Christian Wild

Coral surface area quantification–evaluation of established techniques by comparison with computer tomography

The surface area of scleractinian corals represents an important reference parameter required for various aspects of coral reef science. However, with advancements in detection accuracy and novel approaches for coral surface area quantification, evaluation of established techniques in comparison with state-of-the-art technology gains importance to coral researchers. This study presents an evaluation of methodological accuracy for established techniques in comparison to a novel approach composed of computer tomography (CT) and 3-dimensional surface reconstruction. The skeleton surface area of reef corals from six genera representing the most common morphological growth forms was acquired by CT and subsequently measured by computer-aided 3-dimensional surface reconstruction. Surface area estimates for the same corals were also obtained by application of four established techniques: Simple and Advanced Geometry, Wax Coating and Planar Projection Photography. Comparison of the resulting area values revealed significant differences between the majority (82%) of established techniques and the CT reference. Genus-specific analysis assigned the highest accuracy to geometric approximations (Simple or Advanced Geometry) for the majority of assessed coral genera (maximum accuracy: 104%; Simple Geometry with Montipora sp.). The commonly used and invasive Wax Coating technique reached intermediate accuracy (47–74%) for the majority of genera, but performed outstanding in the measurement of branching Acropora spp. corals (maximum accuracy: 101%), while the Planar Projection Photography delivered genera-wide low accuracy (12–36%). Comparison of area values derived from established techniques and CT additionally yielded approximation factors (AFs) applicable as factors in the mathematical improvement of surface area estimates by established techniques in relation to CT reference accuracy.

Detection and enumeration of microbial cells within highly porous calcareous reef sands

In order to assess and to compare the abundances of prokaryotes in coral sands from three different areas in the Indo-Pacific, a technique was developed and evaluated for enumeration of prokaryotes living on and within calcareous grains. Propidium iodide labelling of prokaryotes and consecutive confocal laser scanning microscopy showed microbial colonisation within pores and small fissures of the coral sands. This embedded microbial colonisation required at least four extractions with weak acetic acid to dissolve the grain surface layer in order to detach 97% of the prokaryotic cells. Microbial enumeration based on this technique revealed that the abundance of prokaryotes in the carbonate sands were not significantly different among the three sites, but were about one order of magnitude higher than reported for silicate sands of a similar grain size spectrum. A possible reason for this high abundance of prokaryotes is the complex surface structure of the biogenic calcareous grains, their correspondingly highly porous matrix and the associated ability of prokaryotes to penetrate into carbonate grains. Our results highlight the role of calcareous reef sands as a substratum with a large specific surface area for prokaryotic colonisation and emphasise the contribution of calcium carbonate reef sands for element cycles in subtropical and tropical ecosystems.

A precise and non-destructive method to calculate the surface area in living scleractinian corals using X-ray computed tomography and 3D modeling

The surface area of corals represents a major reference parameter for the standardization of flux rates, for coral growth investigations, and for investigations of coral metabolism. The methods currently used to determine the surface area of corals are rather approximate approaches lacking accuracy, or are invasive and often destructive methods that are inapplicable for experiments involving living corals. This study introduces a novel precise and non-destructive technique to quantify surface area in living coral colonies by applying computed tomography (CT) and subsequent 3D reconstruction. Living coral colonies of different taxa were scanned by conventional medical CT either in air or in sea water. Resulting data volumes were processed by 3D modeling software providing realistic 3D coral skeleton surface reconstructions, thus enabling surface area measurements. Comparisons of CT datasets obtained from calibration bodies and coral colonies proved the accuracy of the surface area determination. Surface area quantifications derived from two different surface rendering techniques applied for scanning living coral colonies showed congruent results (mean deviation ranging from 1.32 to 2.03%). The validity of surface area measurement was verified by repeated measurements of the same coral colonies by three test persons. No significant differences between all test persons in all coral genera and in both surface rendering techniques were found (independent sample t-test: all n.s.). Data analysis of a single coral colony required approximately 15 to 30 min for a trained user using the isosurface technique regardless of the complexity and growth form of the latter, rendering the method presented in this study as a time-saving and accurate method to quantify surface areas in both living coral colonies and bare coral skeletons.

Coral mucus release and following particle trapping contribute to rapid nutrient recycling in a Northern Red Sea fringing reef

Mucusreleased bycoralscanfunctionasan importantenergycarrier andparticle trapinreefecosystems with strong tidal currents. In fringing reefs with calm conditions, these processes may occur on smaller spatial scales. Observations of coral mucus dynamics in the Northern Red Sea revealed highly particle-enriched and negatively buoyant mucusstringsattachedto,27%ofcoralcoloniesforupto79min.MucusstringsofthescleractiniancoralgenusAcropora exhibited three orders of magnitude higher particulate organic carbon and nitrogen concentrations when compared with freshly released coral mucus, which confirms efficient particle trapping. After detachment from the coral surface, more than95% ofmucus strings rapidlydescended tothe reef seafloor withinless than 1m. Suchmucus-induced transportmay account for 21-25% of the total sedimentary particulate organic matter supply. In situ and laboratory analyses of planktonic and benthic microbial degradation of mucus strings showed high rates of up to 16 and 26% particulate organic carbon h � 1 , respectively. These findings suggest a newly discovered, tight sediment-water coupling mechanism via coral

Benthic community composition affects O2 availability and variability in a Northern Red Sea fringing reef

Many coral reef ecosystems experience shifts in benthic community composition from scleractinian corals to algae. However, consequences of such phase shifts on O2 availability, important for many reef organisms, are unresolved. This study therefore comparatively investigated potential in situ effects of different benthic cover by reef macroalgae and scleractinian corals on water column O2 concentrations in a Northern Red Sea fringing reef. Findings revealed that mean daily O2 concentrations at algae-dominated sites were significantly lower compared to coral-dominated sites. Minimum O2 concentrations were significantly negatively correlated, while diurnal variability in O2 concentration was significantly positively correlated, with increasing benthic cover by algae. In contrast, no correlation with coral cover was found. These results indicate that shifts from corals to benthic algae may likely affect both in situ O2 availability and variability. This may be particularly pronounced in reef systems with low water exchange (e.g. closed lagoons) or under calm weather conditions and suggests potential O2-mediated effects on reef organisms.

Mineralization of particulate organic matter derived from coral-reef organisms in reef sediments of the Gulf of Aqaba

In situ and laboratory incubation experiments in a fringing reef in the Gulf of Aqaba were performed to study degradation rates of particulate organic matter in reef sediments. Coral mucus, clam eggs, and zooxanthellae were used as model particulate organic compounds for these experiments. Aerobic and anaerobic mineralization rates were calculated by dissolved inorganic carbon (DIC) and O2 fluxes from the sediments under different particulate organic matter additions. Fast enhancement (approximately twofold) of O2 and DIC fluxes were found with the addition of coral mucus and clam eggs compared with control incubations without addition. Most of the degradation is believed to have occurred anaerobically rather than aerobically (DIC:O2 ratios were 4.3–28.1). Higher degradation rates of coral mucus and clam eggs were estimated in carbonate sediment than in silicate sediment (1.2–1.6-fold), which was attributed to the different physical and chemical properties of both sediments. Our study shows the significance of the reef sediment as a suitable site for microbial degradation of particulate organic material excreted from different reef community organisms. This may increase the regeneration of nutrients in the reef environment necessary to sustain high biological productivity.

Introduction to the theme section “coral reefs in a changing environment”

This theme section, ‘‘Coral reefs in a changing environment’’, stems from the ASLO Aquatic Sciences Meeting 2009 in Nice, France (January 25–30), where session # 28 (Coral reefs and coral communities in a changing environment) was chaired by Cornelia Maier, Bernhard Riegl and Christian Wild. This session included 12 oral and 9 poster contributions covering a broad range of topics, and attracted a large number of visitors. That the upcoming 2010 European ISRS meeting in Wageningen, the Netherlands, has a similar motto, ‘‘Reefs in a changing environment’’, appears to emphasize the importance and timeliness of the question: How do corals and coral reefs respond to rapid environmental change? Clearly, anthropogenically-induced climate change and pollution alter the chemistry and temperature of seawater at an unprecedented and rapid rate, and constitute a major threat to the survival of tropical, temperate and cold-water coral reefs and coral communities. CO2-induced ocean acidification and warming, along with terrestrial runoff and pollution, negatively affect health, growth and calcification of corals. However, the extent to which these indirect and direct anthropogenic stresses interact and impact corals and coral-generated reef ecosystems needs to be addressed in more detail. The following six contributions to the theme section reflect a high diversity with respect to the scientific contents on both the species and the community level. The manuscripts address the effects of the two groups of parameters linked to global climate change: firstly, indirectly anthropogenically caused effects such as surface temperature (Heyward and Negri 2010; Thompson and Dolman 2010; Naumann et al. 2010) and pH (Ries et al. 2010); and, secondly, directly anthropogenically caused effects such as inorganic nutrient concentrations (Naumann et al. 2010; Tanaka et al. 2010) or natural regular disturbances (Browne et al. 2010) on coral metabolism (Naumann et al. 2010; Tanaka et al. 2010; Ries et al. 2010), coral development (Heyward and Negri 2010) and coral reef community structure and cover (Browne et al. 2010; Thompson and Dolman 2010). In this context, the goals were to identify the relative influence of one or more of the factors related to environmental change. The articles in the theme section focusing on the factor temperature reveal that, below the bleaching threshold, it positively correlates with particulate organic matter (POM) release by scleractinian corals (Naumann et al. 2010), but may also negatively effect the development and distribution of their larvae (Heyward and Negri 2010). Thompson and Dolman (2010) highlight that temperature-induced coral bleaching events, even if they only occur in the frequency and severity of those in the recent past, may lead to a reduction of fast-growing staghorn coral cover in nearshore reefs of the Great Barrier Reef (GBR). This was supported by the finding that mass coral bleaching, in contrast to other stress events such as crown-of-thorns starfish outbreaks and cyclones, resulted in significant changes of benthic community composition and hard coral Communicated by Editor in Chief Prof. Rolf Bak