Reef Framework Research Articles

Crustose coralline algae increased framework and diversity on ancient coral reefs.

Crustose coralline algae (CCA) are key producers of carbonate sediment on reefs today. Despite their importance in modern reef ecosystems, the long-term relationship of CCA with reef development has not been quantitatively assessed in the fossil record. This study includes data from 128 Cenozoic coral reefs collected from the Paleobiology Database, the Paleoreefs Database, as well as the original literature and assesses the correlation of CCA abundance with taxonomic diversity (both corals and reef dwellers) and framework of fossil coral reefs. Chi-squared tests show reef type is significantly correlated with CCA abundance and post-hoc tests indicate higher involvement of CCA is associated with stronger reef structure. Additionally, general linear models show coral reefs with higher amounts of CCA had a higher diversity of reef-dwelling organisms. These data have important implications for paleoecology as they demonstrate that CCA increased building capacity, structural integrity, and diversity of ancient coral reefs. The analyses presented here demonstrate that the function of CCA on modern coral reefs is similar to their function on Cenozoic reefs; thus, studies of ancient coral reef collapse are even more meaningful as modern analogues.

Reef Habitat Type and Spatial Extent as Interacting Controls on Platform-Scale Carbonate Budgets

A coral reefs carbonate budget strongly influences reef structural complexity and net reef growth potential, and thus is increasingly recognised as a key “health” metric. Despite this, understanding of habitat specific budget states, how these scale across reef platforms, and our ability to quantify both framework and sediment production values remains limited. Here we use in-situ census data from an atoll rim reef platform in the central Maldives to quantify rates of both reef framework and sediment production and loss within different platform habitats, and then combine these data with high-resolution habitat maps to quantify contributions to platform wide carbonate budgets. The net reef framework budget for the entire platform is extremely low (0.12 G, where G = Kg CaCO3 m-2 yr-1), with a very high proportion (143,745 kg or 65.1%) of total framework production generated within the platform margin reef zones, despite these comprising only ~8% of platform area. Net platform-scale sediment budgets are higher (1.04 G), but most is produced in the reef and platform margin hardground habitats, of which ~80% derives from parrotfish bioerosion. Significant quantities of new sediment (up to ~1 G derived from the calcareous green algae Halimeda) are produced only in one habitat. All lagoonal habitats have negative or neutral net carbonate budgets. These data demonstrate the marked inter-habitat differences in reef carbonate budgets that occur across reef platforms, and the major dampening effect on overall platform scale budgets when rates are factored for habitat type and size. Furthermore, the data highlights the disproportionately important role that relatively small areas of reef habitat can have on the maintenance of net positive platform scale budgets. Because of the intrinsic link between carbonate production rates and reef-associated landform development and maintenance, these findings also have implications for understanding reef-associated landform stability. In this context the reef island at this site has been highly mobile over the last ~40 years, and we hypothesise that such instability may be being exacerbated by the measured low overall rates of framework and sediment generation.

Coral reefs near the Eocene–Oligocene boundary in the northern Transylvanian Basin, Romania: Composition and paleoenvironmental interpretation

Eocene–Oligocene reefs have been reported in Europe largely from the circum‐Mediterranean region. In this paper, small coral reefs from the northwestern Transylvanian Basin (Romania) are described for the first time. They developed near the Eocene–Oligocene boundary, most probably during the Priabonian, and belong to northernmost outposts of the reef belt developed during this time in Europe. The studied sedimentary successions in Letca and Băbeni–Cuciulat (Sălaj County), up to 55 m thick, belong to the Cozla Formation. The reefs occur within a shallow‐water succession composed mostly of bedded limestones, dominated by bioclastic (coralline) packstones. Low‐relief (constratal) reefs, locally up to 10–15 m in thickness, are spaced cluster (matrix‐supported) reefs. Scleractinian corals are common but poorly diversified (10 species and 8 genera). Branching ramose colonies, branching low‐integrated phaceloid, and sheet‐like (foliaceous) corals dominate. Neither lateral zonation nor vertical succession of reefs was recognized. Corals co‐occur with encrusting and geniculate red algae, but they are of subordinate significance for a nonrigid reef framework. Branching corals baffled or trapped suspended carbonate mud that contributed to the reef growth and ongoing development of topographic relief. Associated fossils are of low to moderate diversity. A relatively low‐energy environment, moderate to high sedimentation rate, and increased turbidity are inferred from carbonate muddy and fine‐grained matrix, dominance of sediment‐resistant corals, their morphology, common occurrence in growth position, as well as low to moderate degree of bioerosion and encrustation. Transylvanian reefs in terms of poor coral diversity, matrix‐supported texture, and turbid‐water sedimentary setting show similarities with many coeval reefs from the circum‐Tethyan area.

Distribution of platform edge reefs and beach as well as their major controlling factors over the Changxing-Feixianguan formations in northeast Sichuan Basin, China

The platform facie, platform edge facie, slope, and basin facie were developed in the Chengkou-Western Hubei Trough zones during the Upper Permian Changxing Formation's Early Triassic Feixianguan period; during this time the platform edge had the most favorable sedimentary facie. The Changxing Formation was composed of three sections in which the second section was rich in framework reef, whereas the third one was rich in micro-reefs at the bottom. Recently, it has been discovered that abundant platform edge shoals are developed in the third member and that oolitic shoals with large thickness are widely distributed. This paper systematically investigated the influence of the changes in reef-building organism system on the development of organic reefs. The results show that the distribution of bank belts is determined through paleogeography framework. The rapid change of relative sea level affects the formation and movement of bank belts. In addition, the regional tectonic activity controls the distribution and transformation of the bank belts. Finally, the prospective zone in the Chengkou-Western Hubei area was evaluated and divided into four favorable zones, in which the Wanyuan-Yunyang area was deemed to be most advantageous.

Seasonal growth and calcification of a reef-building crustose coralline alga on the Great Barrier Reef

Crustose coralline algae (CCA) play a central role in tropical reef ecology, helping to build and stabilise the reef framework and, due to their high Mg-calcite skeleton, are potential indicators for assessing the impacts of ocean acidification. However, basic information on CCA seasonal growth and calcification is relatively unknown on the Great Barrier Reef (GBR). This study provides seasonal baseline rates of vertical growth, marginal (horizontal) growth and calcification of the abundant CCA species Porolithon onkodes, together with the effect of reduced irradiance on these metrics to better understand the natural variability between CCA living in exposed and shaded areas. Seasonal variation was detected in each of the metrics, with maximum vertical growth and calcification observed in spring and marginal extension in autumn. Annual vertical growth rate was 1.45 mm yr(-1), absolute marginal growth rate (surface area) was 11.12 cm(2) yr(-1), and absolute marginal extension rate (diameter) was 24.66 mm yr(-1). Net calcification, determined using the buoyant weight method (includes secondary calcification deposits) was approximately 2.72 to 3.40 g CaCO3 cm(-2) yr(-1), while gross calcification determined using growth metrics (just newly deposited CaCO3) was 0.43 to 0.59 g CaCO3 cm(-2) yr(-1). Shaded treatments yielded higher growth and gross calcification rates compared to exposed in all seasons except summer. This data provides empirical information necessary to monitor the impacts of future climate change on the GBR and to better understand the response of CCA to manipulative experiments on ocean acidification and warming.

Subsurface Permian reef complexes of southern Tunisia: Shelf carbonate setting and paleogeographic implications

2-D seismic reflection sections, borehole data as well as published and unpublished data have been investigated to reconstruct the paleogeography of southern Tunisia during Middle to Late Permian times. Paleogeographical reconstruction based on the integration of petroleum well data and 2-D seismic facies interpretation shows three main depositional areas with very contrasting sedimentary pile. These are 1) a subsiding basin; 2) an outer shelf carbonate, and 3) an inner shelf carbonate. Based on typical electric responses of reef buildups to seismic wave, we shall urge that during Middle Permian times, the outer carbonate shelf was subject of reef barrier development. Lithology evidences from core samples show that reef framework correspond mainly to fossiliferous limestone and dolomite. The WNW-ESE recognized reef barrier led between latitudes 33° 10′ 00″N and 33° 20′ 00″N. The Tebaga of Medenine outcrop constitutes the northern-edge of this barrier. Westward it may be extended to Bir Soltane area whereas its extension eastward is still to be determined. Biogenic buildups took place preferentially over faulted Carboniferous and lower Paleozoic paleohighs resulting likely from the Hercynian orogeny. The subsiding basin is located north of Tebaga of Medenine outcrop where Upper Permian sedimentary sequence is made entirely of 4000 m deep marine green silty shale facies. These are ascribed to unorganized and chaotic reflectors. Inner carbonate shelf facies succession corresponds to a typical interbedding of shallow marine carbonate deposits, shale, dolomite, and anhydrite inducing parallel-layered of strong amplitude and good continuity reflectors. Also within the inner carbonate shelf patch reef or reef pinnacles have been identified based on their seismic signature particularly their low vertical development as compared to reef complexes. Southward, towards Sidi Toui area, the Upper Permian depositional sequence thins out and bears witness of land influences as entailed by the increase of silicoclastic sedimentary supply and the lack of marine fossil.

Coastal plain stratigraphy records tectonic, environmental, and human habitability changes related to sea-level drawdown, ‘Upolu, Sāmoa

AbstractCoastal plain stratigraphy is often over looked in paleo–sea-level reconstructions because carbonate sediments do not precisely constrain former sea level. Pacific Island sedimentology provides an invaluable record of geomorphic and environmental consequences of coastal evolution in response to changes in sea level and local tectonics. A series of coastal auger cores obtained from eastern ʻUpolu reveal a subsurface carbonate sand envelope predominately composed of coral and coralline algae derived from the reef framework. Coupling the sedimentological record with geophysical models of Holocene sea level, we identify a critical value (0.3–1.0 m) during the falling phase of the sea-level high stand (1899–2103 cal yr BP) that represents the transition from a transgressive to a regressive environment and initiates coastal progradation. Correlating the critical value with time, we observe nearly a millennium of coastal plain development is required before a small human population is established. Our findings support previous studies arguing that Sāmoa was colonized by small and isolated groups, as post–mid-Holocene drawdown in regional sea level produced coastal settings that were morphologically attractive for human settlement. As future sea level approaches mid-Holocene high stand values, lessons learned from Pacific Island sedimentological records may be useful in guiding future decisions related to coastal processes and habitat suitability.

Microencruster-microbial framework and synsedimentary cements in the Štramberk Limestone (Carpathians, Czech Republic): Insights into reef zonation

The Stramberk Limestone (Tithonian–lower Berriasian) was developed on a northerly located, isolated intra-Tethyan carbonate platform. It is composed of various facies that can be observed in olistoliths and blocks embedded in the Cretaceous flysch of the Outer Carpathians in Moravia (Czech Republic). Corals, microbialites, microencrusters and synsedimentary cements contributed on various scales to the reef framework. The importance of corals and some microencrusters to the formation of the Stramberk reef complex is well recognized, while other components received less attention in previous studies. Two end members of boundstone types are described from the Kotouc Quarry, near Stramberk. Boundstone type A is dominated by phaceloid (branching-type) corals, encrusted by microbialites and microencrusters, in particular photophile species (“ Lithocodium-Bacinella”, Koskinobullina socialis Cherchi et Schroeder, Iberopora bodeuri Granier et Berthou). Boundstone type B is composed of microencrusters, microbialites and synsedimentary isopachous fibrous cements, while corals are absent or subordinate. Microencrusters [ Crescentiella morronensis (Crescenti) , Labes atramentosa Eliasova , Perturbatacrusta leini Schlagintweit et Gawlick , Radiomura cautica Senowbari-Daryan et Schafer , thin encrusting calcified sponges] are main biotic components of the microencruster-cement boundstone. Some identified microencrusters are known only or mostly from intra-Tethyan carbonate platforms. Except for C. morronensis, other common microencrusters in the coral-microbial boundstone (type A) are rare in the microencruster-cement boundstone (type B). The depositional setting of boundstone type A corresponds to a low-energy environment of an inner platform. Boundstone type B, until now not recognized in the Stramberk Limestone, was developed in a high-energy, upper fore-reef slope environment. Other important facies in the Kotouc Quarry are reef-derived breccias: matrix-supported breccia and clast-supported breccia with radiaxial-fibrous cement (showing some similarities to Triassic “evinosponges” cement), interpreted as being dominantly synsedimentary (pre-burial). The preliminary studies by the present authors, supported by observations under cathodoluminescence, highlight the significance of synsedimentary cementation for the formation of a boundstone framework (type B) and the stabilization of fore-reef, slope deposits.

Geochemical partitioning of lead in biogenic carbonate sediments in a coral reef depositional environment

The fate of trace elements in reef depositional environments has not been extensively investigated. The aim of this study was to determine the partitioning of Pb in sediments of the Veracruz Reef System, and its relation to local environmental sources. Lead was determined in four geochemical fractions: exchangeable (3.8±0.4μgg−1), carbonate (57.0±13.6μgg−1), organic matter (2.0±0.9μgg−1), and mineral (17.5±5.4μgg−1). For the mineral fraction, lead concentrations were higher in those reefs influenced by river discharge or by long-distance transport of terrigenous sediments. The bioavailable concentration of lead (range: 21.9–85.6μgg−1) indicates that the Veracruz Reef System is a moderately polluted area. As expected, the carbonate fraction contained the highest proportion of Pb (70%), and because the reef framework is largely made up of by biogenic carbonate sediments, hence, it is therefore the most important repository of Pb in coral reef depositional environments.

The evolution of the Great Barrier Reef during the Last Interglacial Period

Reef response to Last Interglacial (LIG) sea level and palaeoenvironmental change has been well documented at a limited number of far-field sites remote from former ice sheets. However, the age and development of LIG reefs in the Great Barrier Reef (GBR) remain poorly understood due to their location beneath modern living reefs. Here we report thirty-nine new mass spectrometry U-Th ages from seven LIG platform reefs across the northern, central and southern GBR. Two distinct geochemical populations of corals were observed, displaying activity ratios consistent with either closed or open system evolution. Our closed-system ages (~129–126ka) provide the first reliable LIG ages for the entire GBR. Combined with our open-system model ages, we are able to constrain the interval of significant LIG reef growth in the southern GBR to between ~129–121ka. Using age-elevation data in conjunction with newly defined coralgal assemblages and sedimentary facies analysis we have defined three distinct phases of LIG reef development in response to major sea level and oceanographic changes. These phases include: Phase 1 (>129ka), a shallow-water coralgal colonisation phase following initial flooding of the older, likely Marine Isotope Stage 7 (MIS7) antecedent platform; Phase 2 (~129ka), a near drowning event in response to rapid sea level rise and greater nutrient-rich upwelling and; Phase 3 (~128–121ka), establishment of significant reef framework through catch-up reef growth, initially characterised by deeper, more turbid coralgal assemblages (Phase 3a) that transition to shallow-water assemblages following sea level stabilisation (Phase 3b). Coralgal assemblage analysis indicates that the palaeoenvironments during initial reef growth phases (1 and 2) of the LIG were significantly different than the initial reef growth phases in the Holocene. However, the similar composition of ultimate shallow-water coralgal assemblages and slow reef accretion rates following stabilisation of sea level (phase 3b) suggest that reefs of both ages developed in a similar way during the main phase of relatively stable sea level.

Cryptic biostalactites in a submerged karst cave of the Belize Barrier Reef revisited: Pendant bioconstructions cemented by microbial micrite

Microbial carbonates are important components of postglacial tropical coral reef frameworks. Curious calcium carbonate protrusions in a submarine karst cave in the Belize Barrier Reef platform were initially interpreted as being largely the product of physicochemical precipitation around a meshwork of serpulid tubes. New petrographical, mineralogical, geochronological, and inorganic and organic geochemical data suggest that sulfate-reducing bacteria played a substantial role in carbonate formation, just as in cryptic settings of other modern, tropical coralgal reefs. The bacterial involvement in carbonate authigenesis is indicated by the content and isotopic composition of carbonate-associated sulfate, the presence of non-isoprenoidal sn-1-mono-O-alkyl glycerol monoethers (MAGEs) and terminally-branched fatty acids, and the observation that the majority of molecular fossils assigned to sulfate-reducing bacteria are tightly associated with the carbonate mineral lattice. The carbonate protrusions that were formerly named pseudostalactites, are termed biostalactites here to underline the significance of biological activity during carbonate formation. The Belize biostalactites are composed of four carbonate phases including serpulid tubes, lithified sediment, and microbial carbonate, as well as unconsolidated sediment usually found in macroborings of bivalve molluscs. Lithified sediment and microbial carbonate are fine-grained with clotted, mottled, and peloidal textures. While serpulid tubes are aragonitic, lithified sediment and microbial carbonates are mixtures of high-magnesium calcite and aragonite. Magnesium-calcite content increases with increasing consolidation. Lithified sediment and microbial carbonate are interpreted as fine, biodetrital sediment that was subsequently cemented by authigenic micrite. Scalenohedral high-magnesium calcite and acicular aragonite cements played a subordinate role in cementation. Carbon and oxygen stable isotope ratios of lithified sediment and microbial carbonate exhibit similar ranges as modern reefal microbialites. Our study exemplifies that heterotrophic bacteria can be a crucial agent that contributes to the formation of carbonate build-ups by cementing skeletal frameworks in cryptic environments.

Enhanced macroboring and depressed calcification drive net dissolution at high-CO2 coral reefs.

Ocean acidification (OA) impacts the physiology of diverse marine taxa; among them corals that create complex reef framework structures. Biological processes operating on coral reef frameworks remain largely unknown from naturally high-carbon-dioxide (CO2) ecosystems. For the first time, we independently quantified the response of multiple functional groups instrumental in the construction and erosion of these frameworks (accretion, macroboring, microboring, and grazing) along natural OA gradients. We deployed blocks of dead coral skeleton for roughly 2 years at two reefs in Papua New Guinea, each experiencing volcanically enriched CO2, and employed high-resolution micro-computed tomography (micro-CT) to create three-dimensional models of changing skeletal structure. OA conditions were correlated with decreased calcification and increased macroboring, primarily by annelids, representing a group of bioeroders not previously known to respond to OA. Incubation of these blocks, using the alkalinity anomaly methodology, revealed a switch from net calcification to net dissolution at a pH of roughly 7.8, within Intergovernmental Panel on Climate Change's (IPCC) predictions for global ocean waters by the end of the century. Together these data represent the first comprehensive experimental study of bioerosion and calcification from a naturally high-CO2 reef ecosystem, where the processes of accelerated erosion and depressed calcification have combined to alter the permanence of this essential framework habitat.

Influence of anthropogenic pressure and seasonal upwelling on coral reefs in Nha Trang Bay (Central Vietnam)

AbstractThe growing coastal development, dredging and dumping activities, overfishing and expansion of marine cage culture in Nha Trang Bay (NTB) of Central Vietnam since the beginning of the 2000s have resulted in a dramatic decrease of live coral cover. Surveys conducted in April–May 2013 and the same period in 2014 revealed that with an increase in distance from the outer part of the bay towards the mainland, the rivers’ influxes and dredged areas, coral cover decreased from 75% to 0.6% and species richness from 63 to 5, while the abundance of macroalgae increased from 0% to 56%. These changes correlate with differences in the concentration of suspended sediments on the same gradient. The abundance of the crown‐of‐thorns starfish Acanthaster planci and of the echinoid Diadema setosum significantly increased between the first estimation in 1998 and the survey in 2014, from 0 to 1.7 individuals (ind.) per 100 m−2 and from 50.8 to 94.5 ind. per 100 m−2, respectively, contributing to coral loss and intensive bioerosion of the reef framework in the bay. The large sizes of adult colonies of tabulate Acropora on the remote stations with negligible sedimentation and eutrophication loads were inconsistent with the assumptions that temperature‐induced coral bleaching or cyclones could be the major impacts in Nha Trang Bay. Analysis of the 16‐year thermal history of the bay did not reveal any instances in which the coral thermal bleaching threshold had been exceeded up to the present study. Seasonal upwelling, which occurs annually in the vicinity of Central Vietnam, may contribute to mitigation of thermal anomalies within NTB and to the maintenance of healthy coral communities on the remote reefs with relatively low anthropogenic impact.

Palaeoecology of Ediacaran metazoan reefs

Abstract Terminal Ediacaran metazoan reefs ( c. 548–541 Ma) can be locally substantial and the skeletal metazoans Cloudina riemkeae, C. hartmannae, Namacalathus and Namapoikia produced diverse reef types with complex ecologies in association with varied microbialite support or influence. In the Nama Group, Namibia, metazoan reefs grew in three associations with differing dominant frameworks: (1) monospecific aggregations of Cloudina ; (2) Cloudina–Namacalathus –thrombolite assemblages; and (3) thrombolite-dominated metazoan communities. Cloudina hartmannae formed monospecific reefs up to 7 m wide and 3 m high with no microbialite component. The synoptic relief was probably <1 m. Cloudina riemkeae formed densely aggregating assemblages associated with microbialites and thrombolites, each from 30 to 100 mm high, which successively colonized former generations. Isolated Namacalathus either intergrew with C. riemkeae or formed dense, low-relief, monospecific aggregations succeeding C. riemkeae frameworks. Thrombolite-dominated metazoan reefs reached up to 20 m in height and width, with a synoptic relief of up to several metres. Cloudina and Namacalathus grew closely associated with these framework thrombolites and Namapoikia , which was encrusting and modular, reached up to 1 m in size and occupied neptunian dykes and fissures. Cloudina and Namacalathus also grow cryptically, either as pendent aggregations from laminar crypt ceilings in microbial framework reefs, or as clusters associated with thrombolite attached to neptunian dyke walls.

Low Florida coral calcification rates in the Plio-Pleistocene

Abstract. In geological outcrops and drill cores from reef frameworks, the skeletons of scleractinian corals are usually leached and more or less completely transformed into sparry calcite because the highly porous skeletons formed of metastable aragonite (CaCO3) undergo rapid diagenetic alteration. Upon alteration, ghost structures of the distinct annual growth bands often allow for reconstructions of annual extension ( = growth) rates, but information on skeletal density needed for reconstructions of calcification rates is invariably lost. This report presents the bulk density, extension rates and calcification rates of fossil reef corals which underwent minor diagenetic alteration only. The corals derive from unlithified shallow water carbonates of the Florida platform (south-eastern USA), which formed during four interglacial sea level highstands dated approximately 3.2, 2.9, 1.8, and 1.2 Ma in the mid-Pliocene to early Pleistocene. With regard to the preservation, the coral skeletons display smooth growth surfaces with minor volumes of marine aragonite cement within intra-skeletal porosity. Within the skeletal structures, voids are commonly present along centres of calcification which lack secondary cements. Mean extension rates were 0.44 ± 0.19 cm yr−1 (range 0.16 to 0.86 cm yr−1), mean bulk density was 0.96 ± 0.36 g cm−3 (range 0.55 to 1.83 g cm−3) and calcification rates ranged from 0.18 to 0.82 g cm−2 yr−1 (mean 0.38 ± 0.16 g cm−2 yr−1), values which are 50 % of modern shallow-water reef corals. To understand the possible mechanisms behind these low calcification rates, we compared the fossil calcification rates with those of modern zooxanthellate corals (z corals) from the Western Atlantic (WA) and Indo-Pacific calibrated against sea surface temperature (SST). In the fossil data, we found a widely analogous relationship with SST in z corals from the WA, i.e. density increases and extension rate decreases with increasing SST, but over a significantly larger temperature window during the Plio-Pleistocene. With regard to the environment of coral growth, stable isotope proxy data from the fossil corals and the overall structure of the ancient shallow marine communities are consistent with a well-mixed, open marine environment similar to the present-day Florida Reef Tract, but variably affected by intermittent upwelling. Upwelling along the platform may explain low rates of reef coral calcification and inorganic cementation, but is too localised to account also for low extension rates of Pliocene z corals throughout the tropical WA region. Low aragonite saturation on a more global scale in response to rapid glacial–interglacial CO2 cyclicity is also a potential factor, but Plio-Pleistocene atmospheric pCO2 is generally believed to have been broadly similar to the present day. Heat stress related to globally high interglacial SST only episodically moderated by intermittent upwelling affecting the Florida platform seems to be another likely reason for low calcification rates. From these observations we suggest some present coral reef systems to be endangered from future ocean warming.

Elevated Colonization of Microborers at a Volcanically Acidified Coral Reef.

Experiments have demonstrated that ocean acidification (OA) conditions projected to occur by the end of the century will slow the calcification of numerous coral species and accelerate the biological erosion of reef habitats (bioerosion). Microborers, which bore holes less than 100 μm diameter, are one of the most pervasive agents of bioerosion and are present throughout all calcium carbonate substrates within the reef environment. The response of diverse reef functional groups to OA is known from real-world ecosystems, but to date our understanding of the relationship between ocean pH and carbonate dissolution by microborers is limited to controlled laboratory experiments. Here we examine the settlement of microborers to pure mineral calcium carbonate substrates (calcite) along a natural pH gradient at a volcanically acidified reef at Maug, Commonwealth of the Northern Mariana Islands (CNMI). Colonization of pioneer microborers was higher in the lower pH waters near the vent field. Depth of microborer penetration was highly variable both among and within sites (4.2–195.5 μm) over the short duration of the study (3 mo.) and no clear relationship to increasing CO2 was observed. Calculated rates of biogenic dissolution, however, were highest at the two sites closer to the vent and were not significantly different from each other. These data represent the first evidence of OA-enhancement of microboring flora colonization in newly available substrates and provide further evidence that microborers, especially bioeroding chlorophytes, respond positively to low pH. The accelerated breakdown and dissolution of reef framework structures with OA will likely lead to declines in structural complexity and integrity, as well as possible loss of essential habitat.

Bioerosion by the sea urchin Diadema mexicanum along Eastern Tropical Pacific coral reefs

AbstractBioerosion is a natural process in coral reefs. It is fundamental to the health of these ecosystems. In the Eastern Tropical Pacific (ETP) coral reefs, the most important bioeroders are sponges, bivalves, sea urchins and the fish Arothron meleagris. In the 1980s, El Niño caused high coral mortality and an increase in macroalgal growth. As a result, greater sea urchin bioerosion occurred. This weakened the reef framework. Considering the high vulnerability of the ETP coral reefs, the goal of this study was to determine the current bioerosion impact of the sea urchin Diadema mexicanum along the western coasts of Mexico, El Salvador, Costa Rica and Panamá. The balance between coral bioaccretion and sea urchin bioerosion was also calculated. Between 2009 and 2010, in 12 coral reefs localities, D. mexicanum density, bottom cover and rugosity were quantified along band transects. The daily bioerosion rate was obtained from the amount of carbonates evacuated by sea urchins per unit time. The rate of coral accretion was calculated by multiplying the coral growth rate of the dominant genus by the density of their skeleton and by their specific coral cover. The localities were dissimilar (R = 0.765, P < 0.001) in terms of live coral cover, crustose calcareous algae, turf cover, rugosity index, and density and size of D. mexicanum. At all sites, with the exception of Bahía Culebra (Costa Rica), coral bioerosion was less than coral bioaccretion. Diadema mexicanum plays a dominant role in the balance of carbonates in the ETP, but this depends on reef condition (protection, overfishing, eutrophication) and so the impacts can be either positive or negative.

First observations of the structure and megafaunal community of a large Lophelia reef on the Ghanaian shelf (the Gulf of Guinea)

The distribution of cold-water coral reefs is relatively well known in the North-east Atlantic as compared to the Central-east Atlantic, where only a few documentations exist from low latitudes. In 2012 an initial survey was conducted on a reef situated at 400m depth on the continental shelf off Ghana. The reef corals and fauna were visually documented using a Video Assisted Multi Sampler (VAMS) coupled with an ROV. Here we present the results from three dives on the 1400m long and 70m high reef with an ambient temperatures between 9 and 10°C. The banana shaped reef was oriented perpendicular to the main current, the convex side facing the current and there was no sign of human impact. The great height of the reef is probably a result of undisturbed growth for more than 20,000 years. On the Norwegian continental shelf the largest reefs are around 30m high and have been aged to ~9000 years. The reef morphology resembles that of Northeast Atlantic Lophelia reefs. The main reef building coral was Lophelia pertusa with contribution from Madrepora oculata, Solenosmilia variabilis, and occasional occurrences of Dendrophyllia cf. alternata. The skeleton of Aphrocallistes beatrix (Hexactinellidae) contributed to the reef framework and the reef consisted of 46% coral blocks 22% sediment, 13% coral rubble, 11% sponge skeleton and 8% live corals.A rich megafauna of 31 taxa was recorded and most frequent was Acesta excavate (bivalve), Aphrocallistes beatrix (with an associated Zooanthida on 39% of the colonies), squat lobsters, hydroids and bryozoans. Six fish species were recorded of which the Sebastidae Helicolenus dactylopterus and Nettastoma melanurum were found amongst coral blocks. The reef community showed several similarities with the northern reefs with sponges, Sebastes spp., squat lobsters, and Acesta excavata being common megafauna associates. In contrast the gorgonian corals that are characteristic of the northern reefs seemed to be lacking and Hexactinellidae rather than Demospongia were common on the reef and contributed to the reef framework. Crabs that are uncommon on northern reefs were frequently encountered.

Ecological adaptations and commensal evolution of the Polynoidae (Polychaeta) in the Southwest Indian Ocean Ridge: A phylogenetic approach

The polychaete family polynoid is very large and includes a high diversity of behaviours, including numerous examples of commensal species. The comparison between free-living and commensal behaviours and the evolution of the relationships between commensal species and their hosts are valuable case studies of ecological adaptations. Deep-sea species of Polynoidae were sampled at four seamounts in the Southwest Indian Ridge and twenty specimens from seven species were selected to be analysed. Among them, there were free-living species, living within the three-dimensional framework of cold-water coral reefs, on coral rubble and on mobile sediments, and commensal species, associated with octocorals, hydrocorals (stylasterids), antipatharians and echinoderms (holothurian and ophiuroids). We analysed two mitochondrial (COI, 16S) and two nuclear (18S, 28S) ribosomal genetic markers and their combined sequences were compared with other Genbank sequences to assess the taxonomic relationships within the species under study, and the potential role of hosts in speciation processes. Most basal species of the sub-family Polynoinae are obligate symbionts showing specific morphological adaptations. Obligate and facultative commensal species and free-living species have evolved a number of times, although, according to our results, the obligate coral commensal species appear to be monophyletic.

Early recovery of sponge framework reefs after Cambrian archaeocyath extinction: Zhangxia Formation (early Cambrian Series 3), Shandong, North China

Reefs dominated by the anthaspidellid sponge Rankenella zhangxianensis, the calcimicrobe Epiphyton and the stem-group cnidarian Cambroctoconus orientalis, together with encrusting microstromatolites occur early in the middle Cambrian (Series 3, late Stage 5) of Shandong, eastern China. In the Zhangxia Formation, these in situ components created a tight framework, with centimeter-scale growth cavities mainly filled by fine-grained matrix. Among them, R. zhangxianensis and C. orientalis mutually attached and locally formed metazoan-dominated frameworks. These metazoan-microbial reefs form thin lenses <2m wide within microbial mounds, and probably developed at least a few centimeters of synoptic relief above these surrounding structures. With an age of >505Ma, these Rankenella reefs indicate recovery of framework-building metazoans within ~5millionyears of the archaeocyath reef decline. In structure, they resemble archaeocyath reefs as well as Early Ordovician lithistid sponge-microbial reefs, having conjoined conical macroskeletons thickly veneered by calcimicrobes and microbial carbonate with largely matrix-filled intervening cavities. In combination with other sponge-microbial reefs reported from Australia, Iran, Korea and the USA, they demonstrate that an anthaspidellid sponge-microbial reef consortium was widespread throughout the mid-late Cambrian.