Great Barrier Reef Shelf Research Articles

Relative sea level response to mixed carbonate-siliciclastic sediment loading along the Great Barrier Reef margin

The continental shelf along northeastern Australia is the world's largest mixed carbonate-siliciclastic passive margin and the location of the Great Barrier Reef (GBR). Following sea-level transgression during the last deglaciation, extensive sediment was deposited along the GBR due to neritic carbonate deposition (including shelf edge reefs, Holocene reefs and Halimeda bioherms) and fluvial discharge of terrigenous siliciclastic sediments. Such sediment loading can alter local relative sea level (RSL) by several metres through the sediment isostatic adjustment (SIA) process, a signal that is poorly constrained at the GBR. In this study, we used a glacial isostatic adjustment (GIA) model to develop an ensemble-based sediment loading history for the GBR since Marine Isotope Stage 2 (MIS 2). A Bayesian style framework is adopted to calibrate the sediment history ensemble and GIA model parameters using a sea-level database. According to our results, 1853.7 Gt (1613.1-2078.7 Gt, 95% confidence interval) of sediment have been deposited across the GBR since MIS 2 (28 ka BP), causing spatially variable relative sea-level change with the highest magnitude (0.9-1.1 m) found in the outer shelf of the southern central GBR (18.4-21.6∘ S). Because the SIA-induced RSL rise is unrelated to ice mass loss, failing to correct for this signal will lead to systematic overestimation of grounded ice volume by up to ∼4.3 × 105 km3 during the Last Glacial Maximum. Additionally, we found that spatial variation in sediment loading and coastal environment may explain the different RSL history documented by published fossil coral reef records from Noggin Pass and Hydrographer's Passage. These results highlight the importance of considering SIA for any postglacial sea-level studies adjacent to large sediment systems. Lastly, by quantifying both the GIA and SIA signals, we provide a spatially and temporally complete RSL reconstruction that is well-suited to be used as a boundary condition to study the evolution of the GBR shelf and slope sedimentary system.

Reef growth history at intermediate to mesophotic depths since the end of the Last Glacial period along the Great Barrier Reef shelf edge

Reef communities at intermediate (10 to 30 m) and mesophotic (~ 30 to 150 m) depths occupy large areas of sea floor but little is known about their potential to accrete vertically, their response to sea-level change and other environmental perturbations. In this study, the authors have examined cores from two holes, M0040A and M0041A, drilled by the International Ocean Drilling Program Expedition 325 along the shelf edge of the modern GBR at 131 m water depth. The objective was to investigate reef growth history at palaeo-water depths > 20 m over a time period spanning 30,000 years, from the end of the Last Glacial period through the last deglaciation. Based on changes in lithologies and biotic components, and a robust chronostratigraphic framework supported by 47 radiometric ages, the authors have identified two episodes of reef growth, one during Marine Isotope Stage (MIS) 2 and the other at the onset of the deglaciation, both characterised by abundant microbialite crusts and distinct coral assemblages. Palaeo-water depths range from 30 to 60 m and from 20 to 30 m for the MIS 2 and early deglacial reef sections, respectively. The first episode of reef growth documented in the cores initiated at 27 to 25 ka, possibly in response to increased light availability and change in sedimentation resulting from falling sea-level between 32 and 29 ka from MIS 3 and also to low atmospheric pCO2 at the end of the Last Glacial period. Reef accretion was reduced or ceased sometime between 24 and 19 ka, coinciding with the rapid 20 m sea-level fall of the peak Last Glacial Maximum and minima of SSTs. Reef growth resumed at 19.5 to 18.5 ka, influenced by a period of moderate sea-level rise and increasing sea surface temperatures at the onset of the deglaciation. Reef growth termination at ca. 17 ka correlates with a major episode of reef demise previously identified in adjacent mid and outer terrace cores and linked to reduced water quality combined with rapid deglacial sea-level rise. Vertical accretion (VA) rates were calculated based on two methods: linear visual fitting and Bayesian modelling. The findings show that the highest VA rates are associated with microbialite boundstone. Reef ecosystems dominated by microbialite and corals developed at intermediate and mesophotic depths, and grew vertically at maximum rates of 2 to 5 mm yr-1 depending on the method used, over a period of rapid environmental change during the transition from MIS 3 to MIS 1. Further study needs to explore the potential of modern-type deep coralgal communities to cope with higher rates of sea-level rise predicted this century.

Demographic patterns of the tropical baitfish Spratelloides delicatulus (Order: Clupeiformes) across the Great Barrier Reef shelf and at multiple latitudes

Clupeiformes are the most important food fish in the world, and provide a key trophic link in marine food chains. Here we describe broad scale patterns of clupeiform demographic characteristics of the delicate round herring sprat Spratelloides delicatulus on the Great Barrier Reef (GBR). Sampling was conducted over 10° of latitude and two seasons at multiple distances across the GBR shelf. The oldest S. delicatulus sampled was 152 days and the maximum standard length was 74 mm. Age and length maxima increased with latitude conforming with ‘counter gradient theory’ and these patterns were consistent between years. von Bertalanffy relationships showed that growth rates were highest at Northern GBR sites; growth coefficients ranged from 2–6 K year−1, and were lowest on southern reefs, i.e. ‘tropical gradient of growth’. Daily survivorship ranged from 91–97% day−1 at all sites. Hatching dates estimated from counts of daily otolith increments indicated a prolonged spawning season of at least 9 months. Reproductive development indicated a size-based relationship. Males and females matured at similar sizes ranging from 36–38 mm, but fish from southern sites were 30–40 days older. Tropical clupeiforms live fast and die young, and patterns of abundance, composition and demography followed strong environmental gradients which conformed to some existing models.

Impact of European settlement and land use changes on Great Barrier Reef river catchments reconstructed from long-term coral Ba/Ca records

The increase in sediment and nutrient loads entering the coastal waters of the Great Barrier Reef (GBR) and the associated degradation of water quality represents a major threat to coral reefs. Although the strengthening of preventative management strategies remains a priority, there is a general lack of terrestrial runoff baseline information with respect to the spatial and temporal severity of disturbances associated with ongoing European-style land use practices. Here we use new and existing high-resolution coral Ba/Ca and luminescence records from the central Cairns region to the southern GBR shelf to reconstruct sediment fluxes discharged into the GBR from before European settlement in the 1860s to the present-day. Since the commencement of European settlement in the 1860s we document a tripling of flood-plume suspended sediment loads delivered by the Burdekin River to the GBR lagoon relative to ‘natural’ pre-European baseline levels. We show that this is indicative of a much more extreme degradation of the river catchments than hitherto appreciated with intensified discharge events particularly from the central and southern catchments carrying higher sediment loads. More-over from the 1930s onwards the Burdekin River, the largest source of both sediment and freshwater to the GBR, has also exhibited a progressive northwards expansion of its flood plume. This, together with increased variability of freshwater inputs indicated by coral luminescence records, now shows that the inner GBR not only continues to be impacted by increasing sediment/nutrient loads but is also subject to higher intensity river discharge events due to the loss of ground cover causing increased overland runoff and erosion.

Impact of Typhoon Chan-hom on sediment dynamics and morphological changes on the East China Sea inner shelf

The East China Sea (ECS) inner shelf is highly impacted by tropical cyclones. Due to the limited availability of observations, the sediment dynamics and associated sedimentary process under the impact of typhoons are still poorly understood. In this study, a high-resolution unstructured-grid FVCOM model was employed to investigate the rapid responses of sediment dynamics on the ECS inner shelf to the passage of Typhoon Chan-hom in July 2015. The results indicated that there was an evident asymmetry in sediment dynamics around the typhoon track. During the typhoon, sediments were resuspended in nearshore area and transported downwind with increasing southwestward and seaward sediment flux. The gear-like sediment delivery presented a ratchet effect that induced an evident spatial asymmetry in responses of sediment dynamics and shelf sedimentation to the short-lived typhoon event. The coastal upwelling normally existing in summer was intermittently blocked by the passage of typhoon, and rapidly transited to downwelling, which substantially enhanced the cross-shelf sediment transport. As a result, severe erosions were mainly occurred near the typhoon track within 20-m isobaths, in contrast to newly formed sedimentary layers (~10 cm in thickness) in areas far south of the typhoon track and around the 50-m isobaths. The spatial asymmetry of sedimentary response to the typhoon passage is therefore of importance to locating sediment records that well preserved the typhoon signals. The ratchet effect we proposed provided reasonable interpretations of the spatially asymmetric responses of shelf sedimentations in the northern Gulf of Mexico and the central Great Barrier Reef shelf to tropical cyclones, respectively.

An enigmatic decoupling between heat stress and coral bleaching on the Great Barrier Reef.

Ocean warming threatens the functioning of coral reef ecosystems by inducing mass coral bleaching and mortality events. The link between temperature and coral bleaching is now well-established based on observations that mass bleaching events usually occur when seawater temperatures are anomalously high. However, times of high heat stress but without coral bleaching are equally important because they can inform an understanding of factors that regulate temperature-induced bleaching. Here, we investigate the absence of mass coral bleaching on the Great Barrier Reef (GBR) during austral summer 2004. Using four gridded sea surface temperature data products, validated with in situ temperature loggers, we demonstrate that the summer of 2004 was among the warmest summers of the satellite era (1982–2017) on the GBR. At least half of the GBR experienced temperatures that were high enough to initiate bleaching in other years, yet mass bleaching was not reported during 2004. The absence of bleaching is not fully explained by wind speed or cloud cover. Rather, 2004 is clearly differentiated from bleaching years by the slow speed of the East Australian Current (EAC) offshore of the GBR. An anomalously slow EAC during summer 2004 may have dampened the upwelling of nutrient-rich waters onto the GBR shelf, potentially mitigating bleaching due to the lower susceptibility of corals to heat stress in low-nutrient conditions. Although other factors such as irradiance or acclimatization may have played a role in the absence of mass bleaching, 2004 remains a key case study for demonstrating the dynamic nature of coral responses to marine heatwaves.

Late glacial to deglacial variation of coralgal assemblages in the Great Barrier Reef, Australia

Abstract Integrated Ocean Drilling Program (IODP) Expedition 325 cored submerged reefs along the shelf edge of the Great Barrier Reef (GBR) to study sea-level and environmental changes and their impacts on reef communities and reef growth since the Last Glacial Maximum (LGM). Previous work defined five reef sequences (Reef 1–5) that span the last 30,000 years. Here we examined the variation in coralgal assemblages and their paleoenvironmental settings in late glacial to deglacial sequences from 23 holes cored seaward of the modern GBR in water depths from 46 to 131 m along four transects at three localities: Hydrographers Passage (HYD-01C and HYD-02A), Noggin Pass (NOG 01B), and Ribbon Reef (RIB-02A). We identified three coralline algal assemblages and eight coral assemblages indicating a broad range of reef settings from the shallow reef crest (0–5 m) to the deep forereef slope (>20 m). We document in detail for the first time the distribution and composition of reef communities that grew in the GBR during the LGM from 22,000–19,000 years ago. They included coral taxa that are major reef builders today: Isopora, Acropora gr. humilis, Dipsastraea gr. pallida, Porites, and Montipora. Prior to the fall in sea level to the maximum extent of the LGM, late glacial reef communities developed more proximally (landward) to the modern GBR along the shelf edge. Their distribution and composition reflect influences of the older Pleistocene basement depth and possible terrigenous sediment inputs. Post-LGM deglacial reef growth was vigorous in proximal sites and characterized by the accretion of a very shallow high-energy coralgal assemblage composed of medium to robustly branching Acropora, including A. gr. humilis, and thick algal crusts of Porolithon gr. onkodes associated with vermetid gastropods. More distally, reef growth was variably impacted by terrigenous input following deglacial reflooding of antecedent reef terraces. The coralgal succession and sedimentary facies in Noggin Pass indicate that an early drowning trend was linked to increased turbidity that was likely controlled by shelf morphology (narrow shelf, steep slope) and/or proximity to a paleo-river mouth. The deglacial succession in Ribbon Reef lacks typical shallow-water indicators, which may reflect influences of the particularly steep slope of the northern GBR shelf edge on reef zonation. A major sea-level jump at the onset of the Younger Dryas displaced reef habitats further upslope, forming a barrier reef system mainly composed of robustly branching acroporids distinct from the more distal sites. Our results highlight the importance of sedimentation and shelf morphology in addition to relative sea-level changes in controlling variations in reef community over centennial to millennial timescales.

Depositional environments beneath the shelf-edge slopes of the Great Barrier Reef, inferred from foraminiferal assemblages: IODP Expedition 325

To understand sea-level changes since the Last Glacial Maximum (LGM) and their effects on coral reef systems, the shelf-edge slopes of the Great Barrier Reef (GBR) were cored during the Integrated Ocean Drilling Program (IODP) Expedition 325. Recovered unconsolidated sediments beneath the submerged shelf edge reefs contain abundant foraminiferal tests, which record changes in depositional environments and paleo-water depths. A total of 177 sediment samples were collected from 17 holes along three transects located within two geographical areas (Noggins Pass and Hydrographers Passage), and were analyzed to determine stratigraphic changes in foraminiferal assemblages (2–0.5 mm size fraction). Results show that four foraminiferal assemblages (A, B, C and D) are delineated by multivariate analyses (Q-mode cluster analysis and non-metric multidimensional scaling: NMDS), and these assemblages correspond to a back-reef to reef margin zone (0–10 m deep; Assemblage A), an upper photic zone (10–30 m deep) associated with hard substrates (Assemblage B), an intermediate to lower photic zone (30–90 m deep) characterized by soft substrates (Assemblage C), and a lower photic zone (90–130 m deep) only found in modern shelf slopes (Assemblage D). Gradual shifts in these four foraminiferal assemblages superimposed on a two-dimensional NMDS ordination mainly reflect water-depth gradients and the relative dominance of two substrate types (hard and soft substrates). Pre-LGM (older than Marine Isotope Stage 3: ≥MIS3) sediments along transects at Hydrographers Passage were deposited at intermediate to lower photic zones. In contrast, relatively shallow-water sequences found in ≥MIS3 deposits at Noggins Pass likely owe their origin to either turbidite and/or land slide processes. The lack of Assemblage D in ≥MIS3 deposits from all transects could be related to lowering temperature and/or increasing terrestrial influences (i.e. more light attenuation). Shallowing upward sequences found in LGM (MIS 2) deposits at Hydrographers Passage are likely related to stepwise sea-level falls to the full extent of the LGM. These foraminifer-based paleoenvironmental interpretations suggest that the GBR shelf edge slopes have changed their depositional environments continuously in response to sea-level fluctuations during the last glacial cycle.

Simulated fate of catchment-derived sediment on the Great Barrier Reef shelf

Numerical experiments using a 3D model of fine sediment transport in the Great Barrier Reef region indicate deposition of the bulk mass of catchment sediments from river plumes within a few tens of kilometres from river mouths. A very fine fraction of easily resuspended catchment sediment has a capacity to propagate over much greater distances reaching out into the mid-shelf and outer-shelf regions. The model suggests such particles, instrumental to the development of low density flocs in the marine environment, can play a critical role in altering optical properties of water masses over the shelf during wet years. The mid-term (4 year) impact of Great Barrier Reef catchments on the probability of suspended sediment concentration exceeding the ecologically significant trigger value of 2 mg/L is confined to inshore regions adjacent to river mouth locations.

Evidence of extensive reef development and high coral cover in nearshore environments: implications for understanding coral adaptation in turbid settings.

Mean coral cover has reportedly declined by over 15% during the last 30 years across the central Great Barrier Reef (GBR). Here, we present new data that documents widespread reef development within the more poorly studied turbid nearshore areas (<10 m depth), and show that coral cover on these reefs averages 38% (twice that reported on mid- and outer-shelf reefs). Of the surveyed seafloor area, 11% had distinct reef or coral community cover. Although the survey area represents a small subset of the nearshore zone (15.5 km2), this reef density is comparable to that measured across the wider GBR shelf (9%). We also show that cross-shelf coral cover declines with distance from the coast (R2 = 0.596). Identified coral taxa (21 genera) exhibited clear depth-stratification, corresponding closely to light attenuation and seafloor topography, with reefal development restricted to submarine antecedent bedforms. Data from this first assessment of nearshore reef occurrence and ecology measured across meaningful spatial scales suggests that these coral communities may exhibit an unexpected capacity to tolerate documented declines in water quality. Indeed, these shallow-water nearshore reefs may share many characteristics with their deep-water (>30 m) mesophotic equivalents and may have similar potential as refugia from large-scale disturbances.

Submarine landslides on the Great Barrier Reef shelf edge and upper slope: A mechanism for generating tsunamis on the north-east Australian coast?

Shallow (<200m) submarine landslides influence margin evolution and can produce devastating tsunamis, yet little is known about these processes on mixed siliciclastic–carbonate margins. We have discovered seven landslides along the shelf edge and upper slope of the central Great Barrier Reef (GBR), Australia. The largest shelf edge landslide is investigated in detail and represents a collapse of a 7km long section of the shelf edge at 90m water depth with coarse debris deposited up to 5.5km away on the upper slope down to 250m. The precise timing and triggering mechanisms are uncertain but available chronologic and seismic stratigraphic evidence suggests that this event occurred during the last deglacial sea-level rise between 20 and 14ka. Regional bathymetric data confirms that these shelf edge and upper slope slides are restricted to the central GBR between latitude 18° and 19°S, suggesting a spatial relationship between the extensive Burdekin paleo-fluvial/delta system and shallow landslide activity. This study highlights an important local mechanism for the generation of tsunamis on this margin type, and numerical simulations under present conditions confirm that a 2 to 3m tsunami wave could be produced locally. However, we consider that the risk of such slides, and their resulting tsunamis, to the modern coastline is negligible due to their relatively small size and the capacity of the GBR to dissipate the wave energy.

Mixed Carbonate–Siliciclastic Sedimentation Along the Great Barrier Reef Upper Slope: A Challenge To the Reciprocal Sedimentation Model

Abstract Results of studies involving numerous cores and ODP holes along the Great Barrier Reef (GBR) margin and adjacent Queensland Trough and Queensland Plateau have challenged the use of a reciprocal sedimentation model to describe the sedimentary response of slope and basin settings to glacioeustatic sea-level fluctuations. Upper-slope sedimentation results from the relationships between sea-level fluctuations, antecedent topography, and regional climate that play an important role in the type and amount of sediment deposited on the upper slope during glacial, deglacial, and interglacial times. During the Last Glacial Maximum (LGM, > 20 ka ago) upper-slope sediments generally lacked siliciclastic material and are characterized by very low accumulation rates, whereas early deglacial-time (Termination I, TI) deposits are dominated by a siliciclastic and neritic carbonate pulse. Siliciclastic sedimentation was significantly reduced in the Holocene, while carbonate sedimentation remains elevated. A new borehole, IODP Expedition 325 Hole M0058A (Hole 58A), recovered 82% of a 40.4 m hole on the upper slope east of Noggin Passage on the central GBR margin near Cairns, Australia. Hole 58A provides a detailed sedimentary record during Termination II (TII), Marine Isotope Stage 6/5e (MIS-6/5e), deglacial transition, and through most of interglacial MIS-5. This hole, along with two others (ODP Leg 133 Holes 820A and 819A from the upper slope east of Grafton Passage), show carbonate–siliciclastic cyclicity as the result of glacioeustatic change with the GBR shelf. Sedimentation at Hole 58A is consistent with that of previous studies along the GBR margin (focusing on the LGM to present), and extends the upper-slope sedimentary record back to TII and interglacial MIS-5. A siliciclastic pulse similar to the one during TI occurred during the penultimate deglaciation, TII; however, the maximum neritic aragonite export to the upper slope occurred not during peak MIS-5e highstand when sea level was a few meters above modern position, but subsequently during a time (MIS-5d to 5a) when lowered sea level fluctuated between 30 and 50 m below present sea level. Siliciclastic sediments were reworked and exported to the upper slope when the lowstand fluvial plain was re-flooded, whereas neritic carbonate export to the slope reached a maximum when sea level fell and much of the mid to outer shelf re-entered the photic zone, subsequent to a drowning interval. Thus, this analysis refines the mixed-sedimentation models of upper-slope sedimentation along the central GBR margin during the penultimate deglacial transgression and subsequent interglacial early and late highstand. This study provides further evidence that mixed carbonate–siliciclastic margins do not always behave in a predictable manner and that mixed margins both modern and ancient would benefit from detailed study of sediment transport in the context of sea-level rise and fall.

Phytoplankton, bacterioplankton and virioplankton structure and function across the southern Great Barrier Reef shelf

Bacterioplankton and phytoplankton dynamics, pelagic respiration, virioplankton abundance, and the diversity of pelagic diazotrophs and other bacteria were examined in relation to water-column nutrients and vertical mixing across the southern Great Barrier Reef (GBR) shelf where sharp inshore to offshore gradients in water chemistry and hydrology prevail. A principal component analysis (PCA) revealed station groups clustered geographically, suggesting across-shelf differences in plankton function and structure driven by changes in mixing intensity, sediment resuspension, and the relative contributions of terrestrial, reef and oceanic nutrients. At most stations and sampling periods, microbial abundance and activities peaked both inshore and at channels between outer shelf reefs of the Pompey Reef complex. PCA also revealed that virioplankton numbers and biomass correlated with bacterioplankton numbers and production, and that bacterial growth and respiration correlated with net primary production, suggesting close virus–bacteria–phytoplankton interactions; all plankton groups correlated with particulate C, N, and P. Strong vertical mixing facilitates tight coupling of pelagic and benthic shelf processes as, on average, 37% and 56% of N and P demands of phytoplankton are derived from benthic nutrient regeneration and resuspension. These across-shelf planktonic trends mirror those of the benthic microbial community.

Quantitative indicators of environmental sustainability risk for a tropical shelf trawl fishery

Indicators of trawl exposure were developed for 837 bycatch and benthos species, assemblages and habitats on the Great Barrier Reef shelf, by analysing their spatial distributions (mapped by a previous study) in relation to management zones, overlap with trawl grounds, and the intensity of trawl effort – and estimating the proportion of their distributions exposed to trawling. Exposure to trawl intensity as a swept-coverage was a more sensitive indicator than exposure to trawled grounds or exposure as permitted by management zones. Few habitats and assemblages were highly exposed. About 33 species had high exposure to trawl effort, whereas approximately 70% of the 837 species had low exposure. The indicators for species were extended, using relative catch rates, to estimate the proportion of populations caught annually (exploitation rate). Five species had high estimated exploitation rates and 28 were intermediate, whereas most (>800) species had low exploitation rates. The productivity potential of species to counter the incidental catch was assessed using recovery scores from life history traits. This qualitative approach indicated species at higher relative risk due to trawling. A quantitative indicator of absolute sustainability was estimated using available natural mortality rates to calculate the proportion of fishing mortality at maximum sustainable yield (FMSY). Three species exceeded a limit reference point (≅1.0×FMSY), one species exceeded a first conservative reference point (≅0.8×FMSY) and two others exceeded a second conservative reference point (≅0.6×FMSY). While few species were assessed at high risk, there were uncertainties in the distributions, relative catch rates, and natural mortality rates that required a precautionary response, including considering additional species with high indicator values. The species identified as high risk by the quantitative sustainability indicator and by the qualitative productivity scores corresponded poorly. This raises concerns about the reliability of qualitative approaches often used to conduct risk assessments for data-poor species.

Macroalgal species richness and assemblage composition of the Great Barrier Reef seabed

Understanding the drivers of broad-scale patterns of biodiversity is an overarching goal in ecology. We analysed environmental drivers of macroalgal species richness and composition on the continental shelf seabed of Australia's Great Barrier Reef (GBR), and mapped these patterns to show phycologically diverse and depauperate areas. Although shelf seabed habitats constitute similar to 61% of the GBR Marine Park area, previous floristic studies have been largely confined to intertidal and coral reef areas. Recognising the lack of knowledge of this habitat, the GBR Seabed Biodiversity Project (SBP) surveyed environmental variables and associated biodiversity across the shelf. We used SBP data for 1195 epibenthic sled sites, of which 639 sites recorded 370 macroalgal taxa, including 250 taxa not previously described in the GBR. Regression Random Forests were used to identify the environmental variables that most influence algal richness. Patterns of species composition, or assemblages, were investigated using partitioning around medoids (pam) clustering, and classification Random Forests identified the environmental variables most influential, and shapes of responses, for each assemblage. The 5 assemblages were distinguished based on taxonomy, dominant species, functional form or abundance and species richness. Overall, sediment grain size composition and light availability had the greatest influence on species richness and assemblages, with strong thresholds at 20% mud and at relative benthic irradiance of similar to 0.06 (equivalent to PAR approximate to 120 mu mol m(-2) s(-1)). This study is the first systematic analysis of the macroalgal communities of the GBR shelf seabed, providing valuable information to stimulate future research on taxonomy, productivity and ecosystem services of this habitat.

Oceanography and seabird foraging: within-season impacts of increasing sea-surface temperature on the Great Barrier Reef

Previously we have demonstrated that prey availability to seabirds of the Great Barrier Reef (GBR) decreases in direct association with within-season increases in sea-surface temperature, independent of prevailing El Nino conditions. These negative impacts occur throughout the GBR and affect multiple seabird species. Currently, the oceanic processes driving these impacts or the potential for them to occur in other marine systems are unknown. Here, we use satellite and in situ data obtained during a thermal stress event to identify the within-season links between ocean dynamics and seabird foraging success on the southern GBR. In February 2006, a major mesoscale eddy formed coastward of the East Australian Current flow, adjacent to our study site. In mid-February, strengthening of this eddy caused an intrusion of cool, dense waters at depth across the GBR shelf. This intrusion intensified vertical stratification and caused a pronounced warming of sea-surface layers. Prey availability to seabirds significantly decreased during this period and remained low until eddy intensity decreased and surface waters cooled. Prey availability increased following this episode, clearly indicating that loss of prey was associated with a short-term vertical and/or horizontal redistribution of forage-fish, or subsurface predators, rather than an overall decrease in productivity linked to seasonal-scale El Nino processes.

Do Clouds Save the Great Barrier Reef? Satellite Imagery Elucidates the Cloud-SST Relationship at the Local Scale

Evidence of global climate change and rising sea surface temperatures (SSTs) is now well documented in the scientific literature. With corals already living close to their thermal maxima, increases in SSTs are of great concern for the survival of coral reefs. Cloud feedback processes may have the potential to constrain SSTs, serving to enforce an “ocean thermostat” and promoting the survival of coral reefs. In this study, it was hypothesized that cloud cover can affect summer SSTs in the tropics. Detailed direct and lagged relationships between cloud cover and SST across the central Great Barrier Reef (GBR) shelf were investigated using data from satellite imagery and in situ temperature and light loggers during two relatively hot summers (2005 and 2006) and two relatively cool summers (2007 and 2008). Across all study summers and shelf positions, SSTs exhibited distinct drops during periods of high cloud cover, and conversely, SST increases during periods of low cloud cover, with a three-day temporal lag between a change in cloud cover and a subsequent change in SST. Cloud cover alone was responsible for up to 32.1% of the variation in SSTs three days later. The relationship was strongest in both El Niño (2005) and La Niña (2008) study summers and at the inner-shelf position in those summers. SST effects on subsequent cloud cover were weaker and more variable among study summers, with rising SSTs explaining up to 21.6% of the increase in cloud cover three days later. This work quantifies the often observed cloud cooling effect on coral reefs. It highlights the importance of incorporating local-scale processes into bleaching forecasting models, and encourages the use of remote sensing imagery to value-add to coral bleaching field studies and to more accurately predict risks to coral reefs.

Mixing of relict and modern tests of larger benthic foraminifera on the Great Barrier Reef shelf margin

The distribution of live, well preserved, and relict larger benthic foraminifera (LBF) in sediment samples collected off the shelf margin near Hydrographers Passage in the Great Barrier Reef, Australia, was studied to document taphonomic processes in this setting. In total, eight living species of LBF were found, mostly of the families Amphisteginidae and Nummulitidae, separated into a shallow (platform) assemblage and a deep (terrace) assemblage. The modern (well preserved) shells in the sediment revealed a good representation of this fauna, but differences in depth distribution were lost. The relict fauna is represented by a more diverse fauna, and is composed mostly of taxa occurring in much shallower conditions. We interpret this pattern as an initial downslope transport of the foraminifera from the shelf into deeper waters, and a mixing of foraminifera derived from nearby Pleistocene coral reef structures.

The age and the flushing time of the Great Barrier Reef waters

A numerical model of the Great Barrier Reef (GBR) was verified using water current data from twenty sites, and applied to estimate the flushing time and age of waters. These timescales were calculated under different wind and oceanic inflow conditions. The age of oceanic waters intruding in the GBR was estimated to be between 3 and 5 months on leaving the GBR, depending on the location; the largest residence time prevailed in the southern GBR matrix where the age was the highest within the high density reef matrix, indicating veering of the mean currents around the GBR matrix. The flushing time depends on the size of the domain, and was estimated to be 67 days for the whole central GBR. For the flushing of coastal waters the wind had two effects. Firstly, it increased the flushing times by generating wind-driven currents, transporting some water back to the source; this process is comparable to that of an estuary, where water that leaves the estuary at ebb tide may return at flood tide, a process parameterized by the return coefficient. The return coefficient in the GBR due to wind reversals may be as large as 50% at the timescale of the wind. Secondly, in the southern and central regions of the GBR, the southeasterly tradewind deflected the southward flowing oceanic inflow seaward, away from the inner shelf and towards the outer shelf, making room for a wind-driven current of opposite direction on the inner shelf. Thus the intrusion of oceanic water in the GBR depends on the wind over the GBR. The veering of the mean currents around the GBR reef matrix and the wind over the shelf influencing the oceanic circulation demonstrate small scales (the GBR shelf) influencing the large scale oceanic circulation.

Suspended sediment grain size and mineralogy across the continental shelf of the Great Barrier Reef: Impacts on the physiology of a coral reef sponge

Declining water quality associated with increased suspended sediments has been closely linked to the reduced health status of benthic marine ecosystems and their associated organisms, including marine sponges. The mechanisms driving the impacts of elevated suspended sediments on marine sponges are poorly investigated. This study elucidates spatial and temporal variations in sediment size and mineralogy of inorganic suspended sediments within sponge habitats (reef environments) across the continental shelf of the central Great Barrier Reef (GBR), North-eastern Australia. Inshore sponge habitats were dominated (>80%) by fine-grained suspended sediments consisting of both terrigenous (clay and quartz) and biogenic material (carbonates) with grain sizes <100μm. In contrast, mid- and outer-shelf sponge habitats were dominated by carbonate material with grain sizes >100μm. The abundance and distribution of the common coral reef sponge Rhopaloeides odorabile across the GBR shelf shows a clear correlation between habitat and the distinct patterns in suspended sediment size and mineralogy. Experimental exposure of R. odorabile to clay and carbonate sediments in this study provides the first evidence that the metabolic demand (respiration) of coral reef sponges increases (up to 40%) in response to fine terrigenous (clay) sediments. This physiological response supports sediment load, size, and mineralogy as key factors affecting the distribution and abundance patterns of R. odorabile across the continental shelf of the central GBR.