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Threats from climate change and other human pressures have led to widespread 21 concern for the future of Australia's Great Barrier Reef (GBR) 1 , where increasingly 22 frequent and severe coral bleaching, fishing, and ongoing pollution are 23 undermining long-term persistence of coral-dominated reefs 2,3 . Future resilience 24 of coral-dominated reefs within the GBR will be determined by their ability to 25 resist disturbances and to recover from coral loss, generating intense interest in 26 management actions that can moderate these processes 4-7 . Here we quantify the 27 effect of environmental and human drivers on the resistance and recovery of hard 28 corals to multiple disturbances within the southern and central GBR. Using a 29 composite index for water quality, we find that reefs exposed to poor water quality 30 recover from disturbance more slowly and are more susceptible to outbreaks of 31 crown-of-thorns starfish and coral disease while also being more resistant to 32 coral bleaching. Protection from fishing and increased herbivory were not 33 associated with substantially faster recovery from disturbance. Water quality 34 mediation of a tradeoff between resistance and recovery illustrates that, while 35 reefs in waters of chronically-poor quality contain corals with greater bleaching 36 resistance, there is a net negative impact on recovery and long-term hard coral 37 cover. Given these conditions, we find that 11-23% improvements in water quality 38 will be necessary to bring recovery rates in line with projected increases in coral 39 bleaching among contemporary inshore and mid-shelf reefs. However such 40 reductions are unlikely to buffer projected bleaching effects among outer-shelf 41 GBR reefs dominated by fast growing, thermally sensitive corals, demonstrating 42 practical limits to local management of the GBR against the effects of global 43 warming. 44 45 46The Great Barrier Reef (GBR) has experienced unprecedented losses of hard coral cover 8 . Most 47 coral loss on the GBR has been due to acute disturbances including storms 9,10 , disease 11 , 48 outbreaks of crown-of-thorns starfish Acanthaster spp. (CoTS) 9 , and coral bleaching 8 . Many of 49 these impacts are predicted to become more frequent or intense due to climate change 2,10,12-14 . 50Key to long-term coral-dominance on reefs is whether coral communities can resist coral loss and 51 recover sufficiently quickly between successive disturbances to be resilient and sustain viable 52 populations 15 . However, there are currently few process-based models for quantifying intrinsic 53 rates of increase that accurately characterize recovery. Some of the key drivers thought to 54 influence coral cover recovery include rates of herbivory 16 , coral community composition 17,18 , 55 water quality 19-22 , and protection from fishing 23 . While research into individual drivers is well 56 developed, how cumulative stressors may interact under climate change is not; the potential for 57 non-linear responses to novel ecosystem states creates consid...

Abstract. Tully River flood plume monitoring data for 11 events (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) were used to determine what physical characteristics of the floods (size of flood, direction of plume movement, shape of hydrograph) most influence the flood plume water quality and areal extent. During some events, the maximum area influenced by the Tully flood plumes extended into the Coral Sea. Areal extents depended on wind direction and discharge volume, with large extents more likely during light or northerly winds. Strong gradients in water quality existed away from the Tully mouth during the wet season and the adjacent marine ecosystems were regularly exposed to land-derived material. Flood plumes were grouped into three plume types: primary, secondary and tertiary plumes, based on water-quality characteristics (suspended solids, coloured dissolved organic matter and chlorophyll). The number of reefs and seagrasses exposed to plume waters varied from year to year, and was dependent on the characteristics of the event. Over the 11 years, out of the major 37 reefs and 13 seagrass meadows identified in the Tully marine area, between 11 (30%) and 37 coral reefs (100%) and most of the seagrass meadows were inundated by either a primary or secondary plume every year.

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Surface chlorophyll a concentrations in the Great Barrier Reef (GBR) lagoon were monitored at individual stations for periods of 6 to 12 years. The monitoring program was established to detect spatial and temporal changes in water quality resulting from increased loads of nutrients exported from the catchments adjoining the GBR. Sampling occurred monthly at up to 86 sites that were located in transects across the width of the continental shelf. In the central and southern GBR (16–21°S), there was a persistent cross-shelf chlorophyll a gradient, with higher concentrations near the coast. No cross-shelf gradient was observed in the far northern GBR (12–15°S). Mean chlorophyll a concentrations in the far northern GBR (0.23 µg L–1) were less than half those in the south and central GBR (0.54 µg L–1). Chlorophyll a varied seasonally within regions, with mean summer-wet season (December–April) concentrations ~50% greater than those in the winter-dry season (May–November). Sub-annual, inter-annual and event-related variations in chlorophyll a concentrations were observed in several zones. Multi-year patterns in concentrations suggest that relatively short (5–8 years) time series may give spurious estimates of secular trends. Higher chlorophyll a concentrations in inshore waters south of 16°S were most likely related to the levels of river nutrient delivery associated with agricultural development on adjacent catchments.

Current scientific consensus is that inshore regions of the central and southern Great Barrier Reef, Australia, are at risk of impacts from increased nutrient (as well as sediment and pesticide) loads delivered to Reef waters. Increases in the discharge of water quality contaminants to the Reef are largely a consequence of the expansion of agricultural practices in northern Queensland catchments following European settlement in the 1850s. In particular, the presence of elevated chlorophyll a and nutrient concentrations in many parts of the inshore Great Barrier Reef together with intense and extensive phytoplankton blooms following the discharge of nutrient-rich river flood waters suggest that the central and southern inshore area of the Great Barrier Reef is likely to be significantly impacted by elevated nutrient loads. The biological consequences of this are not fully quantified, but are likely to include changes in reef condition including hard and soft coral biodiversity, macroalgal abundance, hard coral cover and coral recruitment, as well as change in seagrass distribution and tissue nutrient status. Contemporary government policy is centered around promotion and funding of better catchment management practices to minimize the loss of catchment nutrients (both applied and natural) and the maintenance of a Reef wide water quality and ecosystem monitoring program. The monitoring program is designed to assess trends in uptake of management practice improvements and their associated impacts on water quality and ecosystem status over the next 10 years. A draft set of quantitative criteria to assess the eutrophication status of Great Barrier Reef waters is outlined for further discussion and refinement

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The GBR is losing its ability to recover from disturbances, but local management action can help partially restore this ability.