Environmental Records from Great Barrier Reef Corals: Inshore versus Offshore Drivers

Abstract

The biogenic structures of stationary organisms can be effective recorders of environmental fluctuations. These proxy records of environmental change are preserved as geochemical signals in the carbonate skeletons of scleractinian corals and are useful for reconstructions of temporal and spatial fluctuations in the physical and chemical environments of coral reef ecosystems, including The Great Barrier Reef (GBR). We compared multi-year monitoring of water temperature and dissolved elements with analyses of chemical proxies recorded in Porites coral skeletons to identify the divergent mechanisms driving environmental variation at inshore versus offshore reefs. At inshore reefs, water Ba/Ca increased with the onset of monsoonal rains each year, indicating a dominant control of flooding on inshore ambient chemistry. Inshore multi-decadal records of coral Ba/Ca were also highly periodic in response to flood-driven pulses of terrigenous material. In contrast, an offshore reef at the edge of the continental shelf was subject to annual upwelling of waters that were presumed to be richer in Ba during summer months. Regular pulses of deep cold water were delivered to the reef as indicated by in situ temperature loggers and coral Ba/Ca. Our results indicate that although much of the GBR is subject to periodic environmental fluctuations, the mechanisms driving variation depend on proximity to the coast. Inshore reefs are primarily influenced by variable freshwater delivery and terrigenous erosion of catchments, while offshore reefs are dominated by seasonal and inter-annual variations in oceanographic conditions that influence the propensity for upwelling. The careful choice of sites can help distinguish between the various factors that promote Ba uptake in corals and therefore increase the utility of corals as monitors of spatial and temporal variation in environmental conditions.

Sediments and herbivory as sensitive indicators of coral reef degradation

“…The inner shelf location of Orpheus Island makes it more prone to sediment and freshwater inputs, and the most damaging phases of tropical cyclones (DeVantier et al 1997, Walther et al 2013. Despite these potential stressors, Pioneer Bay has previously been demonstrated to be a resilient ecosystem (Bellwood et al 2006a, Hughes et al 2007.…”

Delayed coral recovery in a warming ocean

“…Nutrients in floodwaters have also been implicated in increased frequency of crown‐of‐thorns starfish outbreaks (Fabricius, Okaji, & De'Ath, ), which extends the ecological footprint of terrestrial run‐off beyond the inshore environment. Higher nutrient concentrations may also come from resuspension and upwelling in well‐mixed water masses (Walther, Kingsford, & McCulloch, ; Wooldridge et al., ).…”

“…Higher nutrient concentrations may also come from resuspension and upwelling in well-mixed water masses (Walther, Kingsford, & McCulloch, 2013;Wooldridge et al, 2017).…”

Coupling biogeochemical tracers with fish growth reveals physiological and environmental controls on otolith chemistry

“…Upwelled waters typically leave distinctive elemental signatures in biological carbonates due to differences in water mass chemistry and associated links to primary productivity (e.g., coral [Lea et al 1989], otoliths [Kingsford et al 2009], mollusks [Hatch et al 2013]). For example, where sources of upwelled water are enriched with Ba from deep water (e.g., Galapagos Island, outer Great Barrier Reef, central California coast), coral skeletons (Lea et al 1989, Walther et al 2013) and otoliths (Kingsford et al 2009, Woodson et al 2013 show spikes in concentrations of Ba:Ca during upwelling periods. Coastal upwelling events along Australia's southern coast are distinct, seasonal occurrences (austral summer, December-March; Middleton and Bye 2007) that provide much needed nutrients to the oligotrophic waters of this region (Nieblas et al 2009, van Ruth et al 2010.…”