Reef System Research Articles

Turbidity estimation from an acoustic backscatter signal in a tropical coral reef system.

Turbidity estimation from an acoustic backscatter signal in a tropical coral reef system.

Multistage selective diagenesis increasing poro-perm properties of Anisian sponge-microbial-coral patch reefs and adjacent facies in the Muschelkalk of Upper Silesia, Southern Poland

Carbonate rocks, particularly reefs and circum-reef facies, are significant reservoir rocks whose reservoir properties depend on their primary composition and diagenetic history. This paper reconstructs the influence of multistage selective diagenesis on the pore system evolution and porosity-permeability properties of the Anisian tropical reef system (Karchowice and Diplopora Beds) in the Muschelkalk of Upper Silesia, southern Poland, using field and slab observations, standard and cathodoluminescence petrography, bulk and SIMS carbon and oxygen stable isotope analyses, porosity-permeability plug study, and computer image analysis of macroporosity (>5 mm across). The reef system comprises two distinct levels of sponge-microbial(-coral) reef mounds surrounded by diverse bioclastic, peloidal, coated-grain, and micritic facies rich in Balanoglossites burrows. The strata experienced complex diagenetic alterations related to various fluids (marine, evaporitic, meteoric) and diagenetic environments (shallow substratal, burial, telogenetic), including (in chronological order): 1) early-marine precipitation of isopachous fibrous rims; 2) early dissolution during emersion; 3) brine-reflux dolomitization; 4) patchy silicification; 5) recrystallization; 6) burial calcite cementation by heated marine fluid; 7) deposition of vadose silt; 8) meteoric calcite cementation; 9) chemical compaction; 10) fracturing; and 11) final karstification, dedolomitization, FeO precipitation, and Liesegang ring formation by meteoric water during Cenozoic telogenesis. Many processes were selective and affected the same (more permeable) parts of the rocks. The strata exhibit porosities reaching 50% (combined plug porosity of 6–35% and macroporosity of up to 20%) and plug permeabilities of 0.07–70 mD, with the highest values occurring in the reefs and burrowed facies. The dominant late-stage telogenic dissolution pores originated from selective removal of dolomitized micrite within burrows and between sponge-microbial automicrite as well as dolomite crystals. The study illustrates the transition of the reef system from tight to porous due to multistage selective diagenesis and fluid flow, which enhanced subtle permeability differences between various types of micrite. This is the first case where four different fluids (dolomitizing, early dissolving, calcite-forming, and late dissolving) selectively migrated through the same rock parts at different times and modified them one after another.

Oceanographic‐informed coral out‐planting density for coral reef restoration activities

Out‐planting corals onto coral reefs as a mechanism to increase coral cover on degraded reef systems is an increasing global activity. Current practices typically focus on asexual out‐planting of coral fragments (coral gardening). Newer sexual reproductive approaches are being developed to out‐plant younger corals produced in aquaculture facilities or sea‐based rearing pools to enable assisted evolution or assisted gene flow conservation genetics approaches. Designing out‐planting deployment operations requires decisions to be made on planting density and location in order to maximize future recruitment on both the out‐planted reefs, and on nearby reefs that are connected through oceanographic transport processes. The thinking presented here seeks to unpack how out‐planting density may be influenced, especially by local oceanographic conditions, in order to guide practitioners undertaking restoration projects.

Carbon isotopic record of a platform-to-basin transect through the Permian Reef Complex (Guadalupian) in the Delaware Basin of Texas and New Mexico

The Guadalupian Permian Reef Complex of the Delaware Basin is one of the most studied carbonate reef systems of the Paleozoic. Despite extensive work on the carbonate sedimentology, sequence stratigraphy, and diagenetic history of the Delaware Basin, a high-resolution carbonate carbon isotope record along a platform to basin transect for the Capitanian (264.3–259.5 Ma, the youngest age of the Guadalupian Epoch) does not yet exist. The carbon isotopic record of the Delaware Basin is important because 1) it allows us to test hypotheses about controls on the carbon isotope proxy, 2) it provides constraints on how well modern carbonate platforms like the Great Bahama Banks serve as analogues for ancient carbonate settings, and 3) these types of restricted basins likely played an important role in Permian carbon cycling and the Capitanian extinctions.In this study we present 493 new Capitanian carbonate carbon isotopic values paired with a detailed sedimentological and sequence stratigraphic framework from the platform, slope, toe of slope, and deep basin of the Delaware Basin. The bulk of the new δ13C values fall within the range of previously reported unaltered carbonates from the basin, suggesting that these results record primary environmental processes and were not significantly altered by diagenetic overprinting. With this dataset, we test hypotheses about sources of carbon isotopic variability in shallow carbonate platforms. Our results indicate that in the Delaware Basin there are no systematic and resolvable depth or lateral gradients in carbon isotopic values, that δ13C values do not vary as a function of grain type, and that there is no resolvable relationship between carbon isotopic composition and sea level change. However, we do document statistically significant differences in δ13C distributions among facies associations which we attribute to the isotopic evolution of an upwelling water mass due to direct precipitation of mud along the slope. Our results support the idea that increasing carbon isotopic values through the Capitanian were driven by increased organic carbon burial in restricted basins.

Bacterial and Symbiodiniaceae communities’ variation in corals with distinct traits and geographical distribution

Coral microbiomes play crucial roles in holobiont homeostasis and adaptation. The host’s ability to populate broad ecological niches and to cope with environmental changes seems to be related to the flexibility of the coral microbiome. By means of high-throughput DNA sequencing we characterized simultaneously both bacterial (16S rRNA) and Symbiodiniaceae (ITS2) communities of four reef-building coral species (Mussismilia braziliensis, Mussismilia harttii, Montastraea cavernosa, and Favia gravida) that differ in geographic distribution and niche specificity. Samples were collected in a marginal reef system (Abrolhos, Brazil) in four sites of contrasting irradiance and turbidity. Biological filters governed by the host are important in shaping corals’ microbiome structure. More structured associated microbial communities by reef site tend to occur in coral species with broader geographic and depth ranges, especially for Symbiodiniaceae, whereas the endemic and habitat-specialist host, M. braziliensis, has relatively more homogenous bacterial communities with more exclusive members. Our findings lend credence to the hypothesis that higher microbiome flexibility renders corals more adaptable to diverse environments, a trend that should be investigated in more hosts and reef areas.

Cryptofaunal communities are influenced by benthic cover and fish abundance in a large Caribbean coral reef system

AbstractSmall-sized invertebrates inhabiting hard substrates in coral reefs (a.k.a. cryptofauna) contribute substantially to reef biodiversity, but their patterns of distribution and ecological controls are poorly understood. Here, we characterized the cryptofauna community and explored “bottom-up” and “top-down” controls by benthic cover and fish abundance, respectively. We sampled the cryptofauna inhabiting the reef terrace from 13 sites along 200 km in Jardines de la Reina (Cuba), a well-preserved and protected area in the Caribbean. We counted 23,959 invertebrates of 14 higher taxa, being the most abundant Copepoda (54%), Nematoda (21%), Mollusca (7%), Ostracoda (5%), Polychaeta (5%), and Amphipoda (3%). Richness, abundance, and community structure varied across the reefs without any geographical gradient of distribution. One-third of the variance occurred at site scale (~ 10 km), and half occurred at quadrat scale (~ 1 m). Algal cover promoted cryptofauna richness and abundance likely providing substrate and food, while live coral cover negatively influenced nematode abundances, potentially due to coral defenses. Relationships between cryptofauna and reef fishes were also present, with invertivores and herbivores negatively affecting cryptofauna abundance likely due to direct or indirect predation pressures. This research highlights the important roles of bottom-up and top-down controls, by algal/coral cover and fishes, respectively, on cryptofauna and in extension to coral reef biodiversity. Current threats by climate change are expected to alter these controls on cryptofauna resulting in changes to diversity, trophodynamics and energy flows of coral reefs.

Population genomic structure of the sea urchin Diadema africanum, a relevant species in the rocky reef systems across the Macaronesian archipelagos

The sea urchin Diadema africanum is a macro-herbivore found in the rocky reef systems of the West African region and Macaronesian archipelagos. Over several decades, high densities of this species have generated marine barrens in certain areas at the Canary Islands. In contrast, more recently, during the last few years, the species has suffered mass mortality events that continue to the present day. In this study, we used 9,109 Single Nucleotide Polymorphisms (SNPs) and a fragment of a mitochondrial gene to evaluate the species’ population structure, effects of mass mortalities on its diversity, and potential local adaptation across the Canary Islands and Cabo Verde. Our research provides compelling evidence of low genomic diversity and homogeneity across the studied area for neutral markers, along with recent demographic fluctuations. The high connectivity among distant areas likely allows a rapid recovering of the populations from local mortality events. Interestingly, we also observed genomic sub-structure from 405 SNPs identified as candidate loci under selection for seawater temperature. The lack of divergence among distant sites and the low diversity found can be attributed to the species’ divergence from a small ancestral genomic pool, followed by a contemporary demographic expansion, and ongoinggene flow.

When the Light Goes Out: Distribution and Sleeping Habitat Use of Parrotfishes at Night

Understanding the diurnal and nocturnal activity of organisms is a key topic in behavioral ecology, with implications for population structure and management strategies. In reef systems, parrotfishes play crucial roles as nominal herbivores, but overfishing has led to population collapses with detrimental effects on reef functions. Parrotfish are good models to study diel rhythms, as they are often easily observed while sleeping at night. We assessed the distribution, sleeping substrate selectivity and sleeping areas of parrotfishes during the night in a subtropical rocky reef of the Southwestern Atlantic. We performed replicated visual censuses (50 × 2 m) at different depths to estimate parrotfish abundance, while quantifying the structural complexity of sleeping sites. Among the seven recorded parrotfish species, we focused on the four most abundant to determine their sleeping habitat selectivity. Active telemetry was used for the two most abundant species to define their sleeping site areas and sleeping hotspots. All four species exhibited a preference for sediment as a sleeping substrate, while avoiding zoanthids. Species distribution was influenced by depth, with Scarus zelindae predominantly sleeping in high-complexity substrate at intermediate depths and Sparisoma tuiupiranga sleeping in the rock–sand interface at deeper depths. Tagged Sparisoma axillare and Sparisoma frondosum exhibited similar sleeping areas in terms of size and geographical locations; showing high site fidelity, they returned to the same sleeping sites over consecutive nights. These results represent the first assessment of parrotfish distribution and habitat use at night in the Atlantic. This information is crucial for guiding management actions, particularly in the planning of no-take zones for fishing control and population recovery.

Sabellaria wilsoni Lana and Gruet, 1989 on the Brazilian Amazon coast: Fast growth with high mortality, production, and turnover rate

Given the enormous ecological importance of sabellariid (Annelida: Sabellariidae) reefs for marine environments, data on the life history traits of these reef builders are essential for understanding ecosystem function. We assessed the functional parameters of a population of Sabellaria wilsoni (Annelida: Sabellariidae) on the Brazilian Amazon coast, where this polychaete builds extensive reef systems. The species had fast growth, a relatively short life span (1.30 years), and a high mortality rate (3.87 year−1). The mean annual biomass (58 g AFDM m−2) and production (233 g AFDM m−2 year−1) were very high and resulted in a high production-to-biomass ratio (P/B ratio: 4.02 year−1). The months with the highest productivity were those with the greatest reef coverage and density of worms, which coincided with the periods of reef growth. The high P/B ratio indicates the rapid replacement of biomass in this tropical population. These data support the hypothesis that S. wilsoni is an r-strategist, which is favored by the elevated temperatures and abundance of feeding resources and grains (for tube building) found on the Amazon coast. The present study is the first to produce systematic data on the population dynamics and production of a sabellariid species in the Western Atlantic Ocean. These data provide a baseline for understanding how shallow-water trophic webs function, as well as for monitoring these coastal habitats in the tropics.

Depositional Environment and Ecological Response of Bioconstructions: A Case Study of Southern China (Guizhou Province) in Moscovian-Gzhelian.

From the late Carboniferous to the early Permian, multiple pulses of glaciation and deglaciation have been caused by the LPIA. The Pennsylvanian period experienced phases of recovery, proliferation, and decline, ultimately forming a reef system distinctly different from that of the Mississippian period. During the late Bashkirian to Moscovian, the metazoan reef experienced a limited resurgence, with reef predominantly formed by chaetetid developing in the United States, northern China, and Japan. During the Kasimovian to Gzhelian, the phylloid algal reef dominated the global reef systems. In the late Pennsylvanian, bioconstruction cases and paleoenvironmental proxies in southern Guizhou Province were studied to investigate the composition, recovery, and evolutionary processes of the bioconstructions as well as their response to environmental variations during this period. Several bioconstructions have been reported in the Lumazhai section of Houchang Town, Guizhou Province, southern China, from the Moscovian to the Gzhelian. The upper Carboniferous strata are well-preserved and continuously exposed. The continuous strata, abundant fossils, and diverse bioconstructions provide excellent research materials for exploring the mutual constraints between organisms and their environment. This study identified ten microfacies, whose vertical evolution indicated significant changes in the depositional environment related to relative sea-level fluctuations. Skeletal grains are widely present in these facies. Among them, foraminifera, algae, bryozoans, crinoids, and Tubiphytes are the most common and exhibit distinct distribution characteristics in various environments. Quantitative statistics, CCA and theoretical ecospace have been utilized to examine and interpret environmental impact factors. Quantitative analysis of their relative abundance and distribution patterns provides insights into the complex interactions between organisms and environmental factors. The relative abundances of different organisms and factors controlling their bioconstructions are influenced by relative sea-level changes. CCA analysis reveal that hydrodynamic conditions are the primary influencing factor. Variation trends in average tiering and motility reveal the characteristics of biological communities during environmental changes in phylloid algae and microbial bioconstructions. These bioconstructions are not directly correlated with changes in environmental factors, and the biological communities in phylloid algae mounds and biostromes exhibit similar organism compositions and ecological niches across different environments.

Influence of submarine groundwater discharge on the nutrient dynamics of a fringing-reef lagoon

Study regionThis study investigates nutrient distribution and flux dynamics in a coral reef lagoon in Quintana Roo, Mexico, located on a permeable limestone coast of the Mesoamerican Barrier Reef System. Study FocusEmphasis is placed on submarine groundwater discharge (SGD) as a crucial contributor to nutrient pathways, including ammonium (NH4+), nitrate and nitrite (NOx-), hydrogen silicate (HSiO3-), hydrogen phosphate (HPO42-), and urea. Inputs vary with SGD magnitudes and sources and by proximity to active spring discharges. Groundwater multi-tracer analysis and multiple linear regression identify 226Ra as explaining NH4+ variability due to long-term groundwater processes, while 223Ra predicts NOx-, HSiO3-, and urea due to short-term inputs. No significant relationship was found between HPO42- and any radium isotope, indicating complex behavior in coastal karst aquifers. New hydrological insights for the regionThe findings highlight complex nutrient dynamics in coastal karst settings, with SGD-derived fluxes primarily consisting of dissolved inorganic nitrogen (DIN) and HSiO3-. Although lower in concentration, HPO42- and urea fluxes are significant compared to other karst environments. Radium isotopes distinguish between short-term and long-term, as well as new and recycled nutrient inputs. Groundwater inputs transport fresh nutrients to healthier reefs, whereas processed, recycled inputs were detected near degraded reefs. These insights are essential for understanding global nutrient cycles and coral health, particularly in the context of global change and anthropogenic disturbances affecting coral reef ecosystems.

Coral Reef Water Microbial Communities of Jardines de la Reina, Cuba.

Globally, coral reef ecosystems are undergoing significant change related to climate change and anthropogenic activities. Yet, the Cuban archipelago of Jardines de la Reina (JR) has experienced fewer stressors due to its geographical remoteness and high level of conservation. This study examines the surface and benthic reef water microbial communities associated with 32 reef sites along the JR archipelago and explores the relationship between the community composition of reef microorganisms examined using bacterial and archaeal small subunit ribosomal RNA gene (16S rRNA gene) sequencing compared to geographic, conservation/protection level, environmental, physicochemical, and reef benthic and pelagic community features. Reef nutrient concentrations were low and microbial communities dominated by picocyanobacteria and SAR11 and SAR86 clade bacteria, characteristic of an oligotrophic system. Reef water microbial community alpha and beta diversity both varied throughout the archipelago and were strongly related to geography. Three sites in the western archipelago showed unique microbial communities, which may be related to the hydrogeography and influences of the channels linking the Ana Maria gulf with the Caribbean Sea. Overall, this work provides the first extensive description of the reef microbial ecology of the Caribbean's 'Crown Jewel' reef system and a framework to evaluate the influence of ongoing stressors on the reef microorganisms.

Cushion sea-star removal enhances coral restoration practices and limits background mortality on recovering reefs

The capacity for natural coral reef recovery and the effectiveness of active restoration efforts are often contingent upon uncertain and understudied background variables, such as chronic predation for example. In the Maldives, small coral colonies (< 10 cm), primarily from the genera Pocillopora and Acropora (often found recolonising degraded reefs) are frequently predated on by the spiny cushion sea-star (Culcita schmideliana). Incidentally, these same corals (especially Acroporids) are often prioritised in active reef restoration practices. However, the level of risk these corallivores pose on restoration success has not yet been assessed. Here, we aimed to initially document the population densities of C. schmideliana on a degraded reef system in the Maldives (Kunfunadhoo, Baa Atoll). We then assessed their associated predatory effects on coral recruits and transplants, and explored the benefits of C. schmideliana removal on the survival of these corals. Population densities ranged between 1.2 and 3.3 individuals per 100 m2, which resulted in high predation rates on coral recruits (4 – 20%) and transplants (11 – 43%). Culcita schmideliana predation accounted for the majority of the documented mortality (85%). Where C. schmideliana were removed, a significant increase in survival for recruits (9% higher) and transplants (24% higher) was shown. Further observations linked a cessation of C. schmideliana removal to a significant rise in predation instances for previously protected corals (up to 52%). Our study therefore highlights the severe impacts of C. schmideliana predation and shows Culcita spp. population management as a viable passive solution for effective reef restoration.

The Abrolhos Nominally Herbivorous Coral Reef Fish Acanthurus chirurgus, Kyphosus sp., Scarus trispinosus, and Sparisoma axillare Have Similarities in Feeding But Species-Specific Microbiomes

Coral reefs rely heavily on reef fish for their health, yet overfishing has resulted in their decline, leading to an increase in fast-growing algae and changes in reef ecosystems, a phenomenon described as the phase-shift. A clearer understanding of the intricate interplay between herbivorous, their food, and their gut microbiomes could enhance reef health. This study examines the gut microbiome and isotopic markers (δ13C and δ15N) of four key nominally herbivorous reef fish species (Acanthurus chirurgus, Kyphosus sp., Scarus trispinosus, and Sparisoma axillare) in the Southwestern Atlantic’s Abrolhos Reef systems. Approximately 16.8 million 16S rRNA sequences were produced for the four fish species, with an average of 317,047 ± 57,007 per species. Bacteria such as Proteobacteria, Firmicutes, and Cyanobacteria were prevalent in their microbiomes. These fish show unique microbiomes that result from co-diversification, diet, and restricted movement. Coral-associated bacteria (Endozoicomonas, Rhizobia, and Ruegeria) were found in abundance in the gut contents of the parrotfish species Sc. trispinosus and Sp. axillare. These parrotfishes could aid coral health by disseminating such beneficial bacteria across the reef. Meanwhile, Kyphosus sp. predominantly had Pirellulaceae and Rhodobacteraceae. Four fish species had a diet composed of turf components (filamentous Cyanobacteria) and brown algae (Dictyopteris). They also had similar isotopic niches, suggesting they shared food sources. A significant difference was observed between the isotopic signature of fish muscular gut tissue and gut contents, pointing to the role that host genetics and gut microbes play in differentiating fish tissues.

Multiple Non-Destructive Approaches to Analysis of the Early Silurian Chain Coral Halysites from South China.

Cnidarians are among the most important diploblastic organisms, elucidating many of the early stages of Metazoan evolution. However, Cnidarian fossils from Cambrian deposits have been rarely documented, mainly due to difficulties in identifying early Cnidarian representatives. Halysites, a tabulate coral from Silurian reef systems, serves as a crucial taxon for interpreting Cambrian cnidarians. Traditionally, the biological characteristics of Halysites have been analyzed using methods limited by pretreatment requirements (destructive testing) and the chamber size capacity of relevant analytical instruments. These constraints often lead to irreversible information loss and inadequate data extraction. This means that, to date, there has been no high-resolution three-dimensional mineralization analysis of Halysites. This study aims to introduce novel, non-destructive techniques to analyze the internal structure and chemical composition of Halysites. Furthermore, it seeks to elucidate the relationship between coral organisms and biomineralization in reef settings and to compare Silurian Tabulata with putative Cambrian cnidarians. Techniques such as micro-X-ray fluorescence spectrometry (micro-XRF), micro-X-ray computed tomography (micro-CT), and scanning electron microscopy (SEM) were employed in this research. With the help of high-resolution micro-CT scanning, we identify the growth pattern of Halysites, showing both lateral and vertical development. The lateral multiple-branching growth pattern of Halysites corals is first established herein. The flaggy corallite at the initial stage of branching is also observed. The micro-XRF mapping results reveal the occurrence of septa spines for Halysites, a trait previously thought rare or absent. Additionally, the ratio of coral volume to the surrounding rock was assessed, revealing that Halysites reefs were relatively sparse (volume ratio = ~30%). The cavities between Halysites likely provided more space for other organisms (e.g., rugose corals and bryozoans) when compared to other coral reef types. Additionally, we provide a comparative analysis of post-Cambrian colonial calcareous skeletons, offering insights into the structural features and growth patterns of early skeletal metazoans across the Ediacaran-Cambrian boundary.

Hurricane Irma Linked to Coral Skeletal Density Shifts on the Florida Keys Reef Tract.

Coral reefs are at risk due to various global and local anthropogenic stressors that impact the health of reef ecosystems worldwide. The most recent climate models predict that climate change will increase the frequency and intensity of tropical storms. This increased storm occurrence and strength will likely compromise coral reef structures and habitats for reef-dwelling organisms, including across the Florida Keys Reef Tract (FKRT), the most extensive tropical reef system along the US coast. While several recent studies reveal the chronic impacts of tropical storms on corals, relatively little is known about the effects of major storm events on coral growth and how these effects vary over spatiotemporal scales. Here, I characterize the skeletal growth of two common Caribbean reef-building coral species, Siderastrea siderea and Pseudodiploria strigosa, before and after Hurricane Irma to investigate the storm's impact on coral skeletal growth on inner and outer reefs of the FKRT. Coral cores were extracted from both species at four inner and four outer reef sites in May 2015, before Hurricane Irma struck the Florida Keys in September 2017. Subsequently, 33 micro-cores were collected in May 2019, two years after the storm traversed our previously cored coral colonies. A three-way ANOVA model with storm, species, and reef location as the three factors was used to assess the impact of the storm on each of three growth parameters: skeletal density, linear extension, and calcification rates. Results reveal no difference in the coral annual skeletal growth parameters pre- and post-Hurricane Irma, although previously quantified differences in these growth parameters across species and location were observed. However, analysis of the "yearly" change in annual skeletal growth parameters showed significant differences in skeletal density across groups before and after Hurricane Irma, but not for linear extension and calcification rates. Our findings improve an understanding of the impacts of tropical storms on coral skeletal growth and offer new insights into how we can employ corals' innate growth capacities to help conserve coral reefs under climate change.

Niche expansion of archaeocyaths during their palaeogeographic migration: Evidence from the Chengjiang Biota

Archaeocyaths were among the iconic metazoans of the early Cambrian seas and dominated the earliest Phanerozoic reef system before their dramatic decline toward the end of the early Cambrian time (c. 510 million years ago (Ma)). Over their almost 200-year research history, these organisms have been recognized as stenobionts adapted to a very narrow range of carbonate-dominated environments. Here, for the first time, we report three sorts of archaeocyath-like fossils from the Chengjiang Biota (c. 518 Ma), which was developed on a siliciclastic shelf devoid of any reefal buildups. These fossils are preserved in storm-induced mudstone without evidence of fragmentation during transportation. All of them share the same preservation mode involving replacement by clay minerals and iron oxides but exhibit archaeocyath-type external skeletal features. One species, which used to be interpreted as a dasycladalean alga, shows an aquiferous system and a skeletal ontogeny that is typical of archaeocyaths. It can be assigned to Propriolynthus sp. The other two species are comparable with representatives of the superfamily Ethmocoscinoidea and the family Sigmocyathidae, respectively. Propriolynthus sp. represents the first recognized one-walled archaeocyath in South China and fills a missing link in the migration route of one-walled archaeocyaths, although not in the phase of a reef-builder or dweller. This data reveals new aspects of archaeocyath evolution by niche expansion into siliciclastic-dominated seas at the acme of their diversification.

Fine‐grained sediment production by endolithic sponges on Caribbean coral reefs

AbstractEndolithic sponges are key players in carbonate cycling on coral reef systems. While their bioerosion of reef framework is relatively well studied, their role in biogenic sediment generation is poorly understood. In this study, the sedimentary attributes and production rates of eight Caribbean endolithic sponge species were characterized. The findings revealed notable species‐specific variations in sediment production rates (range: 1.0–6.3 kg CaCO3 m−2 yr−1), alongside consistency in sediment characteristics (modal sizes ranging from 39 to 48 μm). The species‐specific rates were used to explore reef‐scale variations in sponge‐derived sediment generation across 50 reef sites in the Mexican Caribbean. Significant between‐site variability was observed, with estimated annual sediment production ranging from &lt; 0.01 to 0.84 kg m−2 of reef. Production was primarily driven by sponge abundance, with discernible spatial variations in the contributions of different species, which could be related to variability in local environmental conditions. The rapidity at which these sediment producers can utilize dead coral substrates, coupled with their positive response to the changing marine environment, highlights the growing importance of these sponges in modern degraded Caribbean reef systems.

Feeding biology of crown-of-thorns seastars across sites differing in Acropora availability

Crown-of-thorns seastars (COTS, Acanthaster spp.) are a major contributor to coral mortality across the Indo-Pacific and can cause extensive reef degradation. The diet preferences of COTS can influence coral community structure by predation on fast-growing genera such as Acropora and avoidance of rare coral genera. In non-outbreaking populations, this preference can increase species diversity. The feeding biology of Acanthaster cf. solaris was compared at two sites (Shark Alley and Second Lagoon) on One Tree Island reef, located in the southern Great Barrier Reef, to determine whether the availability of Acropora influences differences in COTS movement, feeding preference and feeding rates within the same reef system. Acanthaster cf. solaris were tracked daily for five days across both sites, with measurements of movement, feeding scars and coral composition recorded over this time. While Shark Alley and Second Lagoon have similar live coral cover (40 and 44 % respectively), Shark Alley has significantly lower Acropora availability than Second Lagoon (2 vs 32 %). The feeding rate of COTS was significantly different between Shark Alley and Second Lagoon (259.8 and 733.8 cm2 of coral per day, respectively), but did not differ between seastar size (25–40 cm and >40 cm). Acanthaster cf. solaris showed preference for Pocillopora, Seriatopora, Acropora and Isopora and an avoidance of Porites at both sites. The results suggest that for coral reef sites where Acropora is not dominant, COTS outbreaks may be less likely to initiate, with comparatively low feeding rates found in comparison to coral reefs where Acropora is dominant.

Experimental study on the settling motion of coral grains in still water

Understanding settling motion of coral grains is important in terms of protection of coral reef systems and resotoration of the associated ecosystems. In this paper, a series of laboratory experiments was conducted to investigate the settling motion, using optical microscopy to measure shape parameters of coral grains and the particle-filtering-based object tracking to reconstruct the three-dimensional trajectory. Three characteristic descent regimes, namely, tumbling, chaotic and fluttering, are classified based on the three-dimensional trajectory, the spiral radius variation and the velocity spectrum. It is demonstrated that if one randomly picks up one coral grain, then the probabilities of occurrence of the three regimes are approximately $26\,\%$ , $42\,\%$ and $32\,\%$ , respectively. We have shown that first, the dimensionless settling velocity generally increases with the non-dimensional diameter and Corey shape factor and second, the drag coefficient generally decreases with the Reynolds number and Corey shape factor. Based on this, the applicability of existing models on predicting settling velocity and drag coefficient for coral grains is demonstrated further. Finally, we have proposed extended models for predicting the settling velocity. This study contributes to better understanding of settling motion and improves our predictive capacity of settling velocity for coral grains with complex geometry.