Reef Framework Research Articles

Coral community composition and reef development at the Similan Islands, Andaman Sea, in response to strong environmental variations

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 456:113-126 (2012) - DOI: https://doi.org/10.3354/meps09682 Coral community composition and reef development at the Similan Islands, Andaman Sea, in response to strong environmental variations Gertraud M. Schmidt1,*, Niphon Phongsuwan2, Carin Jantzen1, Cornelia Roder3, Somkiat Khokiattiwong2, Claudio Richter1 1Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, 27568 Bremerhaven, Germany 2Phuket Marine Biological Center, 51 Sakdidet Road, 83000 Phuket, Thailand 34700 King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia *Email: gertraud.schmidt@awi.de ABSTRACT: The Similan Islands, a Thai archipelago in the Andaman Sea located near the shelf break, are subjected to frequent (up to several events per hour) and abrupt changes in physico-chemical conditions, particularly during the dry season (NE monsoon, January through April) and to an intense monsoon season with strong surface wave action (May to October). The exposed west slopes of the islands feature more coral species, but lack a carbonate reef framework. By contrast, the sheltered east sides show a complex reef framework dominated by massive Porites. Our results suggest that the sudden changes in temperature, pH and nutrients (drops of up to 10°C and 0.6 U and increases of up to 9.4 µmol NOx l−1, respectively) due to pulsed upwelling events may rival the importance of surface waves and storms in shaping coral distribution and reef development. KEY WORDS: Benthic−pelagic coupling · Coral community · Similan Islands · Andaman Sea · Upwelling · Internal waves Full text in pdf format Supplementary material PreviousNextCite this article as: Schmidt GM, Phongsuwan N, Jantzen C, Roder C, Khokiattiwong S, Richter C (2012) Coral community composition and reef development at the Similan Islands, Andaman Sea, in response to strong environmental variations. Mar Ecol Prog Ser 456:113-126. https://doi.org/10.3354/meps09682 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 456. Online publication date: June 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Budget of coral-derived organic carbon in a fringing coral reef of the Gulf of Aqaba, Red Sea

The continuous release of organic C-rich material by reef-building corals can contribute substantially to biogeochemical processes and concomitant rapid nutrient recycling in coral reef ecosystems. However, our current understanding of these processes is limited to platform reefs exhibiting a high degree of ecosystem closure compared to the globally most common fringing reef type. This study carried out in the northern Gulf of Aqaba (Red Sea) presents the first quantitative budget for coral-derived organic carbon (COC) in a fringing reef and highlights the importance of local hydrodynamics. Diel reef-wide COC release amounted to 1.1±0.2 kmol total organic carbon (TOC) representing 1–3% of gross benthic primary production. Most COC (73%) was released as particulate organic C (POC), the bulk of which (34–63%) rapidly settled as mucus string aggregates accounting for approximately 28% of total POC sedimentation. Sedimentation of mucus strings, but also dilution of suspended and dissolved COC in reef waters retained 82% of diel COC release in the fringing reef, providing a potentially important organic source for a COC-based food web. Pelagic COC degradation represented 0.1–1.6% of pelagic microbial respiration recycling 32% of diel retained COC. Benthic COC degradation contributed substantially (29–47%) to reef-wide microbial respiration in reef sands, including 20–38% by mucus string POC, and consumed approximately 52% of all retained COC. These findings point out the importance of COC as a C carrier for different reef types. COC may further represent a source of organic carbon for faunal communities colonising reef framework cavities complementing the efficient retention and recycling of COC within fringing reef environments.

Estimating rates of biologically driven coral reef framework production and erosion: a new census-based carbonate budget methodology and applications to the reefs of Bonaire

Census-based approaches can provide important measures of the ecological processes controlling reef carbonate production states. Here, we describe a rapid, non-destructive approach to carbonate budget assessments, termed ReefBudget that is census-based and which focuses on quantifying the relative contributions made by different biological carbonate producer/eroder groups to net reef framework carbonate production. The methodology is presently designed only for Caribbean sites, but has potential to be adapted for use in other regions. Rates are calculated using data on organism cover and abundance, combined with annual extension or production rate measures. Set against this are estimates of the rates at which bioeroding species of fish, urchins and internal substrate borers erode reef framework. Resultant data provide a measure of net rates of biologically driven carbonate production (kg CaCO3 m−2 year−1). These data have potential to be integrated into ecological assessments of reef state, to aid monitoring of temporal (same-site) changes in rates of biological carbonate production and to provide insights into the key ecological drivers of reef growth or erosion as a function of environmental change. Individual aspects of the budget methodology can also be used alongside other census approaches if deemed appropriate for specific study aims. Furthermore, the methodology spreadsheets are user-changeable, allowing local or new process/rate data to be integrated into calculations. Application of the methodology is considered at sites around Bonaire. Highest net rates of carbonate production, +9.52 to +2.30 kg CaCO3 m−2 year−1, were calculated at leeward sites, whilst lower rates, +0.98 to −0.98 kg CaCO3 m−2 year−1, were calculated at windward sites. Data are within the ranges calculated in previous budget studies and provide confidence in the production estimates the methodology generates.

Corals on the slope (Aptian, Maestrat Basin, Spain)

The term “reef” has been frequently misused when applied to fossil coral communities. Our popular but biased view of coral community structure based on the idyllic picture of recent tropical reefs has failed to recognize that, in many fossil examples, alternative states of community structure with no or limited framework may occur. The Aptian colonial scleractinians analysed in the western Maestrat Basin (eastern Spain) constitute an example of non-reef-building coral populations, which thrived in marly slope settings. These corals developed within the photic zone but below the storm wave-base. All colonies are found well-preserved in life position. They are mostly decimetres in size and mainly occur isolated giving rise to a continuous and uniform (dominated by domal and massive forms) unbound growth fabric with a low to medium degree of development (coral skeletal volume = 5–20%). Occasionally, however, colonies growing on top of each other forming small metre-sized bioherms are also present. A total of 21 species were identified. Coral diversity in each sample location varies between three and nine species. These numbers of species are comparable with those exhibited by coeval coral assemblages from other basins of the Tethys, but are comparatively low when compared with diversities exhibited by many Recent and fossil coral communities. The corals studied apparently found optimal ecological conditions for their development on the marly slopes of the western Maestrat Basin. This is primarily expressed in the unusually large dimensions (up to 2.3 m in width) of some of the coral colonies when compared to other Cretaceous occurrences, and in the persistence and resilience of the coral populations. The observed coral genera and species (suborders Archeocaeniina, Faviina, Fungiina and Microsolenina) are very common in the time interval between the Barremian and the Early Albian and most of them have been reported from several other localities in the western and central Tethyan realm. In addition, the coral-bearing levels also contain the poorly known and exotic genera Agrostyliastraea and Procladocora. There are no significant differences at species level or in community structure between the Early and Late Aptian faunas investigated. Therefore, the coral communities as well as the environmental conditions controlling them would have been relatively stable during the time intervals when these corals flourished. An important palaeoecological implication is that comparatively low species diversities and the absence of reef frameworks do not necessarily imply unfavourable environmental conditions for coral growth. Furthermore, this study may serve as an example for the analysis of other level-bottom coral communities displaying a loose growth fabric.

Photobiology of corals from Brazil’s near-shore marginal reefs of Abrolhos

Coral communities were examined from highly turbid near-shore marginal reefs of Abrolhos (Brazil) to test a paradigm previously developed from observations in clear water reefs; specifically, that coral photobiological properties follow a highly conserved linear relationship with optical depth (ζ) via preferential ‘non-photochemical’ over ‘photochemical’ dissipation of absorbed light energy. PAM flourometry in situ was used to examine the photobiology of the most dominant coral species throughout the platform surfaces and bases of Abrolhos’ characteristic ‘chapeirões’ reef framework; however, none of the species consistently adhered to the ‘clear water paradigm’. PAM measurements further demonstrated that species conformed to two different strategies of non-photochemical energy dissipation: transient but relatively rapid for the two closely related endemic species (Mussismilia braziliensis and Mussismilia harttii) as opposed to more persistent for Montastrea cavernosa, Porites astreoides and Siderastrea stellata. Further experiments demonstrated that tolerance to anomalous stress amongst species did not correspond with the non-photochemical energy dissipation strategy present but was consistent with the relative dominance of species within the chapeirões coral communities.

Responses of Cryptofaunal Species Richness and Trophic Potential to Coral Reef Habitat Degradation

Coral reefs are declining worldwide as a result of many anthropogenic disturbances. This trend is alarming because coral reefs are hotspots of marine biodiversity and considered the ‘rainforests of the sea. As in the rainforest, much of the diversity on a coral reef is cryptic, remaining hidden among the cracks and crevices of structural taxa. Although the cryptofauna make up the majority of a reef’s metazoan biodiversity, we know little about their basic ecology or how these communities respond to reef degradation. Emerging research shows that the species richness of the motile cryptofauna is higher among dead (framework) vs. live coral substrates and, surprisingly, increases within successively more eroded reef framework structures, ultimately reaching a maximum in dead coral rubble. Consequently, the paradigm that abundant live coral is the apex of reef diversity needs to be clarified. This provides guarded optimism amidst alarming reports of declines in live coral cover and the impending doom of coral reefs, as motile cryptic biodiversity should persist independent of live coral cover. Granted, the maintenance of this high species richness is contingent on the presence of reef rubble, which will eventually be lost due to physical, chemical, and biological erosion if not replenished by live coral calcification and mortality. The trophic potential of a reef, as inferred from the abundance of cryptic organisms, is highest on live coral. Among dead framework substrates, however, the density of cryptofauna reaches a peak at intermediate levels of degradation. In summary, the response of the motile cryptofauna, and thus a large fraction of the reef’s biodiversity, to reef degradation is more complex and nuanced than currently thought; such that species richness may be less sensitive than overall trophic function.

Species richness of motile cryptofauna across a gradient of reef framework erosion

Coral reef ecosystems contain exceptionally high concentrations of marine biodiversity, potentially encompassing millions of species. Similar to tropical rainforests and their insects, the majority of reef animal species are small and cryptic, living in the cracks and crevices of structural taxa (trees and corals). Although the cryptofauna make up the majority of a reef’s metazoan biodiversity, we know little about their basic ecology. We sampled motile cryptofaunal communities from both live corals and dead carbonate reef framework across a gradient of increasing erosion on a reef in Pacific Panama. A total of 289 Operational Taxonomic Units (OTUs) from six phyla were identified. We used species-accumulation models fitted to individual- and sample-based rarefaction curves, as well as seven nonparametric richness estimators to estimate species richness among the different framework types. All procedures predicted the same trends in species richness across the differing framework types. Estimated species richness was higher in dead framework (261–370 OTUs) than in live coral substrates (112–219 OTUs). Surprisingly, richness increased as framework structure was eroded: coral rubble contained the greatest number of species (227–320 OTUs) and the lowest estimated richness of 47–115 OTUs was found in the zone where the reef framework had the greatest vertical relief. This contradicts the paradigm that abundant live coral indicates the apex of reef diversity.

Catastrophe and the life span of coral reefs

A strong earthquake in the western Caribbean in 2009 had a catastrophic impact on uncemented, unconsolidated coral reefs in the central sector of the shelf lagoon of the Belizean barrier reef. In a set of 21 reef sites that had been observed prior to the earthquake, the benthic assemblages of 10 were eradicated, and one was partially damaged, by avalanching of their slopes. Ecological dynamics that had played out over the previous 23 years, including the mass mortalities of two sequentially dominant coral species and a large increase in the cover of an encrusting sponge, were instantaneously rendered moot in the areas of catastrophic reef-slope failure. Because these prior dynamics also determined the benthic composition and resilience of adjacent sections of reef that remained intact, the history of disturbance prior to the earthquake will strongly influence decadal-scale recovery in the failed areas. Geological analysis of the reef framework yielded a minimum return time of 2000-4000 years for this type of high-amplitude event. Anthropogenic degradation of ecosystems must be viewed against the backdrop of long-period, natural catastrophes, such as the impact of strong earthquakes on uncemented, lagoonal reefs.

Calcifying coral abundance near low-pH springs: implications for future ocean acidification

Rising atmospheric CO2 and its equilibration with surface ocean seawater is lowering both the pH and carbonate saturation state (Ω) of the oceans. Numerous calcifying organisms, including reef-building corals, may be severely impacted by declining aragonite and calcite saturation, but the fate of coral reef ecosystems in response to ocean acidification remains largely unexplored. Naturally low saturation (Ω ~ 0.5) low pH (6.70–7.30) groundwater has been discharging for millennia at localized submarine springs (called “ojos”) at Puerto Morelos, México near the Mesoamerican Reef. This ecosystem provides insights into potential long term responses of coral ecosystems to low saturation conditions. In-situ chemical and biological data indicate that both coral species richness and coral colony size decline with increasing proximity to low-saturation, low-pH waters at the ojo centers. Only three scleractinian coral species (Porites astreoides, Porites divaricata, and Siderastrea radians) occur in undersaturated waters at all ojos examined. Because these three species are rarely major contributors to Caribbean reef framework, these data may indicate that today’s more complex frame-building species may be replaced by smaller, possibly patchy, colonies of only a few species along the Mesoamerican Barrier Reef. The growth of these scleractinian coral species at undersaturated conditions illustrates that the response to ocean acidification is likely to vary across species and environments; thus, our data emphasize the need to better understand the mechanisms of calcification to more accurately predict future impacts of ocean acidification.

Environmental determinants of motile cryptofauna on an eastern Pacific coral reef

Coral reef cryptofauna, which live hidden within reef framework structures, are con- sidered to be the most diverse group of coral reef metazoans. They likely comprise more biomass than all surface fauna, providing food sources for fishes and playing important roles as predators, herbivores, detritivores, filter feeders, and scavengers. In an era of global change, it is necessary to determine how these communities are structured across reef habitats as well as to understand how reef framework degradation will impact the cryptofauna and, by extension, ecosystem func- tion. Artificial reef framework units were constructed from coral rubble to approxi mate framework substrates. Forty replicates were subjected to treatments of differing porosity, flow, and coral cover in a fully crossed ANOVA design. After 2 mo in situ, all motile cryptofauna (>2 mm) were counted, weighed, and identified to the lowest possible level. A total of 11 309 specimens were col- lected, comprising >121 species from 6 separate phyla. Cryptofaunal abundances and biomass were higher in low-porosity crypts and biomass was greater in slow-flow environments, highlight- ing the importance of sheltered low-porosity habitats, such as back-reef rubble plains. The pres- ence of live coral was not found to have a significant effect on the motile cryptofauna occupying the dead coral framework below it, suggesting a high degree of resilience in how framework- dwelling fauna respond to coral mortality. These data support the assertion that artificial reefs are capable of facilitating the accumulation of a diverse cryptic community, independent of live coral, provided they contain suitably porous crypts.

In Tandem Reef Coral and Cryptic Metazoan Declines and Extinctions

Coral reef degradation and loss have been extensively documented worldwide during the last few decades. While much attention has been directed toward the mortality of reef-building corals vis-a-vis various observed disturbances (e.g., bleaching, diseases, overfishing, nutrification), the fate of other reef-associated metazoans, especially invertebrates, has not received sufficient attention. living and dead corals, reef frameworks, and carbonate sediments provide essential habitat niches for a multitude of symbiotic and cryptic species. Thirty-one animal phyla contain species that inhabit coral reefs with known global species richness estimated at 93,000. possibly as many as 1,000,000 reef-associated metazoans occur globally. many of these species are undiscovered because of their cryptic or sibling nature. metazoan reef associates have important functional roles on reefs, e.g., increasing survivorship of coral hosts, aiding in reef framework construction (calcification, consolidation), providing trophic resources, affecting coral mortality (corallivores) and erosion (bioerosion). despite widespread bleaching and mortality, no reef-building corals (scleractinia) have yet to become globally extinct. Three populations of Millepora spp. (Hydroida) were severely impacted in pacific panama during the 1982-83 el nino-southern oscillation event. present status indicates recovery of Millepora intricata milne-edwards and Haime, 1860 to shallow reef zones from relatively deep (10-15 m) refugia. furthermore, two hydrocoral species have suffered regional extinctions in the eastern pacific with populations still present in the Indo-pacific (Millepora platyphylla Hemprich and ehrenberg, 1834) and eastern Indian ocean (Millepora boschmai de Weerdt and Glynn, 1991). Considering the large numbers of obligate symbionts and other coral reef metazoan associates, there is a strong likelihood of large-scale extinctions following the loss of reef-building corals.

Differential sensitivity of coral larvae to natural levels of ultraviolet radiation during the onset of larval competence

Scleractinian corals are the major builders of the complex structural framework of coral reefs. They live in tropical waters around the globe where they are frequently exposed to potentially harmful ultraviolet radiation (UVR). The eggs and early embryonic stages of some coral species are highly buoyant and remain near the sea surface for prolonged periods of time and may therefore be the most sensitive life stages with respect to UVR. Here, we analysed gene expression changes in five developmental stages of the Caribbean coral Montastraea faveolata to natural levels of UVR using high-density cDNA microarrays (10 930 clones). We found that larvae exhibit low sensitivity to natural levels of UVR during early development as reflected by comparatively few transcriptomic changes in response to UVR. However, we identified a time window of high UVR sensitivity that coincides with the motile planula stage and the onset of larval competence. These processes have been shown to be affected by UVR exposure, and the transcriptional changes we identified explain these observations well. Our analysis of differentially expressed genes indicates that UVR alters the expression of genes associated with stress response, the endoplasmic reticulum, Ca(2+) homoeostasis, development and apoptosis during the motile planula stage and affects the expression of neurogenesis-related genes that are linked to swimming and settlement behaviour at later stages. Taken together, our study provides further data on the impact of natural levels of UVR on coral larvae. Furthermore, our results might allow a better prediction of settlement and recruitment rates after coral spawning events if UVR climate data are taken into account.

Los arrecifes coralinos de Golfo Dulce, Costa Rica: aspectos geológicos

The Holocene growth history of the fringing reef a Punta Islotes, Golfo Dulce, Pacif coast of Costa Rica, has been reconstruted based on data obtained from nine core holes and four sediment cores collected along four reef front transects. These cores reveal the thicknees of the reef to be at least 9 m. Radiocarbon date indicate that the reef started to grow about 5500 yr. B.P. over an uneven surface of Cretaceous basalt. Three reef facies - that are represented by reef zones on the present day reef surface- were recognized: reef-flat braching coral, fore-reef massive coral, and fore-reef talus sediment facies. Accumulation rates of the reef-flat facies range from 1,6 to 2,4 m/1000 yr (mean and standard desviation= 1,90 +/- 0,31) of the fore-reef facies from 0,9 to 8,3 m/1000 yr (3,82 +/- 2,52), and of the fore-reef talus facies from 0,07 to 0,3 m/1000 yr.Holocene growth of Punta Islotes fringing reef can be divided into four stages: A. Initial stage, 5500-4000 yr B.P, settling of Pocillopora damicornis and formation of a small fringing reef. B. Establishment of the reef, 4000-1500 yr B.P, continuos growth of the braching coral facies, initiation of the massive coral facies, and drop in the accumulation rates of the fore-reef talues facies. C. Rapid vertical growth stage, 1500-500 yr B.P., accumulation rates of 5 to 8,3 m/1000 yr, growth of most of the reef framework. D. Final stage, 500 yr B.P to the presentm decline accumulation rates, first due to an increase in fresh water, and more recently to the presense of terrigenous sediments related to deforestation on the adjacent shores.Today, the coral reef in the inner section of Golfo Dulce are totally dead or with very low percentages of live coral. Both, living and dead corals of those reefs are being bioeroded mainly by the boring bivalves, Lithophaga spp.

Where’s the ‘reef’? A five year study of serpulid tube bioerosion in a Scottish sea loch

In the British Isles, aggregations of the tubicolous polychaete Serpula vermicularis L. occur only in a few localities in western Scotland and Ireland. In Loch Creran, Argyll, where written observations extend back to the 1880s, build-ups of skeletal debris constituting a true reef framework are notably absent. To investigate the taphonomic processes affecting the residence time of relict tube material, cleaned and weighed skeletal fragments were deployed on panels amongst living worm aggregations. Fragments were either open to the environment or enclosed in mesh cylinders to exclude grazing urchins. Panels were recovered and fragments re-weighed after 1, 2, 3 and 5 yr in situ. Caged fragments typically increased in dry weight over time, while most open fragments remained at steady-state or showed small weight decreases. Change in weight was correlated with the number of living tubeworms which had settled onto the skeletal fragments. Open fragments showed no consistent temporal trend in weight change, suggesting that urchin grazing was not a major bioerosive process over the 5 yr experimental timescale. Data from another locality where ser- pulid aggregations suffered mass mortality between 1984 and 1994 show that tube debris can persist for at least 15 yr in sea loch environments. S. vermicularis aggregations in sea lochs may be relatively transient features, appearing and disappearing over decadal timescales.

Crustose coralline algae can suppress macroalgal growth and recruitment on Hawaiian coral reefs

Crustose coralline algae are important components of tropical reef communities because they promote successful settlement by corals and contribute to solidification of the reef framework. We show experimentally that crustose coralline algae are also capable of suppressing the growth and recruitment potential of an abundant Hawaiian reef macroalga, Ulva fasciata. When mixed communities of crustose coralline algae were absent, relative growth rates of U. fasciata increased by 54.6%. When experimental nutrient additions were used to induce algal spore release, effective recruitment of U. fasciata approached zero only when crustose coralline algae were present. Mixed communities of crustose coralline algae are thus capable of limiting the local abundance of already-established macroalgae by reducing both their growth rate and recruitment success. This experimental observation was confirmed by field surveys. Because crustose coralline species also induce settlement and metamorphosis in a large number of scleractinian coral species, their abundance and species composition are expected to affect the (future) abundance of macroalgae and corals, which are often used to characterize degraded and 'healthy' reefs, respectively.

Breathing of a coral cay: Tracing tidally driven seawater recirculation in permeable coral reef sediments

Coral reefs are characterized by high gross productivity in spite of low nutrient concentrations. This apparent paradox may be partially reconciled if seawater recirculation in permeable sediments over large (meters) and long (hours to days) scales is an important source of recycled nitrogen and phosphorus to coral reefs. In this paper we use radon (222Rn, a natural tracer) to quantify tidally driven pore water (or groundwater) exchange between (1) an offshore coral cay island and its fringing reef lagoon and (2) a reef lagoon and the surrounding ocean. As seawater infiltrates Heron Island at high tide, it acquires a radon signal that can be detected when pore waters emerge from carbonate sands at low tide. A nonsteady state model indicated that vertical pore water upwelling rates (or saline submarine groundwater discharge) were >40 cm/d within the reef lagoon and >100 cm/d outside the lagoon at low tide. Within the lagoon, tidal pumping and temperature‐driven convection were the main driving forces of pore water advection. At low tide, the reef lagoon level is about 1 m higher than the surrounding ocean. As a result, a steep hydraulic gradient develops at the reef edge, driving unidirectional filtration through the reef framework. Groundwaters were highly enriched in nitrate (average of 530 μmol, likely influenced by bird guano) relative to lagoon waters (1.9 μmol). Rough but conservative estimates indicated that groundwater‐derived nitrate fluxes (7.9 mmol/m2/d) can replace the entire lagoon nitrate inventory every <19 days. We speculate that as offshore coral islands “breath” (inhale seawater at high tide and exhale groundwater at low tide), they release nutrients that lead to sustained productivity within coral reefs.

Microbial mounds prior to the Frasnian-Famennian mass extinctions, Hantang, Guilin, South China

Late Frasnian mounds of the Yunghsien Formation, Guilin, South China, developed as part of the Guilin platform, mostly in reef-flat and platform margin settings. Microbial mounds in platform margin settings at Hantang, about 10 km west of Guilin, contain Frasnian biota, such as Stachyodes and Kuangxiastraea and, thus, occur below the Frasnian-Famennian mass extinction boundary. Platform margin facies were dominated by microbes, algae and receptaculitids. Massive corals and stromatoporoids are not common and rarely show reef-building functions as they did in Givetian time. The margin mounds are composed of brachiopod-receptaculitid cementstone, and a variety of boundstones that contain Rothpletzella, Renalcis, thrombolite and stromatolite. Other microbial communities include Girvanella, Izhella, Ortonella and Wetheredella. Solenoporoid algae are abundant locally. Zebra structures and neptunian dykes are well-developed at some intervals. Pervasive early cementation played an important role in lithification of the microbial boundstones and rudstones. Frasnian reefs of many regions of the world were constructed by stromatoporoids and corals, although a shift to calcimicrobe-dominated frameworks occurred before the Famennian. However, the exact ages of many Frasnian margin outcrops are poorly constrained owing to difficulties dating shallow carbonate facies. The Hantang mounds represent a microbe-dominated reef-building community with rare skeletal reef builders, consistent with major Late Devonian changes in reef composition, diversity and guild structure occurring before the end of the Frasnian. A similar transition occurred in the Canning Basin of Western Australia, but coeval successions in North America, Western Europe and the northern Urals are either less well-known or represent different bathymetric settings. The transition in reef-building style below the Frasnian-Famennian boundary is documented here in the two best exposed successions on two continents, which may have been global. Set in the larger context of Late Devonian and Mississippian microbial reef-building, the Hantang mounds help to demonstrate that controls on microbial reef communities differed from those on larger skeletal reef biota. Calcimicrobes replaced stromatoporoids as major reef builders before the Frasnian-Famennian extinction event, and increasing stromatoporoid diversity towards the end of the Famennian did not result in a resurgence of skeletal reef frameworks. Calcimicrobes dominated margin facies through the Famennian, but declined near the Devonian-Carboniferous boundary. Stromatolite and thrombolite facies, which occurred behind the mound margin at Hantang, rose to dominate Mississippian shallow-water reef frameworks with only a minor resurgence of the important Frasnian calcimicrobe Renalcis in the Visean when well-skeletonized organisms (corals) also became volumetrically significant frame builders again.

Roles of clone–clone interactions in building reef frameworks: principles and examples

In living and fossil reefs, rapid upward clone growth provides positive topographic relief; the skeletal framework provides rigidity. Clonal organisms have been the chief frame-builders during most of the Phanerozoic; large clone size, growth habit, growth form, and arrangement of these clones in the framework result from rapid growth rates. Dense skeletal packing enhances rigidity and results in live–live interactions between juxtaposed clones. These interactions are both heterospecific and conspecific; the former mostly involve spatial competition whereas the latter involve clone fusion, self-overgrowth, and fission. We describe three types of fusion: (a) inter-clone fusion of two or more clones, each from a separate propagule; (b) intra-clone fusion of parts of the same clone having its origin from a single propagule; it includes recovery from partial clone degradation and self-overgrowth; (c) quasi-fusion between a live bud/polyp/zooid and a dead part (stem; branch) of the same or a different clone, i.e., a live-dead association.

Microbialite development patterns in the last deglacial reefs from Tahiti (French Polynesia; IODP Expedition #310): Implications on reef framework architecture

The widespread occurrence of microbialites in the last deglacial reef frameworks (16–6KaBP) implies that the accurate study of their development patterns is of prime importance to unravel the evolution of reef architecture through time and to reconstruct the reef response to sea-level variations and environmental changes.The present study is based on the sedimentological and chronological analysis (14C AMS dating) of drill cores obtained during the IODP Expedition #310 “Tahiti Sea Level” on the successive terraces which typify the modern reef slopes from Tahiti. It provides a comprehensive data base to investigate the microbialite growth patterns (i.e. growth rates and habitats), to analyze their roles in reef frameworks and to reconstruct the evolution of the reef framework architecture during sea-level rise.The last deglacial reefs from Tahiti are composed of two distinctive biological communities: (1) the coralgal communities including seven assemblages characterized by various growth forms (branching, robust branching, massive, tabular and encrusting) that form the initial frameworks and (2) the microbial communities developed in the primary cavities of those frameworks, a few meters (1.5 to 6m) below the living coral reef surface, where they heavily encrusted the coralgal assemblages to form microbialite crusts. The dating results demonstrate the occurrence of two distinctive generations of microbialites: the “reefal microbialites” which developed a few hundred years after coralgal communities in shallow-water environments, whereas the “slope microbialites” grew a few thousands of years later in significantly deeper water conditions after the demise of coralgal communities.The development of microbialites was controlled by the volume and the shape of the primary cavities of the initial reef frameworks determined by the morphology and the packing of coral colonies. The most widespread microbialite development occurred in frameworks dominated by branching, thin encrusting, tabular and robust branching coral colonies which built loose and open frameworks typified by a high porosity (>50%). In contrast, their growth was minimal in compact coral frameworks formed by massive and thick encrusting corals where primary cavities yielded a low porosity (~30%) and could not host a significant microbialite expansion.

Cold-water coral mounds on the Pen Duick Escarpment, Gulf of Cadiz: The MiCROSYSTEMS project approach

Here we present a case study of three cold-water coral mounds in a juvenile growth stage on top of the Pen Duick Escarpment in the Gulf of Cadiz; Alpha, Beta and Gamma mounds. Although cold-water corals are a common feature on the adjacent cliffs, mud volcanoes and open slope, no actual living cold-water coral has been observed. This multidisciplinary and integrated study comprises geophysical, sedimentological and (bio)geochemical data and aims to present a holistic view on the interaction of both environmental and geological drivers in cold-water coral mound development in the Gulf of Cadiz. Coring data evidences (past or present) methane seepage near the Pen Duick Escarpment. Several sources and pathways are proposed, among which a stratigraphic migration through uplifted Miocene series underneath the escarpment. The dominant morphology of the escarpment has influenced the local hydrodynamics within the course of the Pliocene, as documented by the emplacement of a sediment drift. Predominantly during post-Middle Pleistocene glacial episodes, favourable conditions were present for mound growth. An additional advantage for mound formation near the top of Pen Duick Escarpment is presented by seepage-related carbonate crusts which might have offered a suitable substrate for coral settling. The spatially and temporally variable character and burial stage of the observed open reef frameworks, formed by cold-water coral rubble, provides a possible model for the transition from cold-water coral reef patches towards juvenile mound. These rubble “graveyards” not only act as sediment trap but also as micro-habitat for a wide range of organisms. The presence of a fluctuating Sulphate–Methane Transition Zone has an important effect on early diagenetic processes, affecting both geochemical and physical characteristics, transforming the buried reef into a solid mound. Nevertheless, the responsible seepage fluxes seem to be locally variable. As such, the origin and evolution of the cold-water coral mounds on top of the Pen Duick Escarpment is, probably more than any other NE Atlantic cold-water coral mound province, located on the crossroads of environmental (hydrodynamic) and geological (seepage) pathways.