Improved predictions of coral bleaching using seasonal baselines and higher spatial resolution

Coral bleaching spread across the southern Great Barrier Reef in January 2006, after sea temperatures reached climatological summer maxima 2 months before normal. Current satellite-derived warning systems were unable to detect severe bleaching conditions in the region because of their use of a constant thermal threshold (summer maximum monthly mean) and low spatial resolution (50 km). Here it is shown that such problems can be ameliorated if the thermal threshold is adjusted for seasonal variation and a 4-km spatial resolution is used. We develop a seasonally and spatially improved thermal threshold for coral bleaching on the basis of a weekly climatology of sea surface temperatures extending from austral spring to late summer, and apply the method to two case-study sites. At both sites, and in particular at the nearshore site that was undetected by the 50-km satellite product, the seasonally adjusted thermal threshold produced a greatly improved consistency between accumulated heating and bleaching severity. The application of thermal stress algorithms that reflect the long-term mean pattern in seasonal variation allows coral bleaching to be forecast with higher precision

Enhanced particle-feeding capacity of corals on turbid reefs (Great Barrier Reef, Australia)

Reef corals occur across a wide range of habitats, from offshore clear waters to nearshore sediment-laden environments. This study tests the hypothesis that corals from turbid nearshore areas have greater capacity to utilise suspended sediment as a food source than conspecifics from less turbid and midshelf areas. The hypothesis was tested on two common and widespread coral species on the Great Barrier Reef (Pocillopora damicornis and Acropora millepora). The particle clearance rates of samples from more turbid reefs were two-fourfold those of conspecifics from less turbid and midshelf reefs. Rates of sediment ingestion were generally a linear function of sediment load indicating no significant saturation within the concentration range of 1–30 mg dry weight ɭ^−1. Estimated assimilation efficiency of particulate ^14C varied between 50 and 80%, and was maximised for midshelf A. millepora at the lowest sediment concentration, suggesting that heterotrophy is more efficient in oligotrophic habitats. Based on feeding-response curves, assimilation efficiencies, and published records of ambient particle concentrations, representatives of these species on turbid inshore reefs are 10–20 times more heterotrophic on suspended sediment than their conspecifics on less turbid and midshelf reefs

Coral mortality following extreme low tides and high solar radiation

Extreme tidal events are one of the most predictable natural disturbances in marine benthic habitats and are important determinants of zonation patterns in intertidal benthic communities. On coral reefs, spring low tides are recurrent disturbances, but are rarely reported to cause mass mortality. However, in years when extremely low tides coincide with high noon irradiances, they have the potential to cause widespread damage. Here, we report on such an event on a fringing coral reef in the central Great Barrier Reef (Australia) in September 2005. Visual surveys of colony mortality and bleaching status of more than 13,000 corals at 14 reef sites indicated that most coral taxa at wave-protected sites were severely affected by the event. Between 40 and 75% of colonies in the major coral taxa (Acropora, Porites, Faviidae, Mussidae and Pocilloporidae) were either bleached or suffered partial mortality. In contrast, corals at wave-exposed sites were largely unaffected (<1% of the corals were bleached), as periodic washing by waves prevented desiccation. Surveys along a 1–9 m depth gradient indicated that high coral mortality was confined to the tidal zone. However, 20–30% of faviid colonies were bleached throughout the depth range, suggesting that the increase in benthic irradiances during extreme low tides caused light stress in deeper water. Analyses of an 8-year dataset of tidal records for the area indicated that the combination of extended periods of aerial exposure and high irradiances occurs during May–September in most years, but that the event in September 2005 was the most severe. We argue that extreme low-tide, high-irradiance events are important structuring forces of intertidal coral reef communities, and can be as damaging as thermal stress events. Importantly, they occur at a time of year when risks from thermal stress, cyclones and monsoon-associated river run-off are minimal

Coral reefs in turbid waters: sediment-induced stresses in corals and likely mechanisms of adaptation

Coral reefs close to the shoreline may repeatedly be exposed to high concentrations of suspended sediments and low light levels, but many such reefs display high coral cover, indicating that the corals have become adapted or acclimated to such events. Here, we discuss a set of mechanisms by which corals might adapt or acclimatise to variable sedimentation and turbidity. Specifically, we note the documented links between key physical parameters in turbid habitats (e.g. turbidity and suspended sediment concentration (SSC), light extinction, grain size and composition) and predicted responses of coral photosynthesis, feeding and respiratory/excretory losses. Based on observed temporal patterns in turbidity and sedimentation, we argue that 1) rapid replenishment of energy reserves during periods between sublethal turbidity events, 2) shifts between phototrophic and heterotrophic dependence, and 3) rapid rates of photo-acclimation are likely mechanisms by which corals maintain a positive energy balance (a proxy of stress or health) on coral reefs in turbid zones

Coral reefs in turbid waters: sediment-induced stresses in corals and likely mechanisms of adaptation

Coral reefs close to the shoreline may repeatedly be exposed to high concentrations of suspended sediments and low light levels, but many such reefs display high coral cover, indicating that the corals have become adapted or acclimated to such events. Here, we discuss a set of mechanisms by which corals might adapt or acclimatise to variable sedimentation and turbidity. Specifically, we note the documented links between key physical parameters in turbid habitats (e.g. turbidity and suspended sediment concentration (SSC), light extinction, grain size and composition) and predicted responses of coral photosynthesis, feeding and respiratory/excretory losses. Based on observed temporal patterns in turbidity and sedimentation, we argue that 1) rapid replenishment of energy reserves during periods between sublethal turbidity events, 2) shifts between phototrophic and heterotrophic dependence, and 3) rapid rates of photo-acclimation are likely mechanisms by which corals maintain a positive energy balance (a proxy of stress or health) on coral reefs in turbid zones

Biotic and abiotic correlates of tissue quality for\ud common scleractinian corals

Reef-building corals provide critical ecosystem functions but are under pressure from a range of stressors, and there is a growing need to understand how environmental factors affect the physiological condition of corals. This study explored the relationships between tissue quality (i.e. energy content per unit surface area) and light intensity, water flow velocity, temperature, colony size and competition for 4 common coral species. A formal model selection procedure revealed that different corals, even species that are closely related and morphologically similar, respond to their local environments in fundamentally different ways. Overall, colony size was the most consistent predictor of tissue quality, and the observed increase in tissue energy content with size may partially explain field observations of reduced mortality for large colonies. However, competitive interactions, light intensity, water flow and temperature also contributed to variation in tissue properties. Warming reduced the tissue quality of massive and foliose corals more than that of branching corals, despite the latter generally being recognised as more susceptible to thermal bleaching. These findings identify species- and habitat-specific responses to baseline environmental variables that can be used to anticipate shifts in coral health under environmental change

A framework for understanding cumulative impacts, supporting environmental decisions and informing resilience based management of the Great Barrier Reef World Heritage Area: Final Report to the Great Barrier Reef Marine Park Authority and Department of the Environment

This project was commissioned by and developed with the Department of the Environment and the Great Barrier Reef Marine Park Authority (GBRMPA) to develop a qualitative framework for understanding cumulative impacts on the resilience of coral reef and seagrass ecosystems to inform decision making in the Great Barrier Reef World Heritage Area (GBRWHA). This report is the result of close collaboration between the Australian Institute of Marine Science (AIMS), GBRMPA, Department of the Environment, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the University of Melbourne. The Great Barrier Reef is facing a range of pressures, and supporting the resilience of the Reef has become a central focus in its management. A key challenge for managers is to understand the cumulative impacts of multiple stressors and incorporating this knowledge into management decisions. The Cumulative Impact and Structured Decision-Making (CISDM) framework represented in this report is designed to assist Great Barrier Reef Marine Park managers and stakeholders in this challenge. The framework uses qualitative and probabilistic modelling to provide a systems-level understanding of how cumulative stressors affect coral reefs and seagrass ecosystems in the GBRWHA. These ecosystems underpin matters of national environmental significance (MNES), including the outstanding universal value (OUV) of the GBRWHA. This report forms one part of the department's program of work funded under the Sustainable Regional Development Program to support the Strategic Assessment of the Great Barrier Reef World Heritage Area.This record includes 2 *.NETA files as mentioned in the report on page 38. These files are specific to an application called Netica. Please see https://www.norsys.com/netica.html for more information

Food availability promotes rapid recovery from thermal stress in a scleractinian coral

Bleaching in corals due to environmental stress represents a loss of energy intake often leading to an increase in mortality risk. Successful coral recovery from severe bleaching events may depend on the rate of replenishment of algal symbiont populations following the period of thermal stress, the supply of an alternative food source, or both. Here, we explore the role of food availability in promoting the survival and recovery of a common coral (Acropora intermedia) following acute experimentally induced thermal stress. Fed corals were provided with live rotifers daily, to maintain densities of zooplankton in tanks that are typical of coral reefs. After a 6-week acclimation phase, heated corals were subjected to a +4 °C thermal anomaly for a 7-day period (bleaching phase) then temperatures were returned to normal for a further 2 weeks (recovery phase). Results demonstrated that heated corals had higher survival when they were provided with heterotrophic food. Fed corals experienced reduced loss of chlorophyll a, relative to unfed corals. During the recovery phase, both fed and unfed corals recovered within a few days; however, fed corals recovered to pre-bleaching phase levels of chlorophyll a, whereas unfed corals stabilized approximately one-third below this level. Protein levels of fed corals declined markedly during the bleaching phase, but recovered all of their losses by the end of the recovery phase. In contrast, unfed corals had low protein levels that were maintained throughout the experiment. To the extent that these results are representative of corals' responses to thermal anomalies in nature, the findings imply that availability of particulate food matter has the potential to increase corals' capacity to survive thermally induced bleaching and to ameliorate its sub-lethal effects. They also support the hypothesis that different rates of heterotrophy are an important determinant of variation in resilience to thermal stress among reef environments

Spatial resilience of the Great Barrier Reef under cumulative disturbance impacts

In the face of increasing cumulative effects from human and natural disturbances, sustaining coral reefs will require a deeper understanding of the drivers of coral resilience in space and time. Here we develop a high-resolution, spatially explicit model of coral dynamics on Australia's Great Barrier Reef (GBR). Our model accounts for biological, ecological and environmental processes, as well as spatial variation in water quality and the cumulative effects of coral diseases, bleaching, outbreaks of crown-of-thorns starfish (Acanthaster cf. solaris), and tropical cyclones. Our projections reconstruct coral cover trajectories between 1996 and 2017 over a total reef area of 14,780 km2 , predicting a mean annual coral loss of -0.67%/year mostly due to the impact of cyclones, followed by starfish outbreaks and coral bleaching. Coral growth rate was the highest for outer shelf coral communities characterized by digitate and tabulate Acropora spp. and exposed to low seasonal variations in salinity and sea surface temperature, and the lowest for inner-shelf communities exposed to reduced water quality. We show that coral resilience (defined as the net effect of resistance and recovery following disturbance) was negatively related to the frequency of river plume conditions, and to reef accessibility to a lesser extent. Surprisingly, reef resilience was substantially lower within no-take marine protected areas, however this difference was mostly driven by the effect of water quality. Our model provides a new validated, spatially explicit platform for identifying the reefs that face the greatest risk of biodiversity loss, and those that have the highest chances to persist under increasing disturbance regimes.Camille Mellin, Samuel Matthews, Kenneth R.N. Anthony, Stuart C. Brown, M. Julian Caley, Kerryn A. Johns, Kate Osborne, Marjetta Puotinen, Angus Thompson, Nicholas H. Wolff, Damien A. Fordham, M. Aaron MacNei