Tree Reef Research Articles

The impact of elevated nutrients on the Holocene evolution of the Great Barrier Reef

The initiation of the Holocene Great Barrier Reef coincided with rapid environmental change as sea level rose and inundated the shelf. Core data from One Tree Reef (southern Great Barrier Reef) shows coral growth started by ∼8.2 ka, but accretion between 8 and 7 ka was slower, occurred in deeper water, and comprised more sediment-tolerant coral communities compared to growth following sea-level stabilization. It has been postulated that environmental stressors (e.g. increased turbidity and nutrients) suppressed and delayed reef growth, however direct data supporting this hypothesis are scarce. Here we combine the isotopic composition of skeletal bound organic nitrogen (δ15N) and Ba/Ca ratios of coral skeletons with published geochemical proxies of terrestrial sediment discharge to constrain Holocene water conditions at One Tree Reef. Between 8 and 7 ka the skeletal δ15N values from multiple corals and genera were elevated (average of 8.45 ± 0.89‰) relative to the early transgression and following sea-level stabilization (average of 7.04 ± 0.82‰). We propose that elevated δ15N in corals reflects the discharge of deep terrestrial soil nitrogen resulting from high runoff. This is supported by Ba/Ca measurements and published rare earth element and yttrium (REE + Y) geochemical proxies in coral and reefal microbialites from the same cores. These data suggest that increased terrigenous discharge of sediment and nutrients did not inhibit reef growth, rather led to the establishment of slower-growing, deeper and more sediment-tolerant coral communities. Understanding the capacity for reef growth under adverse environmental conditions provides insight into thresholds and resilience of the GBR over centennial-millennial timescales.

Movement dynamics, sediment turnover and sheltering behaviours of the nocturnal coral reef sea cucumber, Stichopus cf. monotuberculatus

Animal movement and feeding studies shed light on ecological roles and can inform management strategies for fished species. However, the feeding and movement of nocturnal sea cucumbers have been rarely studied. We determined the movement dynamics, feeding ecology and sheltering behaviour of the nocturnal Stichopus cf. monotuberculatus (dragonfish) at One Tree Reef and Heron Island Reef on the southern Great Barrier Reef. The short-term movement of individual sea cucumbers were tracked, and sediment defecation rates were measured. Displacement rates averaged 3–33 cm h−1 across the four sites, with the sea cucumbers moving more slowly approaching sunrise and within patches of organically rich sediment. The movement paths were moderately tortuous, with larger sea cucumbers moving along straighter paths. Rates of sediment turnover averaged 7.8 g h−1 and were greatest for larger individuals. The faecal casts were organically richer and composed of finer grains than the ambient surface sediments, implying that S. cf. monotuberculatus avoids consuming coarse sediments and feeds on nutritionally rich particles. The sea cucumbers sheltered under hard reef substrata and moved non-directionally on the seascape, with a few individuals homing back to their original refuges. The affinity to reef substrata and short nightly net displacements of S. cf. monotuberculatus have implications for the spatial management of this and other similar species. Larger individuals occupied deeper refuges, implying that projected declines in substratum rugosity on coral reefs due to climate change could impact the size structure of sea cucumber populations, with implications for fisheries.

Who lives in a pear tree under the sea? A first look at tree reefs as a complex natural biodegradable structure to enhance biodiversity in marine systems

Hard substrates play an important role in global marine systems as settlement surface for sessile reef-forming species such as corals, seaweeds, and shellfish. In soft-sediment systems, natural hard substrates such as stones, bedrock and driftwood are essential as they support diverse assemblages of reef-associated species. However, availability of these hard substrates has been declining in many estuaries and shallow seas worldwide due to human impacts. This is also the case in the Dutch Wadden Sea, where natural hard substrates have gradually disappeared due to burial by sand and/or active removal by humans. In addition, driftwood that was historically imported from rivers has been nullified by upstream logging and coastal damming of estuaries. To investigate the historic ecological role of wood presence in the Wadden Sea as settlement substrate and fish habitat, we constructed three meter high artificial reefs made of felled pear trees. Results demonstrate that these reefs rapidly developed into hotspots of biodiversity. Within six months, the tree-reefs were colonized by sessile hard substrate associated species, with a clear vertical zonation of the settled species. Macroalgae and barnacles were more abundant on the lower parts of the reef, while bryozoans were more dominant on the upper branches. In addition, six fish species were observed on the reefs, while only two species were caught on sandy control sites. Moreover, the abundance of fish on the reefs was five times higher. Individuals of the most commonly caught species, the five-bearded rockling Ciliata mustela, were also larger on the reef. These patterns also hold true for common prawn, Palaemon serratus, which were also larger and ten times more numerous on the reefs. Present findings indicate that the reintroduction of tree-reefs as biodegradable, structurally complex hard substrates can increase local marine biodiversity in soft-sediment systems within relatively short time scales.

Quaternary coral, coralline algal and vermetid assemblages as sea-level indicators: a review

Corals, coralline algae and vermetid gastropods are indirect (marine limiting) relative sea-level (RSL) indicators. The precision in sea-level reconstruction based on fossils of those organisms depends on the probable palaeodepth in which they grew. Constraining such palaeodepth depends, in turn, on the available information about the habitats of their living counterparts. Diverse genera, species and species assemblages of corals, coralline algae and vermetid gastropods have historically been proposed as reliable indicators of narrow, shallow depth ranges. However, the increased information on depth distribution of marine benthos in the last two decades has challenged some early assumptions about depth ranges of taxa considered diagnostic of precise palaeodepths. Here, the authors test the reliability of coral, coralline algal and vermetid assemblages that have been extensively used in RSL reconstructions in the light of data from Ocean Biogeographical Information System (OBIS) and other recently published data. In the Indo-Pacific province, these data support the use of the robust-branching and the shallow, high-energy encrusting coral assemblages with a 0 to 10 m uncertainty. In both cases many component species have unimodal distributions and both median and average water depths are shallower than 10 m. The reliability of these coral assemblages as indicative of shallow water depths is strengthened when corals are encrusted by thick plants of the coralline alga Porolithon gr. onkodes. According to OBIS data, coralline algae of this species group in the Indo-Pacific are restricted to very shallow waters (95% probability of occurrence shallower than 0.2 m and in 99.6% of records shallower than 6 m). However, such a narrow depth range and the overall scarce data on coralline algal species in the OBIS database are questionable due to difficulties of coralline algal species identification with the naked eye. A comprehensive survey of the modern distribution of coralline algae at One Tree Reef (southern Great Barrier Reef) indicates that P. gr. onkodes has a log-normal distribution with median depth of less than 5 m and 95% of occurrence probability of thick crusts (> 0.2 mm) shallower than 8.8 m. Data on modern distribution of vermetids are scarce. In the OBIS database, vermetid species are reported from relatively wide depth ranges. However, relatively high densities (> 10 individuals per m2) on coral and coralline algal surfaces only occur from above mean low tide to some 6 m depth. In the Western Atlantic-Caribbean province Acropora palmata is the most precise RSL marker and no additional components of fossil assemblages improve its palaeodepth information. The confident use of coralgal and vermetid assemblages as RSL indicators relies on the identification of fossil corals and coralline algae at the species or species-group level. The scarcity of available data highlights the need for further studies on distribution of coralline algal species and vermetid in modern coral reefs from a variety of oceans and reef settings.

One Tree Reef Foraminifera: a relic of the pre-colonial Great Barrier Reef

Abstract Benthic Foraminifera are important elements of coral reef ecosystems and valuable proxies for monitoring reef health in the face of local, regional and global stressors. Here Foraminifera preserved in a sediment core from the One Tree Reef (OTR) lagoon on the southern Great Barrier Reef (GBR) are used to quantify ecological changes associated with European colonization (early 1800s) and the subsequent industrialization of Australia (1950s onward). Core samples are compared to published benthic grab samples from three reefs to compare spatial and temporal variation. Standard measures of richness, diversity and foraminiferal assemblage composition indicate significant differences between the Recent assemblages of OTR lagoon and those from Heron and Wistari reef lagoons. However, there is no discernible change in the OTR lagoon core assemblages from the last four centuries in any of the richness or diversity indices examined. While benthic grabs likely undersample infaunal Foraminifera relative to core samples, these foraminiferal assemblages indicate the OTR lagoon contains a living example of a pre-colonial GBR lagoon ecosystem. Without cores from Heron and Wistari it is unknown if their foraminiferal assemblages have changed due to recent anthropogenic activity or if they have always been distinct.

Field measurements show rough fore reefs with spurs and grooves can dissipate more wave energy than the reef crest

Coral reefs are widely recognized as effective dissipaters of wave energy. Spurs and grooves (SAG) are common features of fore reefs worldwide and are thought to be particularly efficient at dissipating wave energy. However, very few studies have collected in-situ hydrodynamic data to verify this and understand SAG interactions with hydrodynamic forces. We present in-situ wave data from contrasting SAG sites at Moorea, French Polynesia, and One Tree Reef in the southern Great Barrier Reef, Australia. We measured high rates of wave energy dissipation (up to 0.1 kW/m2) across the SAG zone. Interestingly, under the modal conditions measured, the SAG zones often dissipated wave energy at higher rates than the adjacent reef crest/flat zone. Rates of dissipation were the greatest at sites with high live coral cover in mesotidal environments (i.e., One Tree Reef sites), suggesting the structural complexity of live corals may increase bed friction and that tidal currents may also contribute to dissipation. Correlations between measured dissipation, wave height and depth allowed us to suggest that the SAG zone dissipates more energy under high wave conditions at low tides, while the reef crest/flat dissipates more energy at high tides under small wave conditions. Further study is required to better understand and model the hydrodynamics of SAG zones and the important role they play in reef dynamics and coastal protection.

Holocene microbialite geochemistry records > 6000 years of secular influence of terrigenous flux on water quality for the southern Great Barrier Reef

Anthropocene climate change and water quality degradation represent unprecedented challenges to modern coral reefs. Although declining reef health after European colonization is well documented around the world and increased terrigenous sediment flux is known to have terminated deglacial reefs in the Great Barrier Reef (GBR), longer-term patterns of water quality are poorly understood. Here we present the first direct proxy-based Holocene water quality reconstruction for any reef. The unique geochronological framework provided by cores from Heron and One Tree reefs (offshore, southern GBR) allowed reconstruction of offshore water quality from 8200 to 1800 years before present (BP) using centennially resolved microbialite-based geochemical proxies. Terrigenous sediment-sourced trace elements were measured in microbialites from a well dated succession of reef rock and in paleosol (ancient soil) formed at the Pleistocene-Holocene unconformity. Microbialite-hosted rare earth element and yttrium distributions (e.g., Nd/YbSN = 0.36; Y/Ho = 57) are consistent with precipitation from shallow oxygenated seawater but show a non-linear trend through the Holocene with distinct intervals of higher and lower terrigenous influence relative to average values. Immediately following reef initiation (>8300 years ago) our data suggest increasing terrigenous influence by 8000 ka. Surrounding reef seawater became less affected by terrigenous runoff from ~7000 years ago, but showed marked mid-Holocene variability related to regional climatic factors. Major fluctuations between intervals of high and low relative terrigenous influence correlate well with particular regional and more global climate records. These include local relative sea level fluctuations, fluctuations in Indian-Australian Summer Monsoon (IASM) strength, and dampened El Niño Southern Oscillation (ENSO) frequency corresponding to greater terrigenous influence in the southern GBR at ~7.0, 5.4, and 2.7 ka BP. Water quality then improved significantly after 3200 years BP. More broadly, it is well established that water quality has a major effect on reefs and reef communities, but for past reef history, inferences about water quality are commonly highly speculative. Reefal microbialite geochemistry provides an independent, high-quality proxy for ambient water quality that can be used to directly compare contemporaneous reef growth dynamics and ecological shifts to changing water quality. The high concentrations of trace elements in reefal microbialites, relative to other marine carbonates, provide a very robust, if time averaged, proxy for investigating ancient seawater chemistry, even in offshore reefs, such as Heron Reef. At the same time, the proxy provides a new independent data set for that may aid interpretation and model of climate change relevant to reef growth at centennial to millennial scales. As reefal microbialites are common in many global reef systems, where associated with high quality dating, they may provide useful proxies for investigating secular changes in water quality and associated climatic drivers at various temporal scales in other regions of the world.

Boring bivalve traces in modern reef and deeper-water macroid and rhodolith beds

Macroids and rhodoliths, made by encrusting acervulinid foraminifera and coralline algae, are widely recognized as bioengineers providing relatively stable microhabitats and increasing biodiversity for other species. Macroid and rhodolith beds occur in different depositional settings at various localities and bathymetries worldwide. Six case studies of macroid/rhodolith beds from 0 to 117 m water depth in the Pacific Ocean (northern Central Ryukyu Islands, French Polynesia), eastern Australia (Fraser Island, One Tree Reef, Lizard Island), and the Mediterranean Sea (southeastern Spain) show that nodules in the beds are perforated by small-sized boring bivalve traces (Gastrochaenolites). On average, boring bivalve shells (gastrochaenids and mytilids) are more slender and smaller than those living inside shallow-water rocky substrates. In the Pacific, Gastrochaena cuneiformis, Gastrochaena sp., Leiosolenus malaccanus, L. mucronatus, L. spp., and Lithophaga/Leiosolenus sp., for the first time identified below 20 m water depth, occur as juvenile forms along with rare small-sized adults. In deep-water macroids and rhodoliths the boring bivalves are larger than the shallower counterparts in which growth of juveniles is probably restrained by higher overturn rates of host nodules. In general, most boring bivalves are juveniles that grew faster than the acervulinid foraminiferal and coralline red algal hosts and rarely reached the adult stage. As a consequence of phenotypic plasticity, small-sized adults with slow growth rates coexist with juveniles. Below wave base macroids and rhodoliths had the highest amounts of bioerosion, mainly produced by sponges and polychaete worms. These modern observations provide bases for paleobiological inferences in fossil occurrences.

Mechanisms of spur and groove development and implications for reef platform evolution

Contemporary understanding of Holocene coral reef development is based primarily on sub-surface investigations of reef flat, back reef and lagoon zones. Few studies of Holocene fore reef development exist, constituting a significant gap in our understanding of reef evolution. The spur and groove (SaG) zone is a distinct, understudied, feature of fore reefs worldwide. We review SaG development from previous studies and present 29 new SaG short cores with 52 14C and U–Th ages from six fore reef regions of Heron and One Tree reefs, the first such data from the Great Barrier Reef. Remarkably, we found that SaGs do not necessarily accrete in the same direction as their adjacent reef flat. We identified three modes of reef flat and SaG lateral accretion: Mode 1 – lagoonward accretion of both the reef flat and SaGs; Mode 2 –lagoonward accretion of the reef flat but seaward accretion of the SaGs; Mode 3 - seaward accretion of both the reef flat and SaGs. Most SaG zones (five of the six studied) accreted in a seaward direction (Modes 2 or 3). Hydrodynamic conditions and local topography appear to be the dominant factors determining which mode occurs. Episodic high-energy events are also likely to play an important role in SaG formation. Our findings suggest that traditionally held models of reef evolution whereby lagoonal, mature reefs fill, developing into senile platform reefs, may not hold. Rather, reef flats may continue to expand seaward on their leeward, and semi-exposed fronts to increase in size while maintaining their lagoons.

A new model of Holocene reef initiation and growth in response to sea-level rise on the Southern Great Barrier Reef

The fossil record provides valuable data for improving our understanding of both past and future reef resilience and vulnerability to environmental change. The spatial and temporal pattern of the initiation of the Holocene Great Barrier Reef presents a case study of reef response to rapid sea-level rise. Past studies have been limited by the lack of well-dated and closely spaced reef core transects and have not closely examined the composition of the reef-building communities through time. This study presents 80 new high precision UTh and 5 radiocarbon ages from twelve new cores located along three transects across different geomorphic and hydrodynamic settings of One Tree Reef, southern Great Barrier Reef, to document three distinct stages of Holocene reef development in unprecedented detail. Temporal constraints on changing paleoecological assemblages of coral, coralline algae and associated biota revealed three distinct phases of reef development, consisting of: 1)a fast, shallow and clear-water reef initiation from 8.2 until 8 ka; 2)a shift to slower, deeper and more turbid-water reef growth from 8 to 7 ka; and 3)a return to shallow and rapid branching coral growth in clear-water conditions as the reef “catches up” to sea-level. A minimum lag prior to reef initiation of 700 years was identified, which differs in length depending on reef environment and Pleistocene substrate height. In this new model, reef growth initiated on the topographically lower leeward margin and patch reef, prior to the start of windward margin development, contrary to the traditional reef growth model. While there was a shift to conditions less favorable for reef growth at 8 ka, this did not prevent the slow accretion of more sediment-tolerant coral communities. The majority of the reef reached sea level by ~6 ka. This new conceptual model of Holocene reef growth provides new constraints on changes in paleoenvironment that controlled reef community composition and growth trajectories through sea-level rise following inundation.

Reef Sedimentary Accretion Model (ReefSAM): Understanding coral reef evolution on Holocene time scales using 3D stratigraphic forward modelling

A detailed understanding of the development of coral reefs and their internal structure has important applications for predicting future reef trajectories in light of climate changes, petroleum exploration and reconstructing past environmental changes such as sea level. Numerical computer modelling provides a quantitative means of testing and understanding reef processes and better predicting past and future reef development. However, most existing carbonate stratigraphic forward models (SFMs) focus on the platform scale and several limitations in their simulation of important reef processes and reef-scale features (e.g. patch reefs, sand aprons) have been previously identified. We present the Reef Sedimentary Accretion Model (ReefSAM) – a new carbonate SFM designed to test new approaches to modelling processes important at the reef scale, with the goal of better simulating reef features and evolution at higher temporal and spatial resolutions. Specific developments include a sediment transport model which calculates deposition based on a range of grain sizes, additional controls on coralgal growth including wave energy and surface substrate specificity, and model outputs showing categorized paleo-conditions (composition, paleo-depth and paleo-energy) along with numerous quantitative measures for objective comparison of model results to real-world observational data, which is often more abundant for modern/Holocene reefs than buried ancient reefs. Simulations of the well-studied Holocene One Tree Reef (OTR) in the southern Great Barrier Reef (GBR) showed improvements upon previous modelling attempts in both quantitative measures (e.g. lagoon size or reef sea level catch-up time), and the ability to produce more realistic reef-scale morphological features (e.g. lagoonal patch-reefs, mostly sand-filled lagoon) and reef development characteristics (e.g. transition from mostly vertical aggradation during sea level catch-up to horizontal progradation afterwards). Controls on reef maturity/lagoonal filling identified in previous modelling experiments were corroborated. Sensitivity testing also revealed a very high sensitivity to reef drowning. These results indicate the new modelling approaches are viable and provide a way forward for future Holocene reef evolution model development.

Patterns of Sediment Transport Using Foraminifera Tracers across Sand Aprons on the Great Barrier Reef

ABSTRACT Fellowes, T.E.; Gacutan, J.; Harris, D.L.; Vila-Concejo, A.; Webster, J.M., and Byrne, M., 2017. Patterns of sediment transport using foraminifera tracers across sand aprons on the Great Barrier Reef. Sediment dynamics exert large control over coral reef geomorphological evolution and are vital to understanding past and present geomorphic responses. Large benthic foraminifera (LBF) live in the algal reef flats, and their tests (shells) are transported post-mortem by waves and currents onto back-reef environments, including sand aprons. This study investigated the patterns of transport linking surficial and downcore sediments in samples from three sand aprons with different wave exposures at One Tree Reef on the southern Great Barrier Reef (Australia). Six LBF genera represented up to 32% of the sediments analysed. Lagoonward transport increased LBF test abrasion and sediment bulk density. Sediment grain size and LBF abundance in sediments also decreased with lagoonward transport. Sediment transpo...

Biology and ecology of the vulnerable holothuroid, Stichopus herrmanni, on a high-latitude coral reef on the Great Barrier Reef

Tropical aspidochirotid holothuroids are among the largest coral reef invertebrates, but gaps remain in our understanding of their ecological roles in lagoon sediment habitats, a vast component of coral-reef ecosystems. Stichopus herrmanni, listed as vulnerable (IUCN), is currently a major fishery species on the Great Barrier Reef (GBR) and throughout the Indo-Pacific. It is critical to characterise how this species interacts with its environment to understand how its removal may impact ecosystem functionality. We investigated seasonal variation in movement, bioturbation, feeding and gonad development of S. herrmanni over 3 yr at One Tree Reef, which has been a no-take area for decades. We determined the direct influence of the deposit-feeding activity of S. herrmanni on sediment turnover and granulometry, and on the abundance of infauna and benthic productivity in a comprehensive in situ analysis of tropical holothuroid feeding ecology. This species is highly mobile with identifiable individuals exhibiting site fidelity over 3 yr. With the potential to turn over an estimated 64–250 kg individual−1 yr−1, S. herrmanni is a major bioturbator. Stichopus herrmanni is a generalist feeder and influences trophic interactions by altering the abundance of infauna and microalgae. Stichopus herrmanni exhibited decreased feeding activity and gonad development in winter, the first documentation of a seasonal disparity in the bioturbation activity of a tropical holothuroid. Sediment digestion and dissolution by S. herrmanni has the potential to influence seawater chemistry, a particularly important feature in a changing ocean. Our results provide essential baseline data on the functional roles of this ecologically important species to inform development of ecosystem-based beche-de-mer fisheries management on the GBR.

PASSIVE DEFENSIVE TRAITS ARE NOT GOOD PREDICTORS OF PREDATION FOR INFAUNAL REEF BIVALVES

Predation is a key biotic interaction that shapes ecological communities. Defensive morphological traits are thought to improve prey survivorship, and knowing which traits aid survivorship is critical to understanding predator-prey dynamics and their impact on community composition. Paleontological studies that use optimal foraging theory and single species experiments suggest that shell size and thickness are traits that affect the outcome of mollusk drilling predation. We analyzed the effectiveness of these morphological traits using a tropical molluscan assemblage from the One Tree Reef lagoon (southern Great Barrier Reef, Australia). Shell mass, shape, thickness, and predation frequency were measured in five bivalve species (n = 804) constituting 80% of the predators' diet. Multiple regression determined that shell mass, shell thickness, and the interaction between the two were important predictors of predation. Drilling was more likely in thinner and larger individuals. However, support for the model that included species identity was markedly stronger, indicating that the morphological traits measured are not general predictors of predation across the five prey species we studied. That is, trait-only models are inadequate for explaining predator-prey dynamics at the community level. Alternatively, active behavioral defenses related to infaunality (like burrowing) appear to be stronger drivers of morphology in this system. Care should be taken when extrapolating results from experiments to communities or large temporal scales, as species with similar traits may not necessarily elicit the same response from predators.

Successive phases of Holocene reef flat development: Evidence from the mid- to outer Great Barrier Reef

A re-examination of 46 recently published U/Th reef flat ages from Heron and One Tree reefs in the southern Great Barrier Reef (GBR) identified several distinct Holocene reef growth phases with a clear 2.3-kyr hiatus in lateral reef accretion from 3.9ka to 1.5ka. An analysis of all available published radiocarbon reef flat ages (165) from 27 other mid-outer platform reefs revealed a similar hiatus between 3.6ka and 1.6ka for the northern, south-central and southern GBR. However, no hiatus in reef flat growth was observed in reefs from the central GBR with ages ranging from 7.6ka to 0.9ka. Increased upwelling, turbidity and cyclone activity in response to increased sea-surface temperature (SST's), precipitation and El-Nino Southern Oscillation variability have been ruled out as possible mechanisms of reef turn-off for the mid-outer platform reefs. Rather, a fall (~0.5m) in relative sea level at 4–3.5ka is the most likely explanation for why reefs in the northern and southern regions turned off during this time. Greater hydro-isostatic adjustment of the central GBR and long term subsidence of the Halifax-Basin may have provided greater vertical accommodation for the mid-outer reefs of the central GBR, thus allowing these reefs to continue to accrete vertically despite a fall in sea level~4–3.5ka. Further evidence for greater subsidence in this region includes the lack of senile reefs and dominance of incipient and juvenile reefs in the central GBR. This suggests that these reefs approached sea level considerably later than the northern and southern reefs, consistent with their deeper antecedent substrates. Thus, these results not only provide important information about possible reef flat demise in response to natural environmental factors, but also provide insights into regional subsidence that affected relative sea level along the east Australian margin during the Holocene.

Spatial and temporal variation in reef-scale carbonate storage of large benthic foraminifera: a case study on One Tree Reef

Large benthic foraminifera (LBFs) are a vital component of coral reef carbonate production, often overlooked due to their small size. These super-abundant calcifiers are crucial to reef calcification by generation of lagoon and beach sands. Reef-scale carbonate production by LBFs is not well understood, and seasonal fluctuations in this important process are largely unquantified. The biomass of five LBF species in their algal flat habitat was quantified in the austral winter (July 2013), spring (October 2013), and summer (February 2014) at One Tree Reef. WorldView-2 satellite images were used to characterize and create LBF habitat maps based on ground-referenced photographs of algal cover. Habitat maps and LBF biomass measurements were combined to estimate carbonate storage across the entire reef flat. Total carbonate storage of LBFs on the reef flat ranged from 270 tonnes (winter) to 380 tonnes (summer). Satellite images indicate that the habitat area used by LBFs ranged from 0.6 (winter) to 0.71 km2 (spring) of a total possible area of 0.96 km2. LBF biomass was highest in the winter when algal habitat area was lowest, but total carbonate storage was the highest in the summer, when algal habitat area was intermediate. Our data suggest that biomass measurements alone do not capture total abundance of LBF populations (carbonate storage), as the area of available habitat is variable. These results suggest LBF carbonate production studies that measure biomass in discrete locations and single time points fail to capture accurate reef-scale production by not incorporating estimates of the associated algal habitat. Reef-scale measurements in this study can be incorporated into carbonate production models to determine the role of LBFs in sedimentary landforms (lagoons, beaches, etc.). Based on previous models of entire reef metabolism, our estimates indicate that LBFs contribute approximately 3.9–5.4% of reef carbonate budgets, a previously underappreciated carbon sink.

Dead shell assemblages faithfully record living molluscan assemblages at One Tree Reef

Reef-associated sediments accumulate over time recording the history of biological communities. The agreement between live and dead assemblages has been extensively studied, because discrepancies between the two can reveal taphonomic bias, anthropogenic impact, and/or a decrease in temporal resolution of dead assemblages to centennial scales (i.e., time averaging). Given the potential of sedimentary assemblages to provide temporal perspectives on the history of modern communities, assessments of live-dead agreement in reef mollusk assemblage composition are necessary and timely for understanding rapid environmental change. One Tree Reef (OTR) (southern Great Barrier Reef, Australia) has had very little direct anthropogenic influence over the past four decades, making it a good system for focusing on taphonomic patterns. Live (n=1335) and dead (n=6919) molluscan assemblages were collected from three shallow (6m) carbonate soft-sediment lagoons. Diversity and evenness metrics indicated no significant difference between the live and dead assemblages, and dead assemblage rank order abundance explains 82% of that of the live assemblage. Differences in composition were largely due to sample size, the low probability of sampling rare species and to a lesser degree differences in skeletal durability. Taphonomic factors were responsible for less than 20% of the differences in species composition between live and dead shell assemblages. The live molluscan assemblage sampled was similar to the live assemblage sampled 30years ago. Given that time averaging is approximately 19years in the OTR lagoon, we conclude that the composition of the living assemblage has remained largely unchanged for the last 30years. These findings indicate that dead assemblages preserved in shallow, fully carbonate environments primarily reflect the composition of the original source communities, making them useful to identify recent changes in coral reef areas where anthropogenic impacts are present.

Influence of hydrodynamic energy on Holocene reef flat accretion, Great Barrier Reef

The response of platform reefs to sea-level stabilization over the past 6ka is well established for the Great Barrier Reef (GBR), with reefs typically accreting laterally from windward to leeward. However, these observations are based on few cores spread across reef zones and may not accurately reflect a reef's true accretional response to the Holocene stillstand. We present a new record of reef accretion based on 49 U/Th ages from Heron and One Tree reefs in conjunction with re-analyzed data from 14 reefs across the GBR. We demonstrate that hydrodynamic energy is the main driver of accretional direction; exposed reefs accreted primarily lagoon-ward while protected reefs accreted seawards, contrary to the traditional growth model in the GBR. Lateral accretion rates varied from 86.3m/ka–42.4m/ka on the exposed One Tree windward reef and 68.35m/ka–15.7m/ka on the protected leeward Heron reef, suggesting that wind/wave energy is not a dominant control on lateral accretion rates. This represents the most comprehensive statement of lateral accretion direction and rates from the mid-outer platform reefs of the GBR, confirming great variability in reef flat growth both within and between reef margins over the last 6ka, and highlighting the need for closely-spaced transects.

ENCOUNTER FREQUENCY DOES NOT PREDICT PREDATION FREQUENCY IN TROPICAL DEAD-SHELL ASSEMBLAGES

Abstract Predation is frequently suggested to be a key biotic process that can shape ecological communities and drive coevolution. The premise behind these hypotheses is that predators select prey to ensure maximum gain per unit effort; for example, by selecting species that are more abundant or accessible. In this study, we tested for predator selectivity in a tropical molluscan assemblage by quantifying the influence of relative abundance (encounter frequency) on predation frequencies. We collected macromollusks (> 4 mm) from 15 sites in three soft-sediment reef lagoons at One Tree Reef (southern Great Barrier Reef, Australia). Dead mollusks were counted and identified to species level (61 species, n = 8131), and species predation frequencies were calculated as the proportion of shells with drill holes. We found that in this infauna-dominated community, levels of drilling predation were low (7.14% on average), and there was no evidence that predators selected prey based on encounter frequency. This result was consistent across prey species and lagoons. Thus, drilling predators did not specialize on more accessible prey species and were not a major cause of mortality in this modern macromollusk assemblage. Since drilling gastropods are size selective, lack of selectivity in our samples only applies to the prey size range considered. Detailed studies of prey morphological traits, as well as accounting for predator non-consumptive effects could shed light on the preferences and relevance of drilling gastropods in this soft-sediment carbonate reef assemblage.

Late Holocene sea-level fall and turn-off of reef flat carbonate production: Rethinking bucket fill and coral reef growth models

Relative sea-level rise has been a major factor driving the evolution of reef systems during the Holocene. Most models of reef evolution suggest that reefs preferentially grow vertically during rising sea level then laterally from windward to leeward, once the reef flat reaches sea level. Continuous lagoonal sedimentation (bucket fill) and sand apron progradation eventually lead to reef systems with totally filled lagoons. Lagoonal infilling of One Tree Reef (southern Great Barrier Reef) through sand apron accretion was examined in the context of late Holocene relative sea-level change. This analysis was conducted using sedimentological and digital terrain data supported by 50 radiocarbon ages from fossil microatolls, buried patch reefs, foraminifera and shells in sediment cores, and recalibrated previously published radiocarbon ages. This data set challenges the conceptual model of geologically continuous sediment infill during the Holocene through sand apron accretion. Rapid sand apron accretion occurred between 6000 and 3000 calibrated yr before present B.P. (cal. yr B.P.); followed by only small amounts of sedimentation between 3000 cal. yr B.P. and present, with no significant sand apron accretion in the past 2 k.y. This hiatus in sediment infill coincides with a sea-level fall of ~1-1.3 m during the late Holocene (ca. 2000 cal. yr B.P.), which would have caused the turn-off of highly productive live coral growth on the reef flats currently dominated by less productive rubble and algal flats, resulting in a reduced sediment input to backreef environments and the cessation in sand apron accretion. Given that relative sea-level variations of ~1 m were common throughout the Holocene, we suggest that this mode of sand apron development and carbonate production is applicable to most reef systems.