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Linkages between coral assemblages and coral-based proxies of terrestrial exposure along a cross-shelf gradient of the Great Barrier Reef

Stacy Jupiter1, Guy Marion2, George Roff2, Meegan Henderson2, Ove Hoegh-Guldberg2 and Malcolm McCulloch1

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Centre for Marine Studies, University of Queensland, Brisbane, QLD 4072 Australia

Field studies from recent decades suggest that coral assemblages from sites within proximity to land are shifting in response to both local and diffuse sources of terrestrial pollution. While coastal and nearshore communities have been identified to be at high risk from agricultural runoff in the Mackay section of the Great Barrier Reef, few baseline studies have been performed in this region to assess current coral assemblages and how they may be affected by terrestrial exposure.

We scored transects for benthic cover composition at seven reef locations along a gradient of distance offshore from the Pioneer River mouth. We categorized community structure into major benthic cover types (live hard coral, soft coral, fleshy macroalgae, substratum), dominant coral families, and growth morphology Acroporid corals, the most abundant group. Multivariate analysis of transect data showed that the nearshore reef assemblages at Keswick and St. Bees Islands (~33 km offshore) were categorically different from the other nearshore and midshelf sites due to the high cover of fleshy macroalgae overgrowing dead reef matrix.

Multiple tracers (luminescence intensity, δ15N, barium to calcium ratios) measured from coral core records collected from long-lived Porites colonies showed a gradient of disturbance from terrestrial runoff across the inner-shelf sites. While values of Ba/Ca and δ15N were strongly correlated with terrestrial runoff at Keswick and St. Bees, there was no significant influence of river discharge at the next island offshore (Scawfell at ~50 km). The enriched δ15N during major flood years in Keswick Island corals suggest that coral reef communities are receiving pulses of anthropogenic nutrients (mostly fertiliser-derived nitrogen) sourced from the Pioneer catchment. We therefore suggest that the current community composition at reef sites within 35 km of the Mackay coast is likely to be strongly influenced by repeated influx of sediments and nutrients delivered from flood plumes, which likely affect coral recovery and enable the persistence of late-successional algal stands.