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

Shifts in dominance of benthic communities along a gradient of water temperature and turbidity in tropical coastal ecosystems

Tropical coastal benthic communities will change in species composition and relative dominance due to global (e.g., increasing water temperature) and local (e.g., increasing terrestrial influence due to land-based activity) stressors. This study aimed to gain insight into possible trajectories of coastal benthic assemblages in Raja Ampat, Indonesia, by studying coral reefs at varying distances from human activities and marine lakes with high turbidity in three temperature categories (<31 °C, 31-32 °C, and >32 °C). The benthic community diversity and relative coverage of major benthic groups were quantified via replicate photo transects. The composition of benthic assemblages varied significantly among the reef and marine lake habitats. The marine lakes <31 °C contained hard coral, crustose coralline algae (CCA), and turf algae with coverages similar to those found in the coral reefs (17.4-18.8% hard coral, 3.5-26.3% CCA, and 15-15.5% turf algae, respectively), while the higher temperature marine lakes (31-32 °C and >32 °C) did not harbor hard coral or CCA. Benthic composition in the reefs was significantly influenced by geographic distance among sites but not by human activity or depth. Benthic composition in the marine lakes appeared to be structured by temperature, salinity, and degree of connection to the adjacent sea. Our results suggest that beyond a certain temperature (>31 °C), benthic communities shift away from coral dominance, but new outcomes of assemblages can be highly distinct, with a possible varied dominance of macroalgae, benthic cyanobacterial mats, or filter feeders such as bivalves and tubeworms. This study illustrates the possible use of marine lake model systems to gain insight into shifts in the benthic community structure of tropical coastal ecosystems if hard corals are no longer dominant.

Looking for the sponge loop: analyses of detritus on a Caribbean forereef using stable isotope and eDNA metabarcoding techniques

Coral reefs are biodiverse ecosystems that rely on trophodynamic transfers from primary producers to consumers through the detrital pathway. The sponge loop hypothesis proposes that sponges consume dissolved organic carbon (DOC) and produce large quantities of detritus on coral reefs, with this turn-over approaching the daily gross primary production of the reef ecosystem. In this study, we collected samples of detritus in the epilithic algal matrix (EAM) and samples from potential sources of detritus over two seasons from the forereef at Carrie Bow Cay, Belize. We chose this location to maximize the likelihood of finding support for the sponge loop hypothesis because Caribbean reefs have higher sponge abundances than other tropical reefs worldwide and the Mesoamerican barrier reef is an archetypal coral reef ecosystem. We used stable isotope analyses and eDNA metabarcoding to determine the composition of the detritus. We determined that the EAM detritus was derived from a variety of benthic and pelagic sources, with primary producers (micro- and macroalgae) as major contributors and metazoans (Arthropoda, Porifera, Cnidaria, Mollusca) as minor contributors. None of the sponge species that reportedly produce detritus were present in EAM detritus. The cnidarian signature in EAM detritus was dominated by octocorals, with a scarcity of hard corals. The composition of detritus also varied seasonally. The negligible contribution of sponges to reef detritus contrasts with the detrital pathway originally proposed in the sponge loop hypothesis. The findings indicate a mix of pelagic and benthic sources in the calmer summer and primarily benthic sources in the more turbulent spring.

Recovery of algal turfs following removal

As a consequence of the increasing human footprint on the environment, marine ecosystems are rapidly transforming into new configurations dominated by early-successional and weedy life forms. Algal turfs, in particular, are emerging as a common and widespread configuration of shallow temperate and tropical reefs, and are predicted to transform reef dynamics and ecosystem services. Restoration is an increasingly used approach to mitigate these transformations, with turf removal being proposed as a tool to shift back the competitive balance and facilitate the recovery of initial species, such as forest-forming seaweeds. Yet, our practical understanding of turf recovery trajectories following removal is limited, and removal success may be hindered by strong feedback mechanisms that reinforce turf dominance once turfs are established. Here we investigate the recovery of algal turfs and their properties (mean height, turf biomass and sediment load) to experimental clearance across six turf-dominated reefs at ca. 9 m in subtropical western Australia. Turf cover, mean height, and sediment loads exhibited a rapid recovery following experimental clearing, with all experimental sites reaching pre-clearing turf conditions between 28 and 46 days. This response was mostly driven by the growth of filamentous turf species, whose cover exhibited a positive relationship with sediment load, and are well-known to rapidly recover after disturbance. Turf abundance and turf properties remained relatively constant for the remaining experimental period. Our results suggest that clearing turfs creates only a small time window for recovery of seaweed forests, which limits the effectiveness of turf clearing as a restoration tool. System-specific quantitative evidence on the recovery capacity of turfs may thus be necessary to guide restoration initiatives and develop decision support systems that account for the risks, feasibility, and costs and benefits of restoring turf-dominated systems to previous configurations.

Long-term marine protection enhances kelp forest ecosystem stability

Trophic downgrading destabilizes ecosystems and can drive large-scale shifts in ecosystem state. While restoring predatory interactions in marine reserves can reverse anthropogenic-driven shifts, empirical evidence of increased ecosystem stability and persistence in the presence of predators is scant. We compared temporal variation in rocky reef ecosystem state in New Zealand's oldest marine reserve to nearby fished reefs to examine whether protection of predators led to more persistent and stable reef ecosystem states in the marine reserve. Contrasting ecosystem states were found between reserve and fished sites, and this persisted over the 22-year study period. Fished sites were predominantly urchin barrens but occasionally fluctuated to short-lived turfs and mixed algal forests, while reserve sites displayed unidirectional successional trajectories toward stable kelp forests (Ecklonia radiata) taking up to three decades following protection. This provides empirical evidence that long-term protection of predators facilitates kelp forest recovery, resists shifts to denuded alternate states, and enhances kelp forest stability.

Macroalgae exhibit diverse responses to human disturbances on coral reefs

Scientists and managers rely on indicator taxa such as coral and macroalgal cover to evaluate the effects of human disturbance on coral reefs, often assuming a universally positive relationship between local human disturbance and macroalgae. Despite evidence that macroalgae respond to local stressors in diverse ways, there have been few efforts to evaluate relationships between specific macroalgae taxa and local human-driven disturbance. Using genus-level monitoring data from 1205 sites in the Indian and Pacific Oceans, we assess whether macroalgae percent cover correlates with local human disturbance while accounting for factors that could obscure or confound relationships. Assessing macroalgae at genus level revealed that no genera were positively correlated with all human disturbance metrics. Instead, we found relationships between the division or genera of algae and specific human disturbances that were not detectable when pooling taxa into a single functional category, which is common to many analyses. The convention to use percent cover of macroalgae as an indication of local human disturbance therefore likely obscures signatures of local anthropogenic threats to reefs. Our limited understanding of relationships between human disturbance, macroalgae taxa, and their responses to human disturbances impedes the ability to diagnose and respond appropriately to these threats.

Reef on the edge: resilience failure of marginal patch coral reefs in Eastern Arabian Sea under recurrent coral bleaching, coral diseases, and local stressors

Marked by strong El Niño-Southern Oscillation (ENSO) effects during 2014-2016, global coral reefs underwent mass bleaching. Here, we conducted a comprehensive (2014-2019) study, coinciding with the 2014-16 ENSO, to investigate the response and resilience potential of marginal coral communities to the combined impact of recurrent thermal anomalies and multiple anthropogenic stressors before, during, and after the mass bleaching episodes. Our result unveiled that thermal-stress-driven back-to-back annual coral bleaching episodes caused coral mortality and significantly decimated coral cover, primarily in 2015 and 2016. Subsequent benthic regime shifts toward macroalgal and algal turf colonization, followed by an increase in coral disease prevalence and recruitment failure was observed after the recurrent bleaching episodes. Algal cover increased from 21% in 2014 to 52.90% in 2019, and a subsequent increase in coral disease occurrence was observed from 16% in 2015 to 29% in 2019. The cascading negative effect of multiple stressors magnified coral loss and decreased the coral cover significantly from 45% in 2014 to 20% in 2019. The corals in the intensive recreational diving activity sites showed higher disease prevalence, concurring with high mechanical coral damage. The present study demonstrates that consecutive thermal bleaching episodes combined with local stressors can cause declines in coral cover and promote an undesirable regime shift to algal dominance in marginal coral reef habitats within a short duration. These results are of particular interest given that marginal reefs were traditionally perceived as resilient reef habitats due to their higher survival threshold to environmental changes. The present study indicates that mitigation of local stressors by effective management strategies, in conjunction with globally coordinated efforts to ameliorate climate change, can protect these unique coral reefs.

Turbidity shapes shallow Southwestern Atlantic benthic reef communities

Southwestern Atlantic reefs (Brazilian Province) occur along a broad latitudinal range (∼5°N-27°S) and under varied environmental conditions. We combined large-scale benthic cover and environmental data into uni- and multivariate regression tree analyses to identify unique shallow (<30 m) benthic reef communities and their environmental drivers along the Brazilian Province. Turbidity was the leading environmental driver of benthic reef communities, with the occurrence of two main groups: clear-water (dominated by fleshy macroalgae) and turbid (dominated by turf algae). Seven out of 14 scleractinian coral species were more abundant in the turbid group, thus corroborating the photophobic nature of some Brazilian corals. The most abundant scleractinian in Brazil (Montastraea cavernosa), largely dominated (71-93% of total coral cover) both, the shallow turbid and deeper clear-water reefs. Because these habitat types are widely recognized as potential climate refuges, local threats (e.g. pollution, overfishing) should be averted.

Benthic composition changes on coral reefs at global scales

Globally, ecosystems are being reconfigured by a range of intensifying human-induced stressors. Coral reefs are at the forefront of this environmental transformation, and if we are to secure their key ecosystem functions and services, it is important to understand the likely configuration of future reefs. However, the composition and trajectory of global coral reef benthic communities is currently unclear. Here our global dataset of 24,468 observations spanning 22 years (1997-2018) revealed that particularly marked declines in coral cover occurred in the Western Atlantic and Central Pacific. The data also suggest that high macroalgal cover, widely regarded as the major degraded state on coral reefs, is a phenomenon largely restricted to the Western Atlantic. At a global scale, the raw data suggest decreased average (± standard error of the mean) hard coral cover from 36 ± 1.4% to 19 ± 0.4% (during a period delineated by the first global coral bleaching event (1998) until the end of the most recent event (2017)) was largely associated with increased low-lying algal cover such as algal turfs and crustose coralline algae. Enhanced understanding of reef change, typified by decreased hard coral cover and increased cover of low-lying algal communities, will be key to managing Anthropocene coral reefs.

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.

Benthic cyanobacterial mat formation during severe coral bleaching at Lizard Island: The mediating role of water currents

Cyanobacterial mats are increasingly recognised as a symptom of coral reef change. However, the spatial distribution of cyanobacterial mats during coral bleaching has received limited attention. We explored cyanobacterial mat distribution during a bleaching event at Lizard Island and considered hydrodynamics as a potential modifier. During bleaching cyanobacterial mats covered up to 34% of the benthos at a transect scale, while some quadrats (1 m2) were covered almost entirely (97.5%). The spatial distribution of cyanobacterial mats was limited to areas with slower water currents. Coral cover declined by 44% overall, although cyanobacterial mats were not spatially coupled to the magnitude of coral loss. Overall, the marked increase in cyanobacterial mat cover was an ephemeral spike, not a sustained change, with cover returning to 0.4% within 6 months. Cyanobacterial mats clearly represent dynamic space holders on coral reefs, with a marked capacity to rapidly exploit change, if conditions are right.

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.

How to quantify algal turf sediments and particulates on tropical and temperate reefs: An overview

Algal turfs are the most abundant benthic covering on reefs in many shallow-water marine ecosystems. The particulates and sediments bound within algal turfs can influence a multitude of functions within these ecosystems. Despite the global abundance and importance of algal turfs, comparison of algal turf-bound sediments is problematic due to a lack of standardisation across collection methods. Here we provide an overview of three methods (vacuum sampling, airlift sampling, and TurfPods), and the necessary equipment (including construction suggestions), commonly employed to quantify sediments from algal turfs. We review the purposes of these methods (e.g. quantification of standing stock versus net accumulation) and how methods can vary depending on the research question or monitoring protocol. By providing these details in a readily accessible format we hope to encourage a standardised set of approaches for marine benthic ecologists, geologists and managers, that facilitates further quantification and global comparisons of algal turf sediments.

Limitations to coral recovery along an environmental stress gradient

Positive feedbacks driving habitat-forming species recovery and population growth are often lost as ecosystems degrade. For such systems, identifying mechanisms that limit the re-establishment of critical positive feedbacks is key to facilitating recovery. Theory predicts the primary drivers limiting system recovery shift from biological to physical as abiotic stress increases, but recent work has demonstrated that this seldom happens. We combined field and laboratory experiments to identify variation in limitations to coral recovery along an environmental stress gradient at Ningaloo Reef and Exmouth Gulf in northwest Australia. Many reefs in the region are coral depauperate due to recent cyclones and thermal stress. In general, recovery trajectories are prolonged due to limited coral recruitment. Consistent with theory, clearer water reefs under low thermal stress appear limited by biological interactions: competition with turf algae caused high mortality of newly settled corals and upright macroalgal stands drove mortality in transplanted juvenile corals. Laboratory experiments showed a positive relationship between crustose coralline algae cover and coral settlement, but only in the absence of sedimentation. Contrary to expectation, coral recovery does not appear limited by the survival or growth of recruits on turbid reefs under higher thermal stress, but to exceptionally low larval supply. Laboratory experiments showed that larval survival and settlement are unaffected by seawater quality across the study region. Rather, connectivity models predicted that many of the more turbid reefs in the Gulf are predominantly self seeded, receiving limited supply under degraded reef states. Overall, we find that the influence of oceanography can overwhelm the influences of physical and biological interactions on recovery potential at locations where environmental stressors are high, whereas populations in relatively benign physical conditions are predominantly structured by local ecological drivers. Such context-dependent information can help guide expectations and assist managers in optimizing strategies for spatial conservation planning for system recovery.

Antagonistic effects of seawalls and urban sedimentation on epilithic algal matrix (EAM)-feeding fishes

Marine urbanisation often results in the proliferation of artificial coastal defences and heavy sedimentation, adversely impacting coral reef systems in tropical coastal cities. Knowledge of how motile organisms, such as reef fish, respond to novel human-made habitats and high sedimentation is limited. Here, we examine the role of sloping granite seawalls in supporting reef fishes that utilise the epilithic algal matrix (EAM) as a food resource. We surveyed fish assemblages and feeding activities on seawalls and reef flats, and conducted a field experiment to examine the effects of sediment on EAM feeding rates. Seawalls and reef flats supported distinct fish assemblage composition with significantly greater feeding activity on seawalls. However, reduced feeding activity on EAM with elevated sediment loads suggests that urban sedimentation may limit the utility of this novel feeding ground for nearshore communities. These findings illustrate the complexities and interactive effects of anthropogenic changes driven by coastal urbanisation.

Homogenization and miniaturization of habitat structure in temperate marine forests

Humans are rapidly transforming the structural configuration of the planet's ecosystems, but these changes and their ecological consequences remain poorly quantified in underwater habitats. Here, we show that the loss of forest-forming seaweeds and the rise of ground-covering 'turfs' across four continents consistently resulted in the miniaturization of underwater habitat structure, with seascapes converging towards flattened habitats with smaller habitable spaces. Globally, turf seascapes occupied a smaller architectural trait space and were structurally more similar across regions than marine forests, evidencing habitat homogenization. Surprisingly, such habitat convergence occurred despite turf seascapes consisting of vastly different species richness and with different taxa providing habitat architecture, as well as across disparate drivers of marine forest decline. Turf seascapes contained high sediment loads, with the miniaturization of habitat across 100s of km in mid-Western Australia resulting in reefs retaining an additional ~242 million tons of sediment (four orders of magnitude more than the sediments delivered fluvially annually). Together, this work demonstrates that the replacement of marine forests by turfs is a generalizable phenomenon that has profound consequences for the ecology of temperate reefs.

Nutrient Enrichment Predominantly Affects Low Diversity Microbiomes in a Marine Trophic Symbiosis between Algal Farming Fish and Corals

While studies show that nutrient pollution shifts reef trophic interactions between fish, macroalgae, and corals, we know less about how the microbiomes associated with these organisms react to such disturbances. To investigate how microbiome dynamics are affected during nutrient pollution, we exposed replicate Porites lobata corals colonized by the fish Stegastes nigricans, which farm an algal matrix on the coral, to a pulse of nutrient enrichment over a two-month period and examined the microbiome of each partner using 16S amplicon analysis. We found 51 amplicon sequence variants (ASVs) shared among the three hosts. Coral microbiomes had the lowest diversity with over 98% of the microbiome dominated by a single genus, Endozoicomonas. Fish and algal matrix microbiomes were ~20 to 70× more diverse and had higher evenness compared to the corals. The addition of nutrients significantly increased species richness and community variability between samples of coral microbiomes but not the fish or algal matrix microbiomes, demonstrating that coral microbiomes are less resistant to nutrient pollution than their trophic partners. Furthermore, the 51 common ASVs within the 3 hosts indicate microbes that may be shared or transmitted between these closely associated organisms, including Vibrionaceae bacteria, many of which can be pathogenic to corals.

The study of sediments on coral reefs: A hydrodynamic perspective

There is a rich literature on coral reef sediments. However, this knowledge is spread among research fields, and the extent to which major sediment reservoirs and reservoir connecting processes have been quantified is unclear. We examined the literature to quantify where and how sediments have been measured on coral reefs and, thereby, identified critical knowledge gaps. In most studies, sediments in one reservoir or one sedimentary process were quantified. The measurement of water column sediments (55% of reservoir measurements) and sediment trapping rates (42% of process measurements) were over-represented. In contrast, sediments on reef substrata, and the transition of sediments from the water column to the benthos, were rarely quantified. Furthermore, only ~20% of sediment measurements were accompanied by the quantification of hydrodynamic drivers. Multidisciplinary collaborative approaches offer great promise for advancing our understanding of the connections between sediment reservoirs, and the sedimentary and hydrodynamic processes that mediate these connections.

A centuries-old manmade reef in the Caribbean does not substitute natural reefs in terms of species assemblages and interspecific competition

With increasing maritime activities in the proximity of coral reefs, a growing number of manmade structures are becoming available for coral colonisation. Yet, little is known about the sessile community composition of such artificial reefs in comparison with that of natural coral reefs. Here, we compared the diversity of corals and their competitors for substrate space between a centuries-old manmade structure and the nearest natural reef at St. Eustatius, eastern Caribbean. The artificial reef had a significantly lower species richness and fewer competitive interactions than the natural reef. The artificial reef was dominated by a cover of crustose coralline algae and zoantharians, instead of turf algae and fire corals on the natural reef. Significant differences in species composition were also found between exposed and sheltered sites on both reefs. Our study indicates that even a centuries-old manmade reef cannot serve as a surrogate for natural reefs.

Effect of fine-scale habitat differences on algal colonisation in a coral-dominated subtropical reef

Maintaining the coexistence of algae and corals depends on the interactions between them. We investigated these interactions to assess: (1) recruitment patterns of algal turfs over time in dead areas on live corals; (2) the influence of fine-scale differences in coral-dominated environments on algal colonisation; (3) the influence of coral as a substrate for algal recruitment; (4) the invasion potential of algal turf on live coral tissue. This study compared algal colonisation directly on dead or damaged coral areas with algal colonisation on recruitment plates in coral-dominated or -free areas at 23, 154, and 230 days. We also monitored coral colonies over 1.5 years. Filamentous and articulated coralline algae were primarily evident in the early colonisation, reaching stability after 154 days. On a fine scale, the coral-dominated environment showed an increase in number of algal species and coverage. However, coral substrate was selective, with fewer species recruited to this substrate compared to the artificial plates. Furthermore, the competitive dynamics between corals and algal turfs did not result in a winner over time. Thus, algal turf colonisation was influenced not only by coral substrate but also by the reef environment on a fine scale.

Impacts of water quality on Acropora coral settlement: The relative importance of substrate quality and light

Coral larval settlement patterns are influenced by a vast array of factors; however, the relative roles of individual factors are rarely tested in isolation, leading to confusion about which are most crucial for settlement. For example, direct effects of the light environment are often cited as a major factor influencing settlement patterns, yet this has not been demonstrated under environmentally realistic lighting regimes in the absence of confounding factors. Here we apply programmable multispectral lights to create realistic light spectra, while removing correlating (but not obvious) factors that are common in laboratory settlement experiments. Using two common species of Acropora - key framework builders of the Great Barrier Reef - we find little evidence that light intensity or changes in the spectral profile play a substantial role in larval settlement under most environmentally realistic settings but can under more extreme or artificial settings. We alternatively hypothesise and provide evidence that chronic light conditions and recent sediment exposures that impact benthic substrates (e.g., crustose coralline algae) have a greater impact on settlement success. Under these conditions, there was a decrease of up to 74% settlement success. Management of water quality conditions that impact the quality of benthic-settlement substrates therefore should present a priority area of focus for improving coral recruitment.

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.

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.

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.

Stokes drift in coral reefs with depth-varying permeability

In his famous paper of 1847 (Stokes GG. 1847 On the theory of oscillatory waves. Trans. Camb. Phil. Soc. 8, 441-455), Stokes introduced the drift effect of particles in a fluid that is undergoing wave motion. This effect, now known as Stokes drift, is the result of differences between the Lagrangian and Eulerian velocities of the fluid element and has been well-studied, both in the laboratory and as a mechanism of mass transport in the oceans. On a smaller scale, it is of vital importance to the hydrodynamics of coral reefs to understand drift effects arising from waves on the ocean surface, transporting nutrients and oxygen to the complex ecosystems within. A new model is proposed for a class of coral reefs in shallow seas, which have a permeable layer of depth-varying permeability. We then note that the behaviour of the waves above the reef is only affected by the permeability at the top of the porous layer, and not its properties within, which only affect flow inside the porous layer. This model is then used to describe two situations found in coral reefs; namely, algal layers overlying the reef itself and reef layers whose permeability decreases with depth. This article is part of the theme issue 'Stokes at 200 (part 2)'.

Farming damselfishes shape algal turf sediment dynamics on coral reefs

Farming damselfishes are well known for shaping benthic communities on reefs, in terms of both cultivating algae and increasing productivity. However, their capacity to shape relationships between algal turfs, detritus and sediments remains largely unknown, despite the importance of such relationships on reefs. We therefore examined the relationships between sediment loads and both algal turf length and detritus levels, inside and outside farming damselfish territories, at two reefs on the Great Barrier Reef. We found that, while sediment loads are tightly coupled with algal turfs outside territories, the nature of this coupling was fundamentally different inside damselfish territories, with significantly longer algal turfs and higher detritus levels prevailing, irrespective of sediment loads. These modified algal turf-sediment relationships may be a key factor in explaining, a) the significantly higher productivity levels reported from within farming damselfish territories and b) the ability of farming damselfishes to persist in high-sediment locations.

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.

Damselfishes alleviate the impacts of sediments on host corals

Mutualisms play a critical role in ecological communities; however, the importance and prevalence of mutualistic associations can be modified by external stressors. On coral reefs, elevated sediment deposition can be a major stressor reducing the health of corals and reef resilience. Here, we investigated the influence of severe sedimentation on the mutualistic relationship between small damselfishes (Pomacentrus moluccensis and Dascyllus aruanus) and their coral host (Pocillopora damicornis). In an aquarium experiment, corals were exposed to sedimentation rates of approximately 100 mg cm-2 d-1, with and without fishes present, to test whether: (i) fishes influence the accumulation of sediments on coral hosts, and (ii) fishes moderate partial colony mortality and/or coral tissue condition. Colonies with fishes accumulated much less sediment compared with colonies without fishes, and this effect was strongest for colonies with D. aruanus (fivefold less sediment than controls) as opposed to P. moluccensis (twofold less sediment than controls). Colonies with symbiont fishes also had up to 10-fold less sediment-induced partial mortality, as well as higher chlorophyll and protein concentrations. These results demonstrate that fish mutualisms vary in the strength of their benefits, and indicate that some mutualistic or facilitative interactions might become more important for species health and resilience at high-stress levels.