Water quality mediates resilience on the Great Barrier Reef

Abstract

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...

Spatial resilience of the Great Barrier Reef under cumulative disturbance impacts

“…We used the average frequency of exposure to river plume‐like conditions (PFc) as a proxy for exposure to dissolved nutrients and fine sediments delivered during the wet season (MacNeil et al, ). Based on satellite observations during the 2005–2013 wet seasons, the frequency (i.e., number of weeks per year) of exposure to primary, secondary and tertiary river plumes were estimated at a 1‐km resolution (Petus et al, ).…”

“…We reconstructed coral cover trajectories over the last 22 years (1996–2017) for every 0.01° grid cell based on the parameters estimated from a Gompertz‐based Bayesian hierarchical model of coral growth previously fitted to the LTMP reefs (MacNeil et al, ). This growth model is an adaptation of the Gompertz‐based model of benthic cover developed by Fukaya et al (Fukaya, Okuda, Nakaoka, Hori, & Noda, ) that quantifies the intrinsic growth rate () and strength of density dependence () for sessile species, expressed as coverage of a defined sampling area.…”

Long‐term dynamics and drivers of coral and macroalgal cover on inshore reefs of the Great Barrier Reef Marine Park

“…, Bruno and Valdivia , MacNeil et al. ). Understanding the interactions between these drivers at relevant scales is key to informing effective management of coral reef ecosystems.…”

Shifts in coralline algae, macroalgae, and coral juveniles in the Great Barrier Reef associated with present‐day ocean acidification

“…Understanding the drivers that enhance coastal acidification may help support management decisions toward mitigation and retardation of ecosystem losses. High river nutrient and sediment loads lead to elevated levels of nutrients and suspended sediments in coastal waters, which directly impact macroalgae, coral biodiversity, seagrass, and fish assemblages (De'ath & Fabricius, ; Fabricius et al, ), and reduce coral resistance to other disturbances like disease (MacNeil et al, ). In particular, our data suggest that fine suspended sediments, temperature, and light availability also have a strong impact on reef health.…”

“…Below certain thresholds, turbidity can, however, be beneficial to reefs under some circumstances. For example, suspended sediments can reduce thermal stress on some corals and increase the bleaching tolerance in others by reducing exposure to light, but this is also linked to slower recovery times after disturbances (Fisher, Bessell-Browne, & Jones, 2019;MacNeil et al, 2019;Morgan, Perry, Johnson, & Smithers, 2017).…”