The role of the reef flat in coral reef trophodynamics: Past, present, and future

Quantum of fear: Herbivore grazing rates not affected by reef shark presence

Grazing by nominally herbivorous fishes is widely recognised as a critical ecosystem function on coral reefs. However, several studies have suggested that herbivory is reduced in the presence of predators, especially sharks. Nevertheless, the effects of shark presence on grazing, under natural settings, remains poorly resolved. Using ∼200 h of video footage, we quantify the extent of direct disturbance by reef sharks on grazing fishes. Contrary to expectations, grazing rate was not significantly suppressed due to sharks, with fishes resuming feeding in as little as 4 s after sharks passed. Based on our observations, we estimate that an average m2 area of reef at our study locations would be subjected to ∼5 s of acute shark disturbance during daylight hours. It appears the short-term impact of reef shark presence has a negligible effect on herbivore grazing rates, with the variable nature of grazing under natural conditions overwhelming any fear effects.

Feeding kinematics of a surgeonfish reveal novel functions and relationships to reef substrata

Biting to obtain attached benthic prey characterizes a large number of fish species on coral reefs, and is a feeding mode that contributes to important ecosystem functions. We use high-speed video to reveal the mechanisms used by a surgeonfish, Acanthurus leucosternon, to detach algae. After gripping algae in its jaws, the species pulls it by ventrally rotating both the head and the closed jaws, in a novel use of the intra-mandibular joint. These motions remain in the plane of the fish, reducing the use of a lateral head flick to detach the algae. The novel ability to bite and pull algae off the substrate without bending the body laterally minimizes exposure to high water flows, and may be an adaptation to feeding in challenging reef habitats such as the crest and flat. Therefore, our results could potentially represent a key milestone in the evolutionary history of coral reef trophodynamics.

Bio-physical determinants of sediment accumulation on an offshore coral reef: A snapshot study

Sediments are found on all coral reefs around the globe. However, the amount of sediment in different reservoirs, and the rates at which sediments move between reservoirs, can shape the biological functioning of coral reefs. Unfortunately, relatively few studies have examined reef sediment dynamics, and associated bio-physical drivers, simultaneously over matching spatial and temporal scales. This has led to a partial understanding of how sediments and living reef systems are connected, especially on clear-water offshore reefs. To address this problem, four sediment reservoirs/sedimentary processes and three bio-physical drivers were quantified across seven different reef habitats/depths at Lizard Island, an exposed mid-shelf reef on the Great Barrier Reef. Even in this clear-water reef location a substantial load of suspended sediment passed over the reef; a load theoretically capable of replacing the entire standing stock of on-reef turf sediments in just 8 h. However, quantification of actual sediment deposition suggested that just 2 % of this passing sediment settled on the reef. The data also revealed marked spatial incongruence in sediment deposition (sediment trap data) and accumulation (TurfPod data) across the reef profile, with the flat and back reef emerging as key areas of both deposition and accumulation. By contrast, the shallow windward reef crest was an area of deposition but had a limited capacity for sediment accumulation. These cross-reef patterns related to wave energy and reef geomorphology, with low sediment accumulation on the ecologically important reef crest aligning with substantial wave energy. These findings reveal a disconnect between patterns of sediment deposition and accumulation on the benthos, with the 'post-settlement' fate of sediments dependent on local hydrodynamic conditions. From an ecological perspective, the data suggests key contextual constraints (wave energy and reef geomorphology) may predispose some reefs or reef areas to high-load turf sediment regimes.

Water column contributions to coral reef productivity: overcoming challenges of context dependence

Coral reefs are declining at an unprecedented rate. Effective management and conservation initiatives necessitate improved understanding of the drivers of production because the high rates found in these ecosystems are the foundation of the many services they provide. The water column is the nexus of coral reef ecosystem dynamics, and functions as the interface through which essentially all energy and nutrients are transferred to fuel both new and recycled production. Substantial research has described many aspects of water column dynamics, often focusing on specific components because water column dynamics are highly spatially and temporally context dependent. Although necessary, a cost of this approach is that these dynamics are often not well linked to the broader ecosystem or across systems. To help overcome the challenge of context dependence, we provide a comprehensive review of this literature, and synthesise it through the perspective of ecosystem ecology. Specifically, we provide a framework to organise the drivers of temporal and spatial variation in production dynamics, structured around five primary state factors. These state factors are used to deconstruct the environmental contexts in which three water column sub-food webs mediate 'new' and 'recycled' production. We then highlight critical pathways by which global change drivers are altering coral reefs via the water column. We end by discussing four key knowledge gaps hindering understanding of the role of the water column for mediating coral reef production, and how overcoming these could improve conservation and management strategies. Throughout, we identify areas of extensive research and those where studies remain lacking and provide a database of 84 published studies. Improved integration of water column dynamics into models of coral reef ecosystem function is imperative to achieve the understanding of ecosystem production necessary to develop effective conservation and management strategies needed to stem global coral loss.

A snapshot of sediment dynamics on an inshore coral reef

Sediments are ubiquitous on coral reefs. However, studies of reef sediments have largely focused on isolated reservoirs, or processes, and rarely consider hydrodynamic drivers. We therefore provide a quantitative snapshot of sediment dynamics on a coral reef. Across a depth profile, we simultaneously examined: suspended sediments, sediment deposition and accumulation, and hydrodynamic and biological movement processes. We reveal the marked potential for the water column to deliver sediments. Currents carried 12.6 t of sediment over the 2,314 m2 study area in 6 days. Sediment traps suggested that a surprisingly high percentage of this sediment was potentially deposited (5.2%). Furthermore, wave-driven resuspension and reworking by parrotfishes separated a highly dynamic sediment regime on the shallow reef flat (3 m), from a more stagnant reef slope (4.5 m-12 m). This study provides a comprehensive model of how hydrodynamic forces and on-reef processes may shape sediment dynamics on a coral reef.

Size structure of the coral Stylophora pistillata across reef flat zones in the central Red Sea

Demographic analyses offer insight into the state of a population. Here, we surveyed different reef flat zones (exposed, midreef and sheltered) of six reefs over a cross-shelf gradient to characterize the population structure of Stylophora pistillata, a coral species which dominates reef flats in the central Red Sea. Phototransects were conducted at each reef flat zone, and the density of S. pistillata, the planar area of each colony, and the occurrence of partial mortality were calculated using the program ImageJ. Each colony was also assigned a color morph (yellow, purple or mixed colors). Density and mean size were extremely variable, both among reef flat zones and reefs, but overall, both metrics were lower on the midshelf reefs. The yellow color morph accounted for nearly 90% of colonies surveyed and dominated most reef flats assessed, with the exception of one site where 81% of colonies were purple morphs. There were no spatial trends in the percentage of colonies suffering partial mortality, but overall there is a positive correlation with size class and proportion of colonies with partial mortality. Despite few trends emerging from assessing individual parameters, a PERMANOVA analysis revealed differences among reef flat zones in most of the reefs, highlighting the importance of multivariate analysis. The data presented here serve as a baseline for monitoring and may identify possible future demographic changes to this important coral species in a region increasingly affected by bleaching events.

The rise of biting during the Cenozoic fueled reef fish body shape diversification

We demonstrate that the stunning trophic diversity of modern reef fishes is a relatively recent state driven by a dramatic transformation in representation of major feeding modes. Since the Early Cenozoic, when over 95% of teleost lineages were suction feeders, there has been a steady increase in direct biting feeding modes. A variety of novelties and jaw modifications permitted reef fishes to feed on substrate-bound prey using direct biting and grazing behaviors and opened this rich adaptive zone, which we show elevated rates of body shape evolution. Taken together, our results indicate that recent diversification of the feeding mechanism played a major role in ecologically and phenotypically shaping the modern fauna of reef fishes.

Diversity of feeding mechanisms is a hallmark of reef fishes, but the history of this variation is not fully understood. Here, we explore the emergence and proliferation of a biting mode of feeding, which enables fishes to feed on attached benthic prey. We find that feeding modes other than suction, including biting, ram biting, and an intermediate group that uses both biting and suction, were nearly absent among the lineages of teleost fishes inhabiting reefs prior to the end-Cretaceous mass extinction, but benthic biting has rapidly increased in frequency since then, accounting for about 40% of reef species today. Further, we measured the impact of feeding mode on body shape diversification in reef fishes. We fit a model of multivariate character evolution to a dataset comprising three-dimensional body shape of 1,530 species of teleost reef fishes across 111 families. Dedicated biters have accumulated over half of the body shape variation that suction feeders have in just 18% of the evolutionary time by evolving body shape ∼1.7 times faster than suction feeders. As a possible response to the ecological and functional diversity of attached prey, biters have dynamically evolved both into shapes that resemble suction feeders as well as novel body forms characterized by lateral compression and small jaws. The ascendance of species that use biting mechanisms to feed on attached prey reshaped modern reef fish assemblages and has been a major contributor to their ecological and phenotypic diversification.

Algal turf structure and composition vary with particulate loads on coral reefs

Algal turfs trap and retain particulates, however, little is known about the relationship between particulate accumulation and taxonomic composition of algal turfs. We investigated how particulate mass related to algal turf structure (length and density) and community composition (taxonomic and functional) on two disparate reefs. Particulate mass was positively related to algal turf length. By contrast, the relationship between particulate mass and turf density was more complex and followed a negative parabolic shape; density increased with particulate mass before stabilising and then declining. Community analyses showed taxonomic, but not functional group compositions differed significantly between reefs and with increasing particulate mass. Our results suggest high loads of particulates accumulated in algal turfs are related to a longer, lower density turf structure, typified by filamentous forms such as Cladophora. Changes in algal turf structure and composition could have a variety of bottom-up influences on coral reef ecosystems.

Parrotfish corallivory on stress-tolerant corals in the Anthropocene

Cumulative anthropogenic stressors on tropical reefs are modifying the physical and community structure of coral assemblages, altering the rich biological communities that depend on this critical habitat. As a consequence, new reef configurations are often characterized by low coral cover and a shift in coral species towards massive and encrusting corals. Given that coral numbers are dwindling in these new reef systems, it is important to evaluate the potential influence of coral predation on these remaining corals. We examined the effect of a key group of coral predators (parrotfishes) on one of the emerging dominant coral taxa on Anthropocene reefs, massive Porites. Specifically, we evaluate whether the intensity of parrotfish predation on this key reef-building coral has changed in response to severe coral reef degradation. We found evidence that coral predation rates may have decreased, despite only minor changes in parrotfish abundance. However, higher scar densities on small Porites colonies, compared to large colonies, suggests that the observed decrease in scarring rates may be a reflection of colony-size specific rates of feeding scars. Reduced parrotfish corallivory may reflect the loss of small Porites colonies, or changing foraging opportunities for parrotfishes. The reduction in scar density on massive Porites suggests that the remaining stress-tolerant corals may have passed the vulnerable small colony stage. These results highlight the potential for shifts in ecological functions on ecosystems facing high levels of environmental stress.

Reef Cover, a coral reef classification for global habitat mapping from remote sensing

Coral reef management and conservation stand to benefit from improved high-resolution global mapping. Yet classifications underpinning large-scale reef mapping to date are typically poorly defined, not shared or region-specific, limiting end-users’ ability to interpret outputs. Here we present Reef Cover, a coral reef geomorphic zone classification, developed to support both producers and end-users of global-scale coral reef habitat maps, in a transparent and version-based framework. Scalable classes were created by focusing on attributes that can be observed remotely, but whose membership rules also reflect deep knowledge of reef form and functioning. Bridging the divide between earth observation data and geo-ecological knowledge of reefs, Reef Cover maximises the trade-off between applicability at global scales, and relevance and accuracy at local scales. Two case studies demonstrate application of the Reef Cover classification scheme and its scientific and conservation benefits: 1) detailed mapping of the Cairns Management Region of the Great Barrier Reef to support management and 2) mapping of the Caroline and Mariana Island chains in the Pacific for conservation purposes.

Measurement(s)	habitat
Technology Type(s)	satellite imaging • digital curation
Sample Characteristic - Organism	Anthozoa
Sample Characteristic - Environment	marine biome • coral reef
Sample Characteristic - Location	global

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.14397182

Diel vertical movements and feeding behaviour of blue humphead parrotfish Scarus ovifrons in a temperate reef of Japan

The feeding ecology of scarinine parrotfishes on tropical coral reefs has received considerable attention in the past few decades; nonetheless, relatively few studies have been conducted in high-latitude reefs. Among the Indo-Pacific Scarus species, Scarus ovifrons is unique, being largely restricted to the warm temperate waters of Japan. Nonetheless, there is very little information available on the feeding ecology of this species. In this study, the authors used acoustic telemetry to detect the diel vertical movement patterns of S. ovifrons, video survey to detect its feeding depths and substrata and focal follow survey and genetic analysis to identify algae composition on the feeding scars at Kashiwajima Island, southwestern Japan (32° 46' N, 132° 38' E). Acoustic telemetry revealed that S. ovifrons spent most of its time in shallow water (<10 m) during the day and slept in deeper water (10-15 m) at night. Video and focal follow surveys revealed that most fishes of various sizes regularly took bites on epilithic algae and detrital materials on rocky substrata at depths of <10 m, but large fishes (>40 cm total length) sometimes took bites directly on live corals (Acropora solitaryensis) at the 5 m depth zone where live tabular corals dominated the benthos. Molecular phylogenetic analyses revealed that epilithic algae collected from feeding scars were mainly composed of Rhodophyta, and coralline algae were less often targeted. Overall, this study revealed that S. ovifrons feeds mostly at depths <10 m, and the feeding algae substrata of the species are similar to those of tropical coral reef parrotfishes.

Collapsing ecosystem functions on an inshore coral reef

Ecosystem functions underpin productivity and key services to humans, such as food provision. However, as the severity of environmental stressors intensifies, it is becoming increasingly unclear if, and to what extent, critical functions and services can be sustained. This issue is epitomised on coral reefs, an ecosystem at the forefront of environmental transitions. We provide a functional profile of a coral reef ecosystem, linking time-series data to quantified processes. The data reveal a prolonged collapse of ecosystem functions in this previously resilient system. The results suggest that sediment accumulation in algal turfs has led to a decline in resource yields to herbivorous fishes and a decrease in fish-based ecosystem functions, including a collapse of both fish biomass and productivity. Unfortunately, at present, algal turf sediment accumulation is rarely monitored nor managed in coral reef systems. Our examination of functions through time highlights the value of directly assessing functions, their potential vulnerability, and the capacity of algal turf sediments to overwhelm productive high-diversity coral reef ecosystems.

Relative influence of environmental factors and fishing on coral reef fish assemblages

Understanding whether assemblages of species respond more strongly to bottom-up (availability of trophic resources or habitats) or top-down (predation pressure) processes is important for effective management of resources and ecosystems. We determined the relative influence of environmental factors and predation by humans in shaping the density, biomass, and species richness of 4 medium-bodied (10-40 cm total length [TL]) coral reef fish groups targeted by fishers (mesopredators, planktivores, grazer and detritivores, and scrapers) and the density of 2 groups not targeted by fishers (invertivores, small fish ≤10 cm TL) in the central Philippines. Boosted regression trees were used to model the response of each fish group to 21 predictor variables: 13 habitat variables, 5 island variables, and 3 fishing variables (no-take marine reserve [NTMR] presence or absence, NTMR size, and NTMR age). Targeted and nontargeted fish groups responded most strongly to habitat variables, then island variables. Fishing (NTMR) variables generally had less influence on fish groups. Of the habitat variables, live hard coral cover, structural complexity or habitat complexity index, and depth had the greatest effects on density, biomass, and species richness of targeted fish groups and on the density of nontargeted fishes. Of the island variables, proximity to the nearest river and island elevation had the most influence on fish groups. The NTMRs affected only fishes targeted by fishers; NTMR size positively correlated with density, biomass, and species richness of targeted fishes, particularly mesopredatory, and grazing and detritivorous fishes. Importantly, NTMRs as small as 15 ha positively affected medium-bodied fishes. This finding provides reassurance for regions that have invested in small-scale community-managed NTMRs. However, management strategies that integrate sound coastal land-use practices to conserve adjacent reef fish habitat, strategic NTMR placement, and establishment of larger NTMRs will be crucial for maintaining biodiversity and fisheries.

Algal turf productivity on coral reefs: A meta-analysis

Algal turfs are an abundant and highly productive component of coral reef ecosystems. However, our understanding of the drivers that shape algal turf productivity across studies and among reefs is limited. Based on published studies we considered how different factors may shape turf productivity and turnover rates. Of the factors considered, depth was the primary driver of turf productivity rates, while turnover was predominantly related to turf biomass. We also highlight shortcomings in the available data collected on turf productivity to-date; most data were collected prior to global coral bleaching events, within a limited geographic range, and were largely from experimental substrata. Despite the fact turfs are a widespread benthic covering on most coral reefs, and one of the major sources of benthic productivity, our understanding of their productivity is constrained by both a paucity of data and methodological limitations. We offer a potential way forward to address these challenges.

Ecomorphological divergence and trophic resource partitioning in 15 syntopic Indo-Pacific parrotfishes (Labridae: Scarini)

Adaptive diversification is a product of both phylogenetic constraint and ecological opportunity. The species-rich parrotfish genera Scarus and Chlorurus display considerable variation in trophic cranial morphology, but these parrotfishes are often described as generalist herbivores. Recent work has suggested that parrotfish partition trophic resources at very fine spatial scales, raising the question of whether interspecific differences in cranial morphology reflect trophic partitioning. We tested this hypothesis by comparing targeted feeding substrata with a previously published dataset of nine cranial morphological traits. We sampled feeding substrata of 15 parrotfish species at Lizard Island, Great Barrier Reef, Australia, by following individuals until focused biting was observed, then extracting a bite core 22 mm in diameter. Three indices were parameterized for each bite core: substratum taphonomy, maximum turf height and cover of crustose coralline algae. Parrotfish species were spread along a single axis of variation in feeding substrata: successional status of the substratum taphonomy and epilithic and endolithic biota. This axis of trophic variation was significantly correlated with cranial morphology, indicating that morphological disparity within this clade is associated with interspecific partitioning of feeding substrata. Phylogenetic signal and phylomorphospace analyses revealed that the evolution of this clade involved a hitherto-unrecognized level of trophic diversification.

Implications of 2D versus 3D surveys to measure the abundance and composition of benthic coral reef communities

A paramount challenge in coral reef ecology is to estimate the abundance and composition of the communities residing in such complex ecosystems. Traditional 2D projected surface cover estimates neglect the 3D structure of reefs and reef organisms, overlook communities residing in cryptic reef habitats (e.g., overhangs, cavities), and thus may fail to represent biomass estimates needed to assess trophic ecology and reef function. Here, we surveyed the 3D surface cover, biovolume, and biomass (i.e., ash-free dry weight) of all major benthic taxa on 12 coral reef stations on the island of Curaçao (Southern Caribbean) using structure-from-motion photogrammetry, coral point counts, in situ measurements, and elemental analysis. We then compared our 3D benthic community estimates to corresponding estimates of traditional 2D projected surface cover to explore the differences in benthic community composition using different metrics. Overall, 2D cover was dominated (52 ± 2%, mean ± SE) by non-calcifying phototrophs (macroalgae, turf algae, benthic cyanobacterial mats), but their contribution to total reef biomass was minor (3.2 ± 0.6%). In contrast, coral cover (32 ± 2%) more closely resembled coral biomass (27 ± 6%). The relative contribution of erect organisms, such as gorgonians and massive sponges, to 2D cover was twofold and 11-fold lower, respectively, than their contribution to reef biomass. Cryptic surface area (3.3 ± 0.2 m² m^-2 _{planar reef}) comprised half of the total reef substrate, rendering two thirds of coralline algae and almost all encrusting sponges (99.8%) undetected in traditional assessments. Yet, encrusting sponges dominated reef biomass (35 ± 18%). Based on our quantification of exposed and cryptic reef communities using different metrics, we suggest adjustments to current monitoring approaches and highlight ramifications for evaluating the ecological contributions of different taxa to overall reef function. To this end, our metric conversions can complement other benthic assessments to generate non-invasive estimates of the biovolume, biomass, and elemental composition (i.e., standing stocks of organic carbon and nitrogen) of Caribbean coral reef communities.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00338-021-02118-6.

Algal turf sediments limit the spatial extent of function delivery on coral reefs

The presence of key organisms is frequently associated with the delivery of specific ecosystem functions. Areas with such organisms are therefore often considered to have greater levels of these functions. While this assumption has been the backbone of coral reef ecosystem-based management approaches for decades, we currently have only a limited understanding of how fish presence equates to function on coral reefs and whether this relationship is susceptible to stressors. To assess the capacity of a stressor to shape function delivery we used a multi-scale approach ranging from tens of kilometres across the continental shelf of Australia's Great Barrier Reef, down to centimetres within a reef habitat. At each scale, we quantified the spatial extent of a model function (detritivory) by a coral reef surgeonfish (Ctenochaetus striatus) and its potential to be shaped by sediments. At broad spatial scales, C. striatus presence was correlated strongly with algal turf sediment loads, while at smaller spatial scales, function delivery appears to be constrained by algal turf sediment distributions. In all cases, sediment loads above ~250-500 g m^-2 were associated with a marked decrease in fish abundance or feeding activity, suggesting that a common ecological threshold lies within this range. Our results reveal a complex functional dynamic between proximate agents of function delivery (fish) and the ultimate drivers of function delivery (sediments), which emphasizes: a) weaknesses in the assumed links between fish presence and function, and b) the multi-scale capacity of algal turf sediments to shape reef processes. Unless direct extractive activities (e.g. fishing) are the main driver of function loss on coral reefs, managing to conserve fish abundance is unlikely to yield the desired outcomes. It only addresses one potential driver. Instead, management of both the agents that deliver functions (e.g. fishes), and the drivers that modify functions (e.g. sediments), is needed.

Subconscious Biases in Coral Reef Fish Studies

In complex, diverse ecosystems, one is faced with an exceptionally challenging decision: which species to examine first and why? This raises the question: Is there evidence of subconscious biases in study species selection? Likewise, is there evidence of this bias in selecting methods, locations, and times? We addressed these questions by surveying the literature on the most diverse group of vertebrates (fishes) in an iconic high-diversity ecosystem (coral reefs). The evidence suggests that we select study species that are predominantly yellow. Reef fish studies also selectively examine fishes that are behaviorally bold and in warm, calm, attractive locations. Our findings call for a reevaluation of study species selection and methodological approaches, recognizing the potential for subconscious biases to drive selection for species that are attractive rather than important and for methods that give only a partial view of ecosystems. Given the challenges faced by high-diversity ecosystems, we may need to question our decision-making processes.

Habitat zonation on coral reefs: Structural complexity, nutritional resources and herbivorous fish distributions

Distinct zonation of community assemblages among habitats is a ubiquitous feature of coral reefs. The distribution of roving herbivorous fishes (parrotfishes, surgeonfishes and rabbitfishes) is a particularly clear example, with the abundance of these fishes generally peaking in shallow-water, high-energy habitats, regardless of the biogeographic realm. Yet, our understanding of the factors which structure this habitat partitioning, especially with regards to different facets of structural complexity and nutritional resource availability, is limited. To address this issue, we used three-dimensional photogrammetry and structure-from-motion technologies to describe five components of structural complexity (rugosity, coral cover, verticality, refuge density and field-of-view) and nutritional resource availability (grazing surface area) among habitats and considered how these factors are related to herbivorous fish distributions. All complexity metrics (including coral cover) were highest on the slope and crest. Nutritional resource availability differed from this general pattern and peaked on the outer-flat. Unexpectedly, when compared to the distribution of herbivorous fishes, none of the complexity metrics had a marked influence in the models. However, grazing surface area was a strong predictor of both the abundance and biomass of herbivorous fishes. The strong relationship between grazing surface area and herbivorous fish distributions indicates that nutritional resource availability may be one of the primary factors driving the distribution of roving herbivorous fishes. The lack of a relationship between complexity and herbivorous fishes, and a strong affinity of herbivorous fishes for low-complexity, algal turf-dominated outer-flat habitats, offers some cautious optimism that herbivory may be sustained on future, low-complexity, algal turf-dominated reef configurations.

Trophic innovations fuel reef fish diversification

Reef fishes are an exceptionally speciose vertebrate assemblage, yet the main drivers of their diversification remain unclear. It has been suggested that Miocene reef rearrangements promoted opportunities for lineage diversification, however, the specific mechanisms are not well understood. Here, we assemble near-complete reef fish phylogenies to assess the importance of ecological and geographical factors in explaining lineage origination patterns. We reveal that reef fish diversification is strongly associated with species' trophic identity and body size. Large-bodied herbivorous fishes outpace all other trophic groups in recent diversification rates, a pattern that is consistent through time. Additionally, we show that omnivory acts as an intermediate evolutionary step between higher and lower trophic levels, while planktivory represents a common transition destination. Overall, these results suggest that Miocene changes in reef configurations were likely driven by, and subsequently promoted, trophic innovations. This highlights trophic evolution as a key element in enhancing reef fish diversification.

Sediments ratchet-down coral reef algal turf productivity

Coral reefs are highly productive ecosystems, with much of this productivity arising from the algal turfs which cover the hard reef substratum. This productivity can flow up the food chain through herbivorous fishes, to be harvested by humans as fishable biomass. However, algal turfs exist on a spectrum of forms from short productive algal turfs (SPATs), to long sediment-laden algal turfs (LSATs). The latter are increasingly likely to typify Anthropocene coral reefs, however, we have a limited understanding of their nature and potential productivity. We assessed the nature of algal turfs in terms of length, biomass, relative detritus content, and productivity across a sediment load gradient, from SPATs to LSATs, at two reefs separated by >450 km along Australia's Great Barrier Reef (GBR). Furthermore, to assess the capacity of sediments to shape productivity, we modelled algal turf productivity, as a function of sediment load, across multiple spatial scales in a Bayesian framework. We recorded precipitous declines in both the productivity of algal turfs, and the relative nutritional value of particulates, up to sediment loads of ~100 g m^-2. However, algal turf biomass did not change with sediment loads. This appears to reflect a shift in algal community composition from short, high-biomass, highly-productive algae at low sediment loads, to longer, low-biomass, less productive algae at high sediment loads. Importantly, these relationships provide a robust framework for estimating algal turf productivity on coral reefs. Indeed, when we applied our models to known sediment loads, we reveal that sediment loads alone can explain observed algal turf productivity gradients across multiple spatial scales. In an era of global climate change and coral reef reconfiguration, algal turf sediments may hold the key to maintaining benthic productivity on coral reefs in the Anthropocene.

Human exploitation shapes productivity-biomass relationships on coral reefs

Coral reef fisheries support the livelihoods of millions of people in tropical countries, despite large-scale depletion of fish biomass. While human adaptability can help to explain the resistance of fisheries to biomass depletion, compensatory ecological mechanisms may also be involved. If this is the case, high productivity should coexist with low biomass under relatively high exploitation. Here we integrate large spatial scale empirical data analysis and a theory-driven modelling approach to unveil the effects of human exploitation on reef fish productivity-biomass relationships. We show that differences in how productivity and biomass respond to overexploitation can decouple their relationship. As size-selective exploitation depletes fish biomass, it triggers increased production per unit biomass, averting immediate productivity collapse in both the modelling and the empirical systems. This 'buffering productivity' exposes the danger of assuming resource production-biomass equivalence, but may help to explain why some biomass-depleted fish assemblages still provide ecosystem goods under continued global fishing exploitation.

Algal turf sediments on coral reefs: what's known and what's next

Algal turfs are likely to rise in prominence on coral reefs in the Anthropocene. In these ecosystems the sediments bound within algal turfs will shape ecosystem functions and the services humanity can obtain from reefs. However, while interest is growing in the role of algal turf sediments, studies remain limited. In this review we provide an overview of our knowledge to-date concerning algal turf sediments on coral reefs. Specifically, we highlight what algal turf sediments are, their role in key ecosystem processes, the potential importance of algal turf sediments on Anthropocene reefs, and key knowledge gaps for future research. The evidence suggests that the management of algal turf sediments will be critically important if we are to sustain key functions and services on highly-altered, Anthropocene coral reef configurations.

Quantifying sediment dynamics on an inshore coral reef: Putting algal turfs in perspective

Increased sediment loads within algal turfs, can be highly detrimental to coral reef systems. However, significant knowledge gaps remain in relation to sediment dynamics, especially linking suspended sediments, sedimentation and turf-bound sediments. To examine these links, a series of different methods for quantifying suspended sediments, sedimentation and the accumulation of turf sediments were compared, simultaneously, on an inner-shelf reef. We revealed that the amount and composition of sediment quantified using different methods varied markedly, with commonly employed measures of sedimentation failing to accurately reflect patterns of sediment accumulation in turfs. Our results highlighted the propensity for turfs to trap and retain sediments, with turfs accumulating approximately 2.6 times more sediment than traps, and 6 times more sediment than SedPods, over a seven-day period. This study highlights the major, but often overlooked, role that algal turfs can play in sediment dynamics on coral reefs.

Spatial mismatch in fish and coral loss following 2016 mass coral bleaching

Record-breaking temperatures between 2015 and 2016 led to unprecedented pan-tropical bleaching of scleractinian corals. On the Great Barrier Reef (GBR), the effects were most pronounced in the remote, northern region, where over 90% of reefs exhibited bleaching. Mass bleaching that results in widespread coral mortality represents a major disturbance event for reef organisms, including reef fishes. Using 133 replicate 1 m² quadrats, we quantified short-term changes in coral communities and spatially associated reef fish assemblages, at Lizard Island, Australia, in response to the 2016 mass bleaching event. Quadrats were spatially matched, permitting repeated sampling of fish and corals in the same areas: before, during and 6 months after mass bleaching. As expected, we documented a significant decrease in live coral cover. Subsequent decreases in fish abundance were primarily driven by coral-associated damselfishes. However, these losses, were relatively minor (37% decrease), especially compared to the magnitude of Acropora loss (>95% relative decrease). Furthermore, at a local, 1 m² scale, we documented a strong spatial mismatch between fish and coral loss. Post-bleaching fish losses were not highest in quadrats that experienced the greatest loss of live coral. Nor were fish losses associated with a proliferation of cyanobacteria. Several sites did, however, exhibit increases in fish abundance suggesting substantial spatial movements. These results challenge common assumptions and emphasize the need for caution when ascribing causality to observed patterns of fish loss at larger spatial scales. Our results highlight the potential for short-term resilience to climate change, in fishes, through local migration and habitat plasticity.

Algal turf sediments across the Great Barrier Reef: Putting coastal reefs in perspective

Sediments trapped within algal turfs play a key role in mediating ecosystem processes on reefs. Despite this ecological importance, our understanding of turf-bound sediments on coastal reefs is limited, even though such reefs are heavily exposed to terrestrial sediments. We compared algal turf sediment loads, composition and grain size distributions among coastal reefs along the Great Barrier Reef (GBR) and compared these to turf-bound sediments on reefs across the GBR. Algal turf sediment loads on coastal reefs were on average 9.4 times higher than offshore reefs (range 2.2-16.1 times). Among coastal reefs there was marked variability in the loads (797.87-3681.78 g m^-2), composition (organics 1.0-8.9%; silicates 27.4-93.0%) and grain size of sediments. Our data highlight the potential variability in physical and ecological factors that control sediment dynamics on coastal reefs, including proximity to rivers, and reveal that turfs on coastal reefs are heavily influenced by sediments.

Functional links on coral reefs: Urchins and triggerfishes, a cautionary tale

Urchins are ubiquitous components of coral reefs ecosystems, with significant roles in bioerosion and herbivory. By controlling urchin densities, triggerfishes have been identified as keystone predators. However, the functional linkages between urchins and triggerfishes, in terms of distributional patterns and concomitant effects on ecosystem processes, are not well understood, especially in relatively unexploited systems. To address this we censused urchins and triggerfishes on two cross-shelf surveys on the Great Barrier Reef (GBR) at the same times and locations. We also evaluated the role of urchins in bioerosion. Although urchin abundance and triggerfish biomass varied by 80% and nearly 900% across sites, respectively, this variability was driven primarily by shelf position with no evidence of top-down control on urchins by triggerfishes. Low urchin abundances meant urchins only played a minor role in bioerosion. We highlight the potential variability in functional links, and contributions to ecosystem processes, among regions.