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Loads of suspended sediment, nutrients and pesticides discharged to the Great Barrier Reef (GBR) have increased greatly due to agricultural and urban development of the GBR catchment. As a result, in association with climate change impacts, the ecosystems of the GBR have degraded greatly in recent decades, and the decline in ecosystem health continues. Improved agricultural management practices are now being funded to reverse the decline. However the quantum of reduction in contaminant loads from individual rivers needed to reverse the decline is not known. To estimate load reductions that would reverse the decline we developed what we term Ecologically Relevant Targets (ERTs). These targets are the load reductions of sediment, nutrients and pesticides needed such that GBR lagoonal waters meet the relevant water quality guidelines for the region. The methodology proposed for setting the ERTs is described and examples of ERTs set for the Burdekin Region of the GBR are given. [ABSTRACT FROM AUTHOR] more...

In this paper I challenge the notion that a healthy and resilient coral is (in all cases) a fast-growing coral, and by inference, that a reef characterised by a fast trajectory toward high coral cover is necessarily a healthy and resilient reef. Instead, I explain how emerging evidence links fast skeletal extension rates with elevated coral-algae (symbiotic) respiration rates, most-often mediated by nutrient-enlarged symbiont populations and/or rising sea temperatures. Elevated respiration rates can act to reduce the autotrophic capacity (photosynthesis:respiration ratio) of the symbiosis. This restricts the capacity of the coral host to build and maintain sufficient energy reserves (e.g. lipids) needed to sustain essential homeostatic functions, including sexual reproduction and biophysical stress resistance. Moreover, it explains the somewhat paradoxical scenario, whereby at the ecological instant before the reef-building capacity of the symbiosis is lost, a reef can look visually at its best and be accreting CaCO3 at its maximum. [ABSTRACT FROM AUTHOR] more...

The threats of wide-scale coral bleaching and reef demise associated with anthropogenic climate change are widely known. Here, the additional role of poor water quality in lowering the thermal tolerance (i.e. bleaching ‘resistance’) of symbiotic reef corals is considered. In particular, a quantitative linkage is established between terrestrially-sourced dissolved inorganic nitrogen (DIN) loading and the upper thermal bleaching thresholds of inshore reefs on the Great Barrier Reef, Australia. Significantly, this biophysical linkage provides concrete evidence for the oft-expressed belief that improved coral reef management will increase the regional-scale survival prospects of corals reefs to global climate change. Indeed, for inshore reef areas with a high runoff exposure risk, it is shown that the potential benefit of this ‘local’ management imperative is equivalent to ∼2.0–2.5°C in relation to the upper thermal bleaching limit; though in this case, a potentially cost-prohibitive reduction in end-of-river DIN of >50–80% would be required. An integrated socio-economic modelling framework is outlined that will assist future efforts to understand (optimise) the alternate tradeoffs that the water quality/coral bleaching linkage presents. [Copyright &y& Elsevier] more...

Abstract: We used historical flood plume extent data (modelled) to quantify the typical spatial extent of the summer runoff–seawater mixing zone of the Great Barrier Reef (GBR) lagoon. Spatially explicit analysis of the variability of in situ chlorophyll a concentrations (observed) across the runoff–seawater mixing zone, then allowed us to explore regional differences in the nutrient enrichment impact of runoff events from the various river systems that drain the GBR catchment. We demonstrate the existence of a discernable north–south gradient along the length of the GBR, such that for equivalent runoff:seawater dilutions ratios, lower levels of nutrient enrichment (as indicated by chlorophyll a observations) result from the river systems that drain the relatively undisturbed northern areas of the GBR catchment, compared to more human-impacted central and south areas. We identify a strong correlation between this north–south enrichment gradient and the flood concentration of dissolved inorganic nitrogen (DIN) entrained by the various river systems. By substituting the nutrient enrichment characteristics of the human-impacted river discharges with those of the undisturbed northern rivers, we provide a means to compare the short-term enriching ‘footprint’ for existing runoff intrusions with those that are likely to have occurred under pre-European catchment conditions. We demonstrate that under pre-European conditions, the nutrient enriching impact from river runoff was likely to have been largely constrained within 1–2km of the coast, whereas existing conditions support the impact of reefs some 20–30km off the coast. By using the developed spatial relations, we show that for the heavily human-impacted river systems, reductions in the end-of-river concentrations of DIN in the order 50–80% are needed in order to restore parity with pre-European conditions. We discuss these results in regard to developing end-of-catchment water quality targets for the region. [Copyright &y& Elsevier] more...