A geochemical model for coral reef formation

The conspicuous growth of a reef crest and the resulting differentiation of reef topography into a moat (shallow lagoon), crest and slope have long attracted the interest of scientists studying coral reefs. A geochemical model is here proposed for reef formation, taking into account diffusion-limited and light-enhanced calcification. First, to obtain data on net photosynthesis and calcification rates in the field, a typical coral community was cultured in situ on a reef flat. Using these data, equations including parameters for calcification were then developed and applied in computer simulations to model the development over time of reef profiles and the diffusion of carbon species. The reef topography simulated by the model was in general agreement with reef topography observed in nature. The process of reef growth as shown by the modeling was as follows. Increases in the shore-to-offshore gradients of the concentrations of carbonate species result from calcification by reef biota, giving a lower rate of growth on near-shore parts of the reef than on those further offshore. As a result, original topography is diversified into moat and reef crest for the first time. Reef growth on the reef crest is more rapid than in the inshore moat area, because more light is available at the crest. Reef growth on the near-shore side of the reef is further inhibited by damming of carbon-rich seawater on the seaward side of the reef by the reef crest. Over time, the topographic expression of the reef crest and moat becomes progressively more clearly defined by these geochemical processes.     

Morphologie, Ökologie und Zonierung von Korallenriffen bei Aqaba, (Golf von Aqaba,Rotes Meer)

The coral reefs of the Gulf of Aqaba are among the most northern ones of the world. This study, the first concerning the east coast of this topographically and hydrographically peculiar sea, considers relationships of biophysiographical and structural reef zones to fundamental abiotic environmental factors. An introduction to paleogeography, geology, petrography, topography, climate and hydrography is followed by terminological definitions used to describe the different reef areas. The investigations were carried out on two transects crossing fringing reefs of different shape. Each transect was 20 m wide and run from the shore over nearly 200 m to the fore reef in about 30 m depth. One reef, a “coastal-fringing reef”, represents an unaltered straight reef flat from shore to the reef edge 60 m away; two large pinnacles reach the surface some 125 m off the shore. The other reef, a “lagoon-fringing reef”, is divided into a 100 m wide lagoon of 0.5–2.3 m depth and a reef crest separated from the former by a rear reef. The reef platform of the lagoon-fringing reef is cut by a system of channels and tunnels; the reef edge is about 135 m off shore. Such water depth, substrate, temperature, illumination and water movement were recorded, about 200 common or dominant species (plants and animals) were collected, their distribution plotted and, together with other data and structural items, charted. Indicator species characterize the biophysiographical zones. Their variation as well as that of the structural and substrate zones depend on different zones of water movement. This basic factor also controls other ecological parameters such as food and oxygen supply as well as temperature and salinity gradients between fore reef and shore. From this point of view the ecological requirements of some indicator and other species and conversely the ecological settings of different reef areas are discussed. The different shapes of both reefs are explained on the basis of a “reef development cycle” — a hypothesis applicable to fringing reefs at unchanging sea level and based on the fact that only a small surf-influenced area of “living reef” is able to compensate for reef destruction: While a young coastal fringing reef is growing outwards, its back reef is gradually altered to a reef lagoon by erosion. After stillstand of seaward expansion the reef crest, too, is cut by a channel system eroded by rip currents. This stage is represented by the lagoon-fringing reef. Isolated pinnacles remain as remnants of the former reef crest; young coastal-fringing reefs develop from the shore. This stage is examplified by the first reef studied. Extension, growth intensity, dominant frame building corals, and the number of species of the Aqaba reefs are compared with those of Eilat and with reefs of the middle Red Sea, South India, Southwest-Pacific and Jamaica.     

Evolution of fringing reefs: space and time constraints from the Gulf of Aqaba

This study documents the pattern and rate of reef growth during the late Holocene as revealed by unique geological conditions at the subsiding NW Gulf of Aqaba. We discovered that the modern fringing reef near the city Elat grows on top of a fossil submerged mid-Holocene reef platform. Four coral cores from the fossil platform were dated using the radiocarbon and U-Th methods. The fossil corals range from 5.6±0.1 to 2.4±0.03 ka, constraining the initiation of the modern reef to 2,400 years ago at most. We documented the detailed morphology of the reef using aerial photographs and scuba diving. The survey shows that at its northern end, growth of the 2-km-long reef is inhibited by an active alluvial fan, and it is composed of isolated knolls that are just approaching the sea surface. Towards the south, the knolls are progressively larger and closer together, until they form a continuous reef platform. Along this north-to-south trend we follow the evolution of reef morphology, changes in coral distribution, and the development of a lagoon separated from the open sea. Based on these observations, we suggest a four-stage reef growth model: (a) the reef initiates as coral colonies, forms knolls, and begins to grow upward, limited by the sea surface. (b) Upon reaching the surface, the knolls spread laterally, preferentially parallel to the dominant wave direction assuming an elongated morphology. (c) Continued growth results in adjacent knolls eventually coalescing to form a continuous jagged reef. We interpret the spurs-and-grooves morphology that can be traced across the reef at Elat as remnants of the original trends of knolls. (d) While reef expansion continues, the original knoll trends may be obscured as a massive reef front takes shape. Considering reef growth rates and observations from the modern reef at Elat, this evolution scheme predicts an age range of 10³ years for corals on the reef platform. The range and distribution of radiometric ages we obtained from the fossil reef platform underlying the living Elat reef confirm this hypothesis.     

A Global Estimate of the Number of Coral Reef Fishers

Overfishing threatens coral reefs worldwide, yet there is no reliable estimate on the number of reef fishers globally. We address this data gap by quantifying the number of reef fishers on a global scale, using two approaches - the first estimates reef fishers as a proportion of the total number of marine fishers in a country, based on the ratio of reef-related to total marine fish landed values. The second estimates reef fishers as a function of coral reef area, rural coastal population, and fishing pressure. In total, we find that there are 6 million reef fishers in 99 reef countries and territories worldwide, of which at least 25% are reef gleaners. Our estimates are an improvement over most existing fisher population statistics, which tend to omit accounting for gleaners and reef fishers. Our results suggest that slightly over a quarter of the world’s small-scale fishers fish on coral reefs, and half of all coral reef fishers are in Southeast Asia. Coral reefs evidently support the socio-economic well-being of numerous coastal communities. By quantifying the number of people who are employed as reef fishers, we provide decision-makers with an important input into planning for sustainable coral reef fisheries at the appropriate scale.     

A global coral reef probability map generated using convolutional neural networks

Coral reef research and management efforts can be improved when supported by reef maps providing local-scale details across global extents. However, such maps are difficult to generate due to the broad geographic range of coral reefs, the complexities of relating satellite imagery to geomorphic or ecological realities, and other challenges. However, reef extent maps are one of the most commonly used and most valuable data products from the perspective of reef scientists and managers. Here, we used convolutional neural networks to generate a globally consistent coral reef probability map—a probabilistic estimate of the geospatial extent of reef ecosystems—to facilitate scientific, conservation, and management efforts. We combined a global mosaic of high spatial resolution Planet Dove satellite imagery with regional Millennium Coral Reef Mapping Project reef extents to build training, validation, and application datasets. These datasets trained our reef extent prediction model, a neural network with a dense-unet architecture followed by a random forest classifier, which was used to produce a global coral reef probability map. Based on this probability map, we generated a global coral reef extent map from a 60% threshold of reef probability (reef: probability ≥ 60%, non-reef: probability < 60%). Our findings provide a proof-of-concept method for global reef extent estimates using a consistent and readily updateable methodology that leverages modern deep learning approaches to support downstream users. These maps are openly-available through the Allen Coral Atlas.

     

Sedimentology of the Upper Triassic reef complex at the Hochkönig Massif (Northern Calcareous Alps,Austria)

Summary The Upper Triassic Dachsteinkalk of the Hochk?nig Massif, situated 50 km south of Salzburg in the Northern Calcareous Alps, corresponds to a platform margin reef complex of exceptional thickness. The platform interior limestones form equally thick sequences of the well known cyclic Lofer facies. Sedimentation in the reef complex was not so strongly controlled by low-amplitude sea-level oscillations as was the Lofer facies. The westernmost of the 8 facies of the reef complex is an oncolite-dominated lagoon, in which wave-resistant stromatolite mounds with a relief of a few metres were periodically developed. The transition to the central reef area is accomplished across the back-reef facies. In the back-reef facies patch reefs and calcisponges appear. The proportion of coarse bioclastic sediment increases rapidly over a few hundred metres before the central reef area is encountered. The central reef area consists of relatively widely spaced small patch reefs that did not develop wave-resistant reef framework structures. The bulk of the sediment in the central reef area is coarse bioclastic material, provided by the dense growth of reef organisms and the wave-induced disintegration of patch reefs. Collapse of the reef margin is recorded by the supply of large blocks of patch reef material to the upper reef slope. Additionally, coarse, loose bioclastic debris was supplied to the upper reef slope and this was incorporated into debris flows on the reef slope and turbidites found at the base of the slope and in the off-reef facies. Partially lithified packstones and wackestones of the lower to middle reef slope were modified by mass movement to form breccia and rudstone sheets. The latter reach out hundreds of metres into the off-reef facies environment. A reef profile is presented which was derived by the restoration of strike and dip information. In conjunction with constraints imposed by sedimentary facies related to slope processes, the angle of slope in the reef margin area ranged from 11° to 5°, forming a concave (dished downwards) slope. Water depth estimations require that the central reef area did not develop in water of less than 10 metres depth. At the reef margin water depths were about 30 metres, at the base of the reef slope 200 metres and deepening in the off-reef facies to 250 metres. While previous work on reef complexes from this type of setting suggests growth in a heavily storm-dominated environment, the present author finds little evidence for the storm generation of the fore reef breccias, although there is good evidence for storm-influenced sedimentation and reworking in the central reef area. Post-depositional processes were characterised by continued slope processes causing brecciation and hydraulic injection of red internal sediments downwards into the reef slope and off-reef limestones. Hydrothermal circulation caused a number of phases of post-depositional (diagenetic) brecciation. There appears not to have been an important period of emergence at the Triassic/Jurassic boundary.     

Pleistocene reef environments, constituent grains, and coral community structure: Curaçao, Netherlands Antilles

We investigated the degree to which component grains vary with depositional environment in sediments from three reef habitats from the Pleistocene (125?ka) Hato Unit of the Lower Terrace, Curaçao, Netherlands Antilles: windward reef crest, windward back reef, and leeward reef crest. The windward reef crest sediment is the most distinctive, dominated by fragments of encrusting and branching coralline red algae, coral fragments and the encrusting foraminiferan Carpenteria sp. Windward back reef and leeward reef crest sediments are more similar compositionally, only showing significant differences in relative abundance of coral fragments and Homotrema rubrum. Although lacking high taxonomic resolution and subject to modification by transport, relative abundance of constituent grain types offers a way of assessing ancient skeletal reef community composition, and one which is not limited to a single taxonomic group. The strong correlation between grain type and environment we found in the Pleistocene of Curaçao suggests that constituent grain analysis may be an effective tool in delineating Pleistocene Caribbean reef environments. However, it will not be a sufficient indicator where communities vary significantly within reef environments or where evolutionary and/or biogeographical processes lead to different relationships between community composition and reef environment. Detailed interpretation of geological, biological, and physical characteristics of the Pleistocene reefs of Curaçao reveals that the abundance of the single coral species, Acropora palmata, is not a good predictor of the ecological structure of the ancient reef coral communities. This coral was the predominant species in two of the three reef habitats (windward and leeward reef crest), but the taxonomic composition (based on species relative abundance data) of the reef coral communities was substantially different in these two environments. We conclude that qualitative estimates of coral distribution patterns (presence of a key coral species or the use of a distinctive coral skeletal architecture), when used as a component in a multi-component analysis of ancient reef environments, probably introduces minimal circular reasoning into quantitative paleoecological studies of reef coral community structure.     

The influence of sea level and cyclones on Holocene reef flat development: Middle Island,central Great Barrier Reef

The geomorphology and chronostratigraphy of the reef flat (including microatoll ages and elevations) were investigated to better understand the long-term development of the reef at Middle Island, inshore central Great Barrier Reef. Eleven cores across the fringing reef captured reef initiation, framework accretion and matrix sediments, allowing a comprehensive appreciation of reef development. Precise uranium–thorium ages obtained from coral skeletons revealed that the reef initiated ~7873 ± 17 years before present (yBP), and most of the reef was emplaced in the following 1000 yr. Average rates of vertical reef accretion ranged between 3.5 and 7.6 mm yr^?1. Reef framework was dominated by branching corals (Acropora and Montipora). An age hiatus of ~5000 yr between 6439 ± 19 and 1617 ± 10 yBP was observed in the core data and attributed to stripping of the reef structure by intense cyclones during the mid- to late-Holocene. Large shingle ridges deposited onshore and basset edges preserved on the reef flat document the influence of cyclones at Middle Island and represent potential sinks for much of the stripped material. Stripping of the upper reef structure around the outer margin of the reef flat by cyclones created accommodation space for a thin (<1.2 m) veneer of reef growth after 1617 ± 10 yBP that grew over the eroded mid-Holocene reef structure. Although limited fetch and open-water exposure might suggest the reef flat at Middle Island is quite protected, our results show that high-energy waves presumably generated by cyclones have significantly influenced both Holocene reef growth and contemporary reef flat geomorphology.     

The large-scale influence of the Great Barrier Reef matrix on wave attenuation

Offshore reef systems consist of individual reefs, with spaces in between, which together constitute the reef matrix. This is the first comprehensive, large-scale study, of the influence of an offshore reef system on wave climate and wave transmission. The focus was on the Great Barrier Reef (GBR), Australia, utilizing a 16-yr record of wave height from seven satellite altimeters. Within the GBR matrix, the wave climate is not strongly dependent on reef matrix submergence. This suggests that after initial wave breaking at the seaward edge of the reef matrix, wave energy that penetrates the matrix has little depth modulation. There is no clear evidence to suggest that as reef matrix porosity (ratio of spaces between individual reefs to reef area) decreases, wave attenuation increases. This is because individual reefs cast a wave shadow much larger than the reef itself; thus, a matrix of isolated reefs is remarkably effective at attenuating wave energy. This weak dependence of transmitted wave energy on depth of reef submergence, and reef matrix porosity, is also evident in the lee of the GBR matrix. Here, wave conditions appear to be dependent largely on local wind speed, rather than wave conditions either seaward, or within the reef matrix. This is because the GBR matrix is a very effective wave absorber, irrespective of water depth and reef matrix porosity.     

Estimation of photosynthesis and calcification rates at a fringing reef by accounting for diurnal variations and the zonation of coral reef communities on reef flat and slope: a case study for the Shiraho reef, Ishigaki Island, southwest Japan

Seven coral reef communities were defined on Shiraho fringing reef, Ishigaki Island, Japan. Net photosynthesis and calcification rates were measured by in situ incubations at 10 sites that included six of the defined communities, and which occupied most of the area on the reef flat and slope. Net photosynthesis on the reef flat was positive overall, but the reef flat acts as a source for atmospheric CO₂, because the measured calcification/photosynthesis ratio of 2.5 is greater than the critical ratio of 1.67. Net photosynthesis on the reef slope was negative. Almost all excess organic production from the reef flat is expected to be effused to the outer reef and consumed by the communities there. Therefore, the total net organic production of the whole reef system is probably almost zero and the whole reef system also acts as a source for atmospheric CO₂. Net calcification rates of the reef slope corals were much lower than those of the branching corals. The accumulation rate of the former was approximately 0.5 m kyr⁻¹ and of the latter was ~0.7–5 m kyr⁻¹. Consequently, reef slope corals could not grow fast enough to keep up with or catch up to rising sea levels during the Holocene. On the other hand, the branching corals grow fast enough to keep up with this rising sea level. Therefore, a transition between early Holocene and present-day reef communities is expected. Branching coral communities would have dominated while reef growth kept pace with sea level rise, and the reef was constructed with a branching coral framework. Then, the outside of this framework was covered and built up by reef slope corals and present-day reefs were constructed.     

A DESCRIPTION OF THE CORAL STRUCTURE OF TUTIA REEF (PANGANYIKA TERRITORY, EAST AFRICA), AND ITS FISH FUNA

The origin and structure of Tutia Reef, a coral reef off the Tanganyika coast, is described. It is concluded that the reef is formed in part from the cutting back flat of the older reef 14 ft above the level of the present reef, and in part from seaward growth of the reef.
Based on lines of quadrats down three sides of the reef an assessment of the cover of living coral on the reef is made, and the distribution of the dominant corals of the reef described. It is suggested that, contrary to the views of Crossland (1902, 1903), many East African coral reefs are undergoing seaward growth.
The distribution of the fishes on the reef is described, and a correlation is shown with the distribution of living coral on the reef. Many species of fishes have restricted distribution on the reef.
The feeding relationships of the fishes are discussed, and the total fish biomass divided into separate feeding categories. Tutia Reef has a low biomass of herbivorous fishes compared with the results from Eniwetok Atoll obtained by Odum & Odum (1955). This is considered to be due to the fact that Tutia Reef exhibits characteristics of an outer reef, deriving much of its energy from plankton, not those of an enclosed and almost autotrophic logoon.     

Herbivorous fish assemblages and herbivory pressure on Laamu Atoll, Republic of Maldives

While herbivory is recognized as a fundamental process structuring coral reef communities, herbivore assemblages and processes are poorly described for reefs in the Indian Ocean region. We quantified herbivorous fish assemblage structure (abundance and diversity) in Laamu Atoll, Republic of Maldives, in four reef habitat types: faro reef flats, faro reef slopes, inner and outer atoll reef slopes (20 sites in total). Herbivorous fish assemblages, representing a total of 30 species, grouped strongly by habitat type, with the highest absolute abundance observed on faro reef flats and lowest abundance on inside atoll rim reef slopes. Removal of Thalassia seagrass blades by ambient herbivore assemblages was used in a bioassay to assess relative herbivory pressure among four habitat types (eight sites). Also, at one site a choice herbivory assay was performed to assess herbivore preference among four benthic plants across three depth zones. Relative herbivory, as indicated by Thalassia assays, was highest on inside atoll rim reef slopes and lowest on outside atoll rim reef slopes. Thalassia consumption did not correspond to overall herbivorous fish abundance, but corresponded more closely with parrotfish abundance. In the choice assays, herbivores showed strong preferences among plant types and consumption of most plant types was higher at mid-depth than in the shallow reef flat or deep reef knoll zones.     

Cycles of coral reef ‘turn‐on’, rapid growth and ‘turn‐off’ over the past 8500 years: a context for understanding modern ecological states and trajectories

Human activities threaten reef ecosystems globally, forcing ecological change at rates and scales regarded as unprecedented in the Holocene. These changes are so profound that a cessation of reef accretion (reef ‘turn‐off’) and net erosion of reef structures is argued by many as the ultimate and imminent trajectory. Here, we use a regional scale reef growth dataset, based on 76 core records (constrained by 211 radiometric dates) from 22 reefs along and across the inner‐shelf of the Great Barrier Reef, Australia, to examine the timing of different phases of reef initiation (‘turn‐on’), growth and ‘turn‐off’ during the Holocene. This dataset delineates two temporally discrete episodes of reef‐building over the last 8500 years: the first associated with the Holocene transgression‐early highstand period [～8.5–5.5 k calibrated years bp (cal ybp )]; the second since ～2.3 k cal ybp . During both periods, reefs accreted rapidly to sea level before entering late evolutionary states – states naturally characterized by reduced coral cover and low accretion potential – and a clear hiatus occurs between these reef‐building episodes for which no records of reef initiation exist. These transitions mimic those projected under current environmental disturbance regimes, but have been driven entirely by natural forcing factors. Our results demonstrate that, even through the late Holocene, reef health and growth has fluctuated through cycles independent of anthropogenic forcing. Consequently, degraded reef states cannot de facto be considered to automatically reflect increased anthropogenic stress. Indeed, in many cases degraded or nonaccreting reef communities may reflect past reef growth histories (as dictated by reef growth–sea level interactions) as much as contemporary environmental change. Recognizing when changes in reef condition reflect these natural ‘turn‐on’– growth –‘turn‐off’ cycles and how they interact with on‐going human disturbance is critical for effective coral reef management and for understanding future reef ecological trajectories.     

Two waves of colonization straddling the K–Pg boundary formed the modern reef fish fauna

Living reef fishes are one of the most diverse vertebrate assemblages on Earth. Despite its prominence and ecological importance, the origins and assembly of the reef fish fauna is poorly described. A patchy fossil record suggests that the major colonization of reef habitats must have occurred in the Late Cretaceous and early Palaeogene, with the earliest known modern fossil coral reef fish assemblage dated to 50 Ma. Using a phylogenetic approach, we analysed the early evolutionary dynamics of modern reef fishes. We find that reef lineages successively colonized reef habitats throughout the Late Cretaceous and early Palaeogene. Two waves of invasion were accompanied by increasing morphological convergence: one in the Late Cretaceous from 90 to 72 Ma and the other immediately following the end-Cretaceous mass extinction. The surge in reef invasions after the Cretaceous–Palaeogene boundary continued for 10 Myr, after which the pace of transitions to reef habitats slowed. Combined, these patterns match a classic niche-filling scenario: early transitions to reefs were made rapidly by morphologically distinct lineages and were followed by a decrease in the rate of invasions and eventual saturation of morphospace. Major alterations in reef composition, distribution and abundance, along with shifts in climate and oceanic currents, occurred during the Late Cretaceous and early Palaeogene interval. A causal mechanism between these changes and concurrent episodes of reef invasion remains obscure, but what is clear is that the broad framework of the modern reef fish fauna was in place within 10 Myr of the end-Cretaceous extinction.     

A late Devonian (Frasnian) coral-bafflestone reef at Houshan in Guilin, South China

Summary Givetian to early Carboniferous sediments of South China are characterized by carbonates. Middle and Late Devonian strata are best developed in the Guilin area. Reefs and organic shoals are recorded by various lithofacies types indicating the existence of an extended carbonate platform and a change of the composition of reef communities in time. Starting in the late Devonian, stromatoporoids and corals were replaced by algae that subsequently played an important role together with stromatoporoids, receptaculitids and fasciculate rugose corals in reef communities. In Houshan, 5 km west of Guilin, a coral-bafflestone reef occurs in the Frasnian strata, situated near an offshore algal-stromatoporoid reef. The coral reef was formed in a back-reef area adjacent to the inner platform margin. The coral-bafflestone reef is unique among the late Devonian reefs of South China with regard to the biotic composition. The reef is composed of fasciculate colonies ofSmithiphyllum guilinense n. sp. embedded within in packstones and wackestones. The height of colonies reaches 1 m. The community is low-diverse. The species ofSmithiphyllum occurring in the Frasnian reef complexes of Guilin exhibit a distinct facies control:Smithiphyllum guilinense occurs in or near to margin facies and formed bafflestone, constituting a coral reef whereasSmithiphyllum occidentale Sorauf, 1972 andSmithiphyllum sp.—characterized by small colonies with thin corallites—are restricted to the back-reef and marginal slope facies. The bush-like coral colonies baffled sediments. Algae and stromatoporoids (mainlyStachyodes) are other reef biota. Reef-dwelling organisms are dominated by brachiopods. The reefs are composed from base to top of five lithofacies types: 1) cryptalgal micrite, 2) peloidal packstone, 3) stromatactis limestone, 4) coral-bafflestone, and 5) pseudopeloidal packstone. The reef complex can be subdivided into back-reef subfacies, reef flat and marginal subfacies, and marginal fore-slope subfacies. The Houshan coral-bafflestone reef is not a barrier reef but a coral patch reef located near the inner margin of a carbonate platform.     

Plaeocene reef sediments from the maiella carbonate platform,Italy

Summary Upper Cretaceous and Paleocene reef limestones from the Maiella carbonate platform show how reefs evolved during a time of faunal turn-over. Biostratigraphy and facies analysis of the reef limestones reveal the details of reef growth, composition, and age. Rudists disappeared as reef builders from the Maiella platform shortly before the Cretaceous/Tertiary boundary. Small coral-algal reefs became established in the Danian to Late Thanetian. These scleractinian-red algal dominated boundstones and framestones represent two periods of reef sedimentation and the subsequent interruption of reef growth by emersion and erosion, controlled primarily by fluctuations of relative sea-level. The coral-algal reefs evolved as the taxonomic composition of reef organisms changed. The Paleocene reef sediments are preserved as large slide blocks and as boulders redeposited from the shallow-water platform onto the slope during the course of the Paleocene.     

Spatial variation in mechanical properties of coral reef substrate and implications for coral colony integrity

The physical structure of coral reefs plays a critical role as a barrier to storm waves and tsunamis and as a habitat for living reef-building and reef-associated organisms. However, the mechanical properties of reef substrate (i.e. the non-living benthos) are largely unknown, despite the fact that substrate properties may ultimately determine where organisms can persist. We used a geo-mechanical technique to measure substrate material density and strength over a reef hydrodynamic gradient. Contrary to expectation, we found a weak relationship between substrate strength and wave-induced water flow: flow rates decline sharply at the reef crest, whereas substrate properties are relatively constant over much of the reef before declining by almost an order of magnitude at the reef back. These gradients generate a novel hump-shaped pattern in resistance to mechanical disturbances for live corals, where colonies closer to the back reef are prone to dislodgement because of poorly cemented substrate. Our results help explain an intermediate zone of higher taxonomic and morphological diversity bounded by lower diversity exposed reef crest and unstable reef back zones.     

A comparison of the movements of two species of gadoid in the vicinity of an underwater reef

Two saithe (35 and 38 cm) and two pollack (43 and 44 cm) were tracked simultaneously for 170 h. During the day, saithe generally patrolled over the whole of an underwater reef as part of a school, making occasional excursions off the reef to another smaller reef 250m distant. At night, saithe movements were largely limited to the reef. Pollack covered less than 50% of the reef during the study period, moving only small distances off the reef. Pollack swam more slowly than saithe during the day, but at the same speed at night.     

The role of bryozoans in fossil reefs—an example from the Middle Devonian of the Western Sahara

Stenolaemate bryozoans with their stable calcitic skeletons play a significant role in reef building. In the Middle Devonian Sabkhat Lafayrina reef complex (Western Sahara), bryozoans are abundant and diverse. Although they do not form part of the principal framework of reefs, bryozoans are involved significantly in reef growth, especially in the initial stage. In this way, bryozoans are important with respect to initiating reef growth. They contribute greatly to sediment stabilization, making it possible for principal reef builders to grow on hardened and stabilized substrates, and also play sediment-baffling and sediment-filling roles. The aim of this study is to document the diversity of bryozoans in a Middle Devonian reef complex and to estimate their potential for initiation and contribution to reef structures.