extension rates were detected and associated with floods and bleaching events. Responses of Acropora and Porites indicated stability and historical resilience in reef communities of Keppel Islands despite a complex history of disturbances in this region. Overall this study reveals that floods have played a significant role in the dynamics of Keppel reefs and provides evidence that past environmental conditions and disturbance regimes in the southern GBR are significantly influenced by the strength of La Niña and modulated by the PDO. Therefore future trajectories and resilience responses for the inshore reefs of the GBR could be largely mediated by changing climatic regimens.
High precision U-series dating from coral rubble deposits in One Tree Island were additionally used to interpret the formation of storm-induced deposits in the southern GBR. U-series ages verified that previous dating of storm deposits by radiocarbon techniques are reliable and useful to reconstruct cyclone history over millennial time scales. There was no evidence to link the formation and deposition of storm ridges to a single climatic event. Instead, U-series dates suggest that deposits have been reworked over time by low intensity cyclones or unusual wind conditions.