Black reefs: iron-induced phase shifts on coral reefs

The Line Islands are calcium carbonate coral reef platforms located in iron-poor regions of the central Pacific. Natural terrestrial run-off of iron is non-existent and aerial deposition is extremely low. However, a number of ship groundings have occurred on these atolls. The reefs surrounding the shipwreck debris are characterized by high benthic cover of turf algae, macroalgae, cyanobacterial mats and corallimorphs, as well as particulate-laden, cloudy water. These sites also have very low coral and crustose coralline algal cover and are call black reefs because of the dark-colored benthic community and reduced clarity of the overlying water column. Here we use a combination of benthic surveys, chemistry, metagenomics and microcosms to investigate if and how shipwrecks initiate and maintain black reefs. Comparative surveys show that the live coral cover was reduced from 40 to 60% to <10% on black reefs on Millennium, Tabuaeran and Kingman. These three sites are relatively large (>0.75 km²). The phase shift occurs rapidly; the Kingman black reef formed within 3 years of the ship grounding. Iron concentrations in algae tissue from the Millennium black reef site were six times higher than in algae collected from reference sites. Metagenomic sequencing of the Millennium Atoll black reef-associated microbial community was enriched in iron-associated virulence genes and known pathogens. Microcosm experiments showed that corals were killed by black reef rubble through microbial activity. Together these results demonstrate that shipwrecks and their associated iron pose significant threats to coral reefs in iron-limited regions.     

Atlantic coral reefs: the transplantation alternative

Although all of the world’s coral reef regions have suffered degradation due to direct and indirect human influences, only the Western Atlantic reefs have declined to the extent that their continued existence appears to be in jeopardy. Of a once flourishing reef system, only about 10% is still alive and it is depauperate in terms of the food web diversity necessary to maintain a stable and productive ecosystem. The large carnivores and herbivores have become so scarce that they have failed to control the populations of smaller animals and plants, so that almost all the trophic levels have been disrupted. At the same time, the impacts of many other factors have contributed to the process of degradation resulting in a true crisis. The extent of damage, and the general impairment of regeneration, is such that a hope for natural recovery appears to be unrealistic. It is suggested that a recovery could be achieved through transplantation of corals and other reef species from areas where more diverse, relatively stable ecosystems still exist. Available data on the introduction of exotic species into marine ecosystems indicate that such species are generally accommodated and do not cause extinctions among the native species. Although some introduced species are considered to be pests, others have proved to be beneficial, and all have apparently increased the biodiversity of the invaded areas.     

Global change and coral reefs: impacts on reefs, economies and human cultures

Coral reefs have reconstituted themselves after previous large sea-level variations, and climate changes. For the past 6000 years of unusually stable sea-level, reefs have grown without serious interruptions. During recent decades, however, new stresses threaten localized devastation of many reefs. A new period of global climate change is occurring, stimulated by anthropogenic increases in greenhouse gases. Coral reefs will cope well with predicted sea-level rises of 4.5 cm per decade, but reef islands will not. Higher sea levels will provide corals with greater room for growth across reef flats, but there are no foreseeable mechanisms for reef island growth to keep pace with sea-level rise, therefore many low islands may ultimately become uninhabitable. Climate change will introduce localized variations in weather patterns, but changes to individual reefs cannot be predicted. Reefs on average should cope well with regional climate change, as they have coped with similar previous fluctuations. Air temperature increases of 0.2–0.3 °C/decade will induce slower increases in sea-surface temperatures, which may cause localized, or regional increases in coral bleaching. Changes in rainfall will impact on reefs near land masses. Likewise, increased storms and variations in El Nino Southern Oscillation (ENSO) may stress some reefs, but not others. The greatest impact of climate change will be a synergistic enhancement of direct anthropogenic stresses (excessive sediment and pollution from the land; over-fishing, especially via destructive methods; mining of coral rock and sand; and engineering modifications), which currently cause most damage to coral reefs. Many of the world's reefs have been degraded and more will be damaged as anthropogenic impacts increase under the ‘demophoric’ increases in population (demos) and economic (phoric) activity. This biotic and habitat loss will result in severe economic and social losses. Reefs, however, have considerable recovery powers and losses can be minimized by effective management of direct human impacts and reducing indirect threats of global climate change.     

Trampling on coral reefs: tourism effects on harpacticoid copepods

Human trampling is a common type of disturbance associated with outdoor recreational activities in coastal ecosystems. In this study, the effect of trampling on the meiofaunal harpacticoid copepod assemblage inhabiting turfs on a coral reef was investigated. In Porto de Galinhas, northeastern Brazil, reef formations near the beach are one of the main touristic destinations in the country. To assess trampling impact, two areas were compared: a protected area and an area subject to intensive tourism. Densities of total Harpacticoida and of the most abundant harpacticoid species showed strong reductions in the trampled area. An analysis of covariance revealed that the loss of phytal habitat was not the main source of density reductions, showing that trampling affected the animals directly. In addition, multivariate analysis demonstrated differences in the structure of harpacticoid assemblages between areas. Of the 43 species identified, 12 were detected by the Indicator Species Analyses as being indicators of the protected or trampled areas. Moreover, species richness was reduced in the area open to tourism. At least 25 harpacticoids are new species for science, of these, 20 were more abundant or occurred only in the protected area, while five were more abundant or occurred only in the trampled area; thus, our results highlight the possibility of local extinction of still-unknown species as one of the potential consequences of trampling on coral reefs.     

Taphonomy of coral reefs: a review

This review summarises the major factors that affect the post-mortem history of skeletons in a coral reef environment. Skeletal material is traced from life, through death, breakdown, transport, burial and diagenesis to its final fossil form. The fact that most reef sediments are of skeletal composition poses problems of concentration or dilution of individual grain types in taphonomic analysis of reefs. Rates of supply of grains vary, not only with organism abundance and skeletal growth rates, but also with rates of physical and biological breakdown to transportable sediment. Physical and organic processes affect sedimentary structures and textures by mixing or segregating skeletal grains, though biogenic processes normally dominate in the protected setting of reef lagoons. The soft and hard substrates associated with reefs present different media for calcium carbonate accumulation and post-depositional disturbance, for example, loose sediments suffer bioturbation and rocks surfaces suffer bioerosion. The wide range of durability of skeletons and their susceptibility to diagenesis contribute further to the complexities of the preservation of coral reefs.     

Mesopredator trophodynamics on thermally stressed coral reefs

Coral Reefs - Ecosystems are becoming vastly modified through disturbance. In coral reef ecosystems, the differential susceptibility of coral taxa to climate-driven bleaching is predicted to shift...     

Estimating three-dimensional surface areas on coral reefs

One of the main obstacles for biological assessments of coral reefs over large spatial scales is the ability to link data obtained at the laboratory scale to spatially large data sets. This is particularly the case when trying to assess the ecological function of microbial processes following dramatic large-scale events such as mass coral bleaching. To be able to infer ecological function of field corals from laboratory measurement standardised to surface area it is imperative to be able to measure the actual surface area of corals in-situ. There have been several approaches proposed to estimate the three-dimensional surface area of field corals. While these have been shown to be reliable for simple coral growth forms, large degrees of error are introduced when applying them to complex growth forms. This paper refines a technique for calculating the three-dimensional surface area based on the projected surface area, with errors associated with complex growth forms reduced to < 5%. Once developed, the simple mathematical relationship (called the surface index) can be used to estimate the three-dimensional surface area of field corals from photograph or video imagery, allowing physiological parameters of corals determined at the sub-colony scale to whole colony and spatially large data sets of coral reefs. The effectiveness of using laser scanning techniques to derive three-dimensional images of corals is also discussed.     

Caribbean reefs of the Anthropocene: Variance in ecosystem metrics indicates bright spots on coral depauperate reefs

Dramatic coral loss has significantly altered many Caribbean reefs, with potentially important consequences for the ecological functions and ecosystem services provided by reef systems. Many studies examine coral loss and its causes—and often presume a universal decline of ecosystem services with coral loss—rather than evaluating the range of possible outcomes for a diversity of ecosystem functions and services at reefs varying in coral cover. We evaluate 10 key ecosystem metrics, relating to a variety of different reef ecosystem functions and services, on 328 Caribbean reefs varying in coral cover. We focus on the range and variability of these metrics rather than on mean responses. In contrast to a prevailing paradigm, we document high variability for a variety of metrics, and for many the range of outcomes is not related to coral cover. We find numerous “bright spots,” where herbivorous fish biomass, density of large fishes, fishery value, and/or fish species richness are high, despite low coral cover. Although it remains critical to protect and restore corals, understanding variability in ecosystem metrics among low‐coral reefs can facilitate the maintenance of reefs with sustained functions and services as we work to restore degraded systems. This framework can be applied to other ecosystems in the Anthropocene to better understand variance in ecosystem service outcomes and identify where and why bright spots exist.     

Forecasting intensifying disturbance effects on coral reefs

Anticipating future changes of an ecosystem's dynamics requires knowledge of how its key communities respond to current environmental regimes. The Great Barrier Reef (GBR) is under threat, with rapid changes of its reef‐building hard coral (HC) community structure already evident across broad spatial scales. While several underlying relationships between HC and multiple disturbances have been documented, responses of other benthic communities to disturbances are not well understood. Here we used statistical modelling to explore the effects of broad‐scale climate‐related disturbances on benthic communities to predict their structure under scenarios of increasing disturbance frequency. We parameterized a multivariate model using the composition of benthic communities estimated by 145,000 observations from the northern GBR between 2012 and 2017. During this time, surveyed reefs were variously impacted by two tropical cyclones and two heat stress events that resulted in extensive HC mortality. This unprecedented sequence of disturbances was used to estimate the effects of discrete versus interacting disturbances on the compositional structure of HC, soft corals (SC) and algae. Discrete disturbances increased the prevalence of algae relative to HC while the interaction between cyclones and heat stress was the main driver of the increase in SC relative to algae and HC. Predictions from disturbance scenarios included relative increases in algae versus SC that varied by the frequency and types of disturbance interactions. However, high uncertainty of compositional changes in the presence of several disturbances shows that responses of algae and SC to the decline in HC needs further research. Better understanding of the effects of multiple disturbances on benthic communities as a whole is essential for predicting the future status of coral reefs and managing them in the light of new environmental regimes. The approach we develop here opens new opportunities for reaching this goal.     

Herbivory on coral reefs: algal susceptibility to herbivorous fishes

Summary The susceptibility of several tropical algal species to fish grazing was studied on the Belizean barrier reef off the Caribbean coast of Central America. Short-term transplant experiments indicate that plant species vary markedly in their rates of biomass loss to grazing by a shallow-water guild of herbivorous fishes. Algal species transplanted from habitats with low grazing pressure are highly susceptible to grazing, while species occurring in habitats with high herbivore densities are highly resistant to grazing. Algal species show differential susceptibility to grazing by two major components of the tropical herbivore guild, Acanthurus (surgeonfishes) and Sparisoma (parrotfishes).Variability in plant susceptibility to grazing by herbivorous fishes was not clearly correlated with morphological or chemical characteristics that have been previously suggested as plant defenses against herbivory. Plants found to be highly resistant to fish grazing, such as Halimeda, exhibit both morphological characteristics and secondary chemical compounds which do appear to reduce herbivory. In contrast, species of Caulerpa, Sargassum, Turbinaria, and Padina, which also possess alleged morphological and/or chemical defenses, are nevertheless highly susceptible to fish grazing.     

Biological destruction of coral reefs

The major agents of biological destruction of coral reefs can be divided into grazers, etchers and borers. Each of these groups is reviewed on a world wide basis, together with the mechanisms by which they destroy the coral substrate. Rates of bioerosion attributed to major agents of grazers, etchers and borers are given, together with limitations of some of the measurements. Recent work is highlighting the variability in rates of bioerosion both over time and space. Factors which may be responsible for this variability are discussed. Bioerosion is a major factor influencing reef morphology and the ways in which this is achieved is discussed in some detail. Although the review concentrates mainly on present day reefs, some attempt is made to consider the impact of bioerosion on older reefs.     

Marine pollution and coral reefs

Coral reefs are exposed to many anthropogenic stresses increasing in impact and range, both on local and regional scales. The main ones discussed here are nutrient enrichment, sewage disposal, sedimentation, oil-related pollution, metals and thermal pollution. The stress comprising the main topic of this article, eutrophication, is examined from the point of view of its physiological and ecological mechanisms of action, on a number of levels. Nutrient enrichment can introduce an imbalance in the exchange of nutrients between the zooxanthellae and the host coral, it reduces light penetration to the reef due to nutrient- stimulated phytoplankton growth, and, most harmful of all, may bring about proliferation of seaweeds. The latter rapidly outgrow, smother and eventually replace, the slow-growing coral reef, adapted to cope with the low nutrient concentrations typical in tropical seas.
Eutrophication seldom takes place by itself. Sewage disposal invariably results in nutrient enrichment, but it also enriches the water with organic matter which stimulates proliferation of oxygen-consuming microbes. These may kill corals and other reef organisms, either directly by anoxia, or by related hydrogen sulfide production. Increased sediment deposition is in many cases associated with other human activities leading to eutrophication, such as deforestation and topsoil erosion.
Realistically achievable goals to ensure conservation, and in some instances, rehabilitation of coral reefs are listed.     

珊瑚礁生态修复研究进展

珊瑚礁生态系统有着很高的生物多样性和重要的生态功能。20世纪80年代以后全球范围内珊瑚礁的大面积退化引起了人们广泛的关注。简述了世界珊瑚礁资源现状,破坏原因,生态修复方法以及我国的珊瑚礁资源现状和修复策略等。国际上通用的生态修复策略主要是根据珊瑚的两种繁殖方式进行的,此外再配合人为的适度干扰,增加珊瑚的成活率。方法主要有：珊瑚移植、Gardening、人工渔礁、底质稳固、幼体附着等以及对相关利益者的宣传,海岸带的保护等。我国珊瑚礁退化严重,但是由于缺乏相关的科技资料报道和技术支持,缺乏系统的研究,使得珊瑚礁的生态修复成绩甚微,今后应在该领域开展更多的工作。     

Aggression among sea urchins on Caribbean coral reefs

The existence and nature of intra- and interspecific aggression were examined for five species of sea urchins inhabiting Caribbean coral reefs. The studies took place in the San Blas Islands of Panama and involved the following species: Echinometra lucunter (Linnaeus), E. viridis Agassiz, Diadema antillarum Phillipi, Lytechinus williamsi Chesher, and Eucidaris tribuloides (Lamarck). An intruder was placed next to an undisturbed resident and the behaviors and responses of both individuals were followed. All pairwise combinations of resident species and intruder species were tested except for combinations involving D. antillarum as intruder.

For E. viridis and E. lucunter, agonistic interactions occurred commonly (46–79% of trials) between conspecifics and congeners. Almost all of the agonistic encounters involved pushing. Additionally, biting occurred in 8–25 % of the trials. Residents were most often the aggressors and usually succeeded in retaining their location. Intruders only succeeded in forcing residents out of their positions if the intruder was equal to or larger in size than the resident. Surveys of an undisturbed population of E. viridis during the daytime indicated that 16% of the individuals were engaged in intraspecific agonistic interactions at any one time.

D. antillarum exhibited biting behavior against both species of Echinometra in 23–24% of the trials. Biting attacks against L. williamsi and E. tribuloides occurred rarely. L. williamsi only once demonstrated pushing behavior and never was observed biting another sea urchin. E. tribuloides occasionally exhibited pushing and biting behaviors, both as residents and as intruders, and was twice observed biting Echinometra.

These studies suggest that two kinds of agonistic interactions may commonly occur among Caribbean reef-dwelling sea urchins: (1) intraspecific and interspecific aggression among Echinometra, and (2) predatory/aggressive attacks against Echinometra by D. antillarum. The former may result in greater dispersion of Echinometra relative to food and shelter resources and control the spatial distribution and concentration of Echinometra grazing pressure within an area. The attacks by D. antillarum may result in the restriction to, or higher densities of, Echinometra in crevices and rugose microhabitats that provide shelter from the larger-bodied D. antillarum.     

Not worth the risk: apex predators suppress herbivory on coral reefs

Apex predators are known to exert strong ecological effects, either through direct or indirect predator–prey interactions. Indirect interactions have the potential to influence ecological communities more than direct interactions as the effects are propagated throughout the population as opposed to only one individual. Indirect effects of apex predators are well documented in terrestrial environments, however there is a paucity of information for marine environments. Furthermore, manipulative studies, as opposed to correlative observations, isolating apex predator effects are lacking. Coral reefs are one of the most diverse ecosystems, providing a useful model system for investigating the ecological role of apex predators and their influence on lower trophic levels. Using predator models and transplanted macroalgae we examined the indirect effects of predators on herbivore foraging behaviour. We show that the presence of a model reef shark or large coral‐grouper led to a substantial reduction in bite rate and species richness of herbivorous fishes and an almost absolute localized cessation of macroagal removal, due to the perceived risk of predation. A smaller‐sized coral‐grouper also reduced herbivore diversity and activity but to a lesser degree than the larger model predators. These indirect effects of apex predators on the foraging behaviour of herbivores may have flow‐on effects on the biomass and distribution of macroalgae, and the functioning of coral reef ecosystems. This highlights that the ecological interactions and processes that contribute to ecosystem resilience may be more complex than previously assumed.     

Spatial dynamics of benthic competition on coral reefs

The community structure of sedentary organisms is largely controlled by the outcome of direct competition for space. Understanding factors defining competitive outcomes among neighbors is thus critical for predicting large-scale changes, such as transitions to alternate states within coral reefs. Using a spatially explicit model, we explored the importance of variation in two spatial properties in benthic dynamics on coral reefs: (1) patterns of herbivory are spatially distinct between fishes and sea urchins and (2) there is wide variation in the areal extent into which different coral species can expand. We reveal that the size-specific, competitive asymmetry of corals versus fleshy algae highlights the significance of spatial patterning of herbivory and of coral growth. Spatial dynamics that alter the demographic importance of coral recruitment and maturation have profound effects on the emergent structure of the reef benthic community. Spatially constrained herbivory (as by sea urchins) is more effective than spatially unconstrained herbivory (as by many fish) at opening space for the time needed for corals to settle and to recruit to the adult population. Further, spatially unconstrained coral growth (as by many branching coral species) reduces the number of recruitment events needed to fill a habitat with coral relative to more spatially constrained growth (as by many massive species). Our model predicts that widespread mortality of branching corals (e.g., Acropora spp) and herbivorous sea urchins (particularly Diadema antillarum) in the Caribbean has greatly reduced the potential for restoration across the region.