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.     

Competition between corals and algae on coral reefs: a review of evidence and mechanisms

Despite widespread acceptance that competition between scleractinian corals and benthic algae is important to the structure of coral reef communities, there is little direct experimental evidence that corals and algae do compete, and very little data on the processes and causality of their interactions. Most available evidence is observational or correlative, with intrinsic risks of confounded causality. This paper reviews and categorises the available evidence, concluding that competition between corals and algae probably is widespread on coral reefs, but also that the interaction varies considerably. Widespread replacement of corals by algae may often indicate coral mortality due to external disturbances, rather than competitive overgrowth, but may lead to competitive inhibition of coral recruitment, with consequences for reef recovery. We list eight specific processes by which corals and algae may affect each other, and suggest life history properties that will influence which of these interactions are possible. We propose a matrix for algal effects on corals, which lists the subset of processes possible for each combination of coral life form and algal functional group. This table provides a preliminary framework for improved understanding and interpretation of coral-algal interactions.     

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.     

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.     

Ecology: a different route to recovery for coral reefs

Worldwide, many coral reef ecosystems have undergone regime shifts, changing from domination by coral to domination by algae. New work indicates that the return path is surprisingly different from the forward one.     

Standing stock of microzooplankton on coral reefs: a preliminary study

The standing stock of microzooplankton on mid-shelf coral reefsin the central Great Barrier Reef was investigated based onsamples collected with a Niskin bottle in February and October1989 The observed net decrease in standing stock of microzooplanktonacross the reef flat suggests that a lack of information ontheir standing stock on coral reefs has led to an underestimateof the allochthonous energy input to coral reef systems.     

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.     

Regenerative functions and microbial ecology of coral reefs: Labelled bacteria in a coral reef microcosm

Representative coral reef organisms and substrata assembled in a laboratory microcosm removed radioactively labelled bacteria from water circulated over them. A similar experiment with a reef clam and its algal-encrusted base gave similar results. Biochemical fractionation of selected organisms in these experiments suggested digestion and possible assimilation of bacterial proteins. In view of previous results concerning the microbial ecology of coral reefs, it is suggested that reef infaunal metazoa are adapted to utilize internal sedimentary processes and regenerative functioning through suspension- (and deposit-)feeding mechanisms. A model ecosystem is presented to suggest the possible feedback of these mechanisms as they operate within a reef.     

Tropical marine fisheries and the future of coral reefs: a brief review with emphasis on Southeast Asia

Coral Reefs -  Rapidly growing human population and economic inequities are placing increasing demands on tropical marine fisheries. Coral reef fisheries constitute an important source of...     

Bioindicators of changes in water quality on coral reefs: review and recommendations for monitoring programmes

Effective environmental management requires monitoring programmes that provide specific links between changes in environmental conditions and ecosystem health. This article reviews the suitability of a range of bioindicators for use in monitoring programmes that link changes in water quality to changes in the condition of coral-reef ecosystems. From the literature, 21 candidate bioindicators were identified, whose responses to changes in water quality varied spatially and temporally; responses ranged from rapid (hours) changes within individual corals to long-term (years) changes in community composition. From this list, the most suitable bioindicators were identified by determining whether responses were (i) specific, (ii) monotonic, (iii) variable, (iv) practical and (v) ecologically relevant to management goals. For long-term monitoring programmes that aim to quantify the effects of chronic changes in water quality, 11 bioindicators were selected: symbiont photophysiology, colony brightness, tissue thickness and surface rugosity of massive corals, skeletal elemental and isotopic composition, abundance of macro-bioeroders, micro- and meiobenthic organisms such as foraminifera, coral recruitment, macroalgal cover, taxonomic richness of corals and the maximal depth of coral-reef development. For short-term monitoring programmes, or environmental impact assessments that aim to quantify the effects of acute changes in water quality, a subset of seven of these bioindicators were selected, including partial mortality. Their choice will depend on the specific objectives and the timeframe available for each monitoring programme. An assessment framework is presented to assist in the selection of bioindicators to quantify the effects of changing water quality on coral-reef ecosystems.     

Recovery of meiofauna following a short-term disturbance on coral reefs

The present study experimentally determined the short-term effect of different trampling intensities on the phytal habitat and associated meiofauna, and assessed the recovery of these communities on a coral reef. The experiment was conducted within the protected area on the reef of Porto de Galinhas Beach, northeastern Brazil. Within three areas, 30 × 30 cm² plots were subjected to different experimental trampling intensities: 0 (control), 32 (low intensity) and 79 (high intensity) footsteps applied during 3 consecutive days. Samples were taken before trampling started and then 1 day and 1, 2 and 3 months after the end of trampling. Multivariate and univariate analyses were used to test for changes in the phytal substrate, the densities of the main meiofaunal major taxa, and the diversity, comparing the trampling intensities and the time periods. The immediate impact of trampling negatively affected the phytal substrate, causing large reductions in sediment content, total substrate weight, and algal turf height. Trampling also drastically decreased the density of the associated meiofauna and modified the community structure. The major meiofauna groups showed different patterns of recovery during the first and second months. However, 3 months after trampling ended, no impact was found for densities of total meiofauna and for most major taxa, and no differences were observed in the community structure among all trampling intensities. Finally, the results for trampling susceptibility and resilience allowed us to extend the discussion to management strategies for the use of reef areas for recreation.     

Embracing a world of subtlety and nuance on coral reefs

Climate change will homogenise the environment and generate a preponderance of mediocre reefs. Managing seascapes of mediocrity will be challenging because our science is ill prepared to deal with the ‘shades of grey’ of reef health; we tend to study natural processes in the healthiest reefs available. Yet much can be gained by examining the drivers and implications of even subtle changes in reef state. Where strong ecological interactions are discovered, even small changes in abundance can have profound impacts on coral resilience. Indeed, if we are to develop effective early warnings of critical losses of resilience, then monitoring must place greater emphasis on measuring and interpreting changes in reef recovery rates. In terms of mechanism, a more nuanced approach is needed to explore the generality of what might be considered ‘dogma’. A more nuanced approach to science will serve managers needs well and help minimise the rise of mediocrity in coral reef ecosystems.

     

Latitudinal variation in coral communities in eastern Australia: a qualitative biophysical model of factors regulating coral reefs

The processes underlying the distributional limits of both corals and coral reefs can be elucidated by examining coral communities at high latitudes. Coral-dominated communities in eastern Australia cover a latitudinal range of >2,500 km, from the northern Great Barrier Reef (11°S) to South West Rocks (31.5°S). Patterns of coral species richness from 11 locations showed a clear separation between the Great Barrier Reef and subtropical sites, with a further abrupt change at around 31°S. Differences in community structure between the Great Barrier Reef and more southern sites were mainly attributable to higher cover of massive corals, branching Acropora, dead coral and coralline algae on the Great Barrier Reef, and higher cover of macroalgae and bare rock at more southern sites. The absence of some major reef-building taxa (i.e., staghorn Acropora and massive Porites) from most subtropical sites coincided with the loss of reef accretion capacity. Despite high cover of hard corals in communities at up to 31°S, only Lord Howe Island contained areas of reef accretion south of the Great Barrier Reef. Factors that have been hypothesized to account for latitudinal changes in coral community structure include water temperature, aragonite saturation, light availability, currents and larval dispersal, competition between corals and other biota including macroalgae, reduced coral growth rates, and failure of coral reproduction or recruitment. These factors do not operate independently of each other, and they interact in complex ways.     

Science and management of coral reefs: problems and prospects

Conclusion It should be recognised that many principles of reef management do not need further research, as they involve changing human behaviour and activities in order to remove or reduce impacts on reefs. Much of the time of a reef manager is taken up with social, economic and political issues: the integration of reef management into broad coastal zone management objectives; the development of community participation and co-management; and the organisation of training and education pro-grammes so that people in countries where reefs are located are able to take responsibility for their sustainable management.Perhaps the main obstacle to be overcome is poor communication (Harmon 1994). Many reef scientists are already strongly convinced of the need to communicate their results and the implications of these for management and conservation policy (Hatcher et al. 1989), but they may however need to understand that reef managers are not always able or willing to act on their advice because of political, economic or social factors.Pure research is increasingly being conducted within a framework of goals identified as important to society. Funding is invariably easier to obtain if it can be demonstrated that the research will have some ultimate benefit in management terms, and much research is being commissioned because of the need for practical solutions. As the complexity of management becomes more apparent and managers themselves call for more scientific support and advice, the role that science has to play in perceiving and defining problems, understanding the mechanisms involved and strategically assessing potential solutions, becomes more central. Often, only a slight adjustment to a project is required in order for data to be collected that is of direct value to a reef manager.Partnerships built between scientists and managers engaged in adaptive management efforts may lead to more rapid progress in managing reefs and may banish the science and management dichotomy once and for all.     

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...