Declining coral calcification in massive Porites in two nearshore regions of the northern Great Barrier Reef

Temporal and spatial variation in the growth parameters skeletal density, linear extension and calcification rate in massive Porites from two nearshore regions of the northern Great Barrier Reef (GBR) were examined over a 16‐year study period. Calcification rates in massive Porites have declined by approximately 21% in two regions on the GBR ～450 km apart. This is a function primarily of a decrease in linear extension (～16%) with a smaller decline in skeletal density (～6%) and contrasts with previous studies on the environmental controls on growth of massive Porites on the GBR. Changes in the growth parameters were linear over time. Averaged across colonies, skeletal density declined over time from 1.32 g cm^?3 (SE = 0.017) in 1988 to 1.25 g cm^?3 (0.013) in 2003, equivalent to 0.36% yr^?1 (0.13). Annual extension declined from 1.52 cm yr^?1 (0.035) to 1.28 cm yr^?1 (0.026), equivalent to 1.02% yr^?1 (0.39). Calcification rates (the product of skeletal density and annual extension) declined from 1.96 g cm^?2 yr^?1 (0.049) to 1.59 g cm^?2 yr^?1 (0.041), equivalent to 1.29% yr^?1 (0.30). Mean annual seawater temperatures had no effect on skeletal density, but a modal effect on annual extension and calcification with maxima at ～26.7 °C. There were minor differences in the growth parameters between regions. A decline in coral calcification of this magnitude with increasing seawater temperatures is unprecedented in recent centuries based on analysis of growth records from long cores of massive Porites. We discuss the decline in calcification within the context of known environmental controls on coral growth. Although our findings are consistent with studies of the synergistic effect of elevated seawater temperatures and pCO₂ on coral calcification, we conclude that further data on seawater chemistry of the GBR are required to better understand the links between environmental change and effects on coral growth.     

Densitometry from digitized images of X-radiographs: Methodology for measurement of coral skeletal density

Skeletons of massive coral colonies contain annual density bands that are revealed by X-radiography of slices cut along growth axes. These bands allow measurement of skeletal growth parameters such as annual extension rate and annual calcification rate. Such measurements have been important in understanding coral growth, in assessing environmental impacts and in recovering proxy environmental information. Measurements of coral calcification rate from annual density banding require measurements of skeletal density along tracks across skeletal slices and, until now, such density measurements have depended upon specialized and expensive equipment. Here, we describe a straightforward, inexpensive and accurate technique for measuring skeletal density from digitized images of X-radiographs of coral skeletal slices. An aragonitic step-wedge was included in each X-radiograph of a coral slice together with two aluminium bars positioned along the anode-cathode axis. Optical density was measured along tracks across the X-ray images of these different objects. The aragonite step-wedge provided a standard for converting optical density to skeletal density. The aluminium bars were used to correct for the heel effect—a variation in the intensity of the X-ray beam along the anode-cathode axis that would, otherwise, introduce large errors into measurements of skeletal density. Exposure was found to vary from X-radiographs to X-radiograph, necessitating the inclusion of the calibration standards in each X-radiograph of a coral slice. Results obtained using this technique compared well with results obtained by direct gamma densitometry of skeletal slices.     

Phenotypic plasticity for skeletal growth,density and calcification of <Emphasis Type="Italic">Porites lobata</Emphasis> in response to habitat type

A reciprocal transplant experiment (RTE) of the reef-building coral Porites lobata between shallow (1.5 m at low tide) back reef and forereef habitats on Ofu and Olosega Islands, American Samoa, resulted in phenotypic plasticity for skeletal characteristics. Transplants from each source population (back reef and forereef) had higher skeletal growth rates, lower bulk densities, and higher calcification rates on the back reef than on the forereef. Mean annual skeletal extension rates, mean bulk densities, and mean annual calcification rates of RTE groups were 2.6–9.8 mm year⁻¹, 1.41–1.44 g cm⁻³, and 0.37–1.39 g cm⁻² year⁻¹ on the back reef, and 1.2–4.2 mm year⁻¹, 1.49–1.53 g cm⁻³, and 0.19–0.63 g cm⁻² year⁻¹ on the forereef, respectively. Bulk densities were especially responsive to habitat type, with densities of transplants increasing on the high energy forereef, and decreasing on the low energy back reef. Skeletal growth and calcification rates were also influenced by source population, even though zooxanthella genotype of source colonies did not vary between sites, and there was a transplant site x source population interaction for upward linear extension. Genetic differentiation may explain the source population effects, or the experiment may have been too brief for phenotypic plasticity of all skeletal characteristics to be fully expressed. Phenotypic plasticity for skeletal characteristics likely enables P. lobata colonies to assume the most suitable shape and density for a wide range of coral reef habitats.     

Extension growth rates in two coral species from high-latitude reefs of Bermuda

Mean annual growth rates (skeletal linear extension) in the hermatypic coralsPorites astreoides Lamarck andDiploria labyrinthiformis (L.) were investigated mainly by X-radiography from a variety of localities at various depths on the high-latitude coral reefs of Bermuda. Growth rates of both species show an inverse curvilinear relationship with depth, with highest growth rates in the shallow inshore waters of Castle Harbour and lowest at the edge of the Bermuda platform and on the adjacent fore-reef slope. Annual density bands form seasonal couplets, with narrow, high density bands appearing to form in the spring-summer months and wider, low density bands over the rest of the year in both species. Comparison of the growth rates ofP. astreoides from Bermuda with those from lower latitude West Indian localities, particularly Jamaica, indicates an inverse relationship with latitude and a similar inverse curvilinear relationship with depth at both geographic locations. Growth rate-locality differences in Bermuda for both species are suggested to be controlled mainly by local differences in wave energy and food supply and possibly seasonal water temperature fluctuations; growth rate-depth differences by decreasing illumination with depth; and growth rate-latitudinal differences by reduction in winter water temperatures and light levels with increasing latitude.     

Facilitation in Caribbean coral reefs: high densities of staghorn coral foster greater coral condition and reef fish composition

Recovery of the threatened staghorn coral (Acropora cervicornis) is posited to play a key role in Caribbean reef resilience. At four Caribbean locations (including one restored and three extant populations), we quantified characteristics of contemporary staghorn coral across increasing conspecific densities, and investigated a hypothesis of facilitation between staghorn coral and reef fishes. High staghorn densities in the Dry Tortugas exhibited significantly less partial mortality, higher branch growth, and supported greater fish abundances compared to lower densities within the same population. In contrast, partial mortality, branch growth, and fish community composition did not vary with staghorn density at the three other study locations where staghorn densities were lower overall. This suggests that density-dependent effects between the coral and fish community may only manifest at high staghorn densities. We then evaluated one facilitative mechanism for such density-dependence, whereby abundant fishes sheltering in dense staghorn aggregations deliver nutrients back to the coral, fueling faster coral growth, thereby creating more fish habitat. Indeed, dense staghorn aggregations within the Dry Tortugas exhibited significantly higher growth rates, tissue nitrogen, and zooxanthellae densities than sparse aggregations. Similarly, higher tissue nitrogen was induced in a macroalgae bioassay outplanted into the same dense and sparse aggregations, confirming greater bioavailability of nutrients at high staghorn densities. Our findings inform staghorn restoration efforts, suggesting that the most effective targets may be higher coral densities than previously thought. These coral-dense aggregations may reap the benefits of positive facilitation between the staghorn and fish community, favoring the growth and survivorship of this threatened species.     

Effects of corallivory and coral colony density on coral growth and survival

A suite of processes drive variation in coral populations in space and time, yet our understanding of how variation in coral density affects coral performance is limited. Theory predicts that reductions in density can send coral populations into a predator pit, where concentrated corallivory maintains corals at low densities. In reality, how variation in coral density alters corallivory rates is poorly resolved. Here, we experimentally quantified the effects of corallivory and coral density on growth and survival of small colonies of the staghorn coral Acropora pulchra. Our findings suggest that coral density and corallivory have strong but independent effects on coral performance. In the presence of corallivores, corals suffered high but density-independent mortality. When corallivores were excluded, however, vertical extension rates of colonies increased with increasing densities. While we found no evidence for a predator pit, our results suggest that spatio-temporal variation in corallivore and coral densities can fundamentally alter population dynamics via strong effects on juvenile corals.

     

Structure and growth rates of the high-latitude coral: <Emphasis Type="Italic">Plesiastrea versipora</Emphasis>

The high-latitude coral species Plesiastrea versipora was investigated to identify growth rates in colonies over 1 m in diameter. Six colonies from two temperate gulfs (latitudes of 33°–35°S) in Southern Australia were examined using X-ray, luminescence and ²³⁸U/²³⁰Th dating techniques. Annual density bands were present in each coral but varied in width and definition, suggesting different linear extension and calcification rates. Differences in density band width were observed at the local scale (amongst colonies on the same reef) and regional scales (between the two gulfs). Extension rates of the P. versipora colonies examined in this study varied between 1.2 and 7 mm per year, which are amongst the slowest growth rates reported for hermatypic corals. As only one of the six P. versipora colonies had obvious luminescent banding, we conclude that luminescent banding is not an accurate chronological marker in this species of temperate water coral. Coral age estimates derived from counting density bands in X-radiographs ranged from 90 to 320 years for the six colonies studied. U-Th ages from the same colonies determined by high-precision multi-collector inductively coupled plasma mass spectrometer established radiometric ages between 105 and 381 years. The chronological variation in absolute ages between the two techniques varied between 2 and 19% in different colonies, with the lowest growth rates (~1 mm) displaying the greatest variation between density band age and radiometric U-Th age. This result implies that the age of P. versipora and other slow-growing corals cannot be determined accurately from density bands alone. The outcome of this research demonstrates that P. versipora may be useful as a paleoclimate archive, recording several centuries in a single colony in high-latitude environments (corals found in latitudes greater than 30° in either hemisphere), where other well-established coral climate archives, such as Porites, do not occur.     

River runoff reconstructions from novel spectral luminescence scanning of massive coral skeletons

Inshore massive corals often display bright luminescent lines that have been linked to river flood plumes into coastal catchments and hence have the potential to provide a long-term record of hinterland precipitation. Coral luminescence is thought to result from the incorporation of soil-derived humic acids transported to the reef during major flood events. Corals far from terrestrial sources generally only exhibit dull relatively broad luminescence bands, which are attributed to seasonal changes in coral density. We therefore tested the hypothesis that spectral ratios rather than conventional luminescence intensity provide a quantitative proxy record of river runoff without the confounding effects of seasonal density changes. For this purpose, we have developed a new, rapid spectral luminescence scanning (SLS) technique that splits emission intensities into red, green and blue domains (RGB) for entire cores with an unprecedented linear resolution of 71.4 μm. Since humic acids have longer emission wavelength than the coral aragonite, normalisation of spectral emissions should yield a sensitive optical humic acid/aragonite ratio for humic acid runoff, i.e., G/B ratio. Indeed, G/B ratios rather than intensities are well correlated with Ba/Ca, a geochemical coral proxy for sediment runoff, and with rainfall data, as exemplified for coral records from Madagascar. Coral cores also display recent declining trends in luminescence intensity, which are also reported in corals elsewhere. Such trends appear to be associated with a modern decline in skeletal densities. By contrast, G/B spectral ratios not only mark the impact of individual cyclones but also imply that humic acid runoff increased in Madagascar over the past few decades while coral skeletal densities decreased. Consequently, the SLS technique deconvolves the long-term interplay between humic acid incorporation and coral density that have confounded earlier attempts to use luminescence intensities as a proxy for river runoff.     

Seasonal growth bands in Famennian rugose coral Scruttonia kunthi and their environmental significance

Large colonies of rugose coral Scruttonia kunthi occurring in the upper Famennian of Sudetes (southern Poland) reveal distinct growth banding in their skeletons. They were investigated for internal structural characteristics and stable isotopic composition. The skeletal tissue consists of alternating light and dark bands which differ in thickness, density and morphology of structural elements, and in occurrence of corallite contraction and rejuvenescense. Darker parts with densely arranged thick skeletal elements are thin in comparison to lighter parts. In addition, they include frequently offsets and contraction of corallites. A couplet of dense and less dense bands is interpreted to represent most probably an annual cycle. The calculated growth rate for Scruttonia kunthi varied from 6 mm/yr to 12 mm/yr. Growth-band formation was influenced environmentally. Oxygen isotopic data provide an evidence that high-density bands were formed in the season of higher environmental stress, with relatively warmer temperatures and higher sedimentation rates. Carbon isotopic signatures are very uniform, and thus enigmatic. They indicate that at least growth rate of the skeleton and seawater temperature had no influence on the coral δ¹³C.     

Declining coral skeletal extension for forereef colonies of Siderastrea siderea on the Mesoamerican Barrier Reef System, Southern Belize

Background

Natural and anthropogenic stressors are predicted to have increasingly negative impacts on coral reefs. Understanding how these environmental stressors have impacted coral skeletal growth should improve our ability to predict how they may affect coral reefs in the future. We investigated century-scale variations in skeletal extension for the slow-growing massive scleractinian coral Siderastrea siderea inhabiting the forereef, backreef, and nearshore reefs of the Mesoamerican Barrier Reef System (MBRS) in the western Caribbean Sea.

Methodology/Principal Findings

Thirteen S. siderea cores were extracted, slabbed, and X-rayed. Annual skeletal extension was estimated from adjacent low- and high-density growth bands. Since the early 1900s, forereef S. siderea colonies have shifted from exhibiting the fastest to the slowest average annual skeletal extension, while values for backreef and nearshore colonies have remained relatively constant. The rates of change in annual skeletal extension were −0.020±0.005, 0.011±0.006, and −0.008±0.006 mm yr⁻¹ per year [mean±SE] for forereef, backreef, and nearshore colonies respectively. These values for forereef and nearshore S. siderea were significantly lower by 0.031±0.008 and by 0.019±0.009 mm yr⁻¹ per year, respectively, than for backreef colonies. However, only forereef S. siderea exhibited a statistically significant decline in annual skeletal extension over the last century.

Conclusions/Significance

Our results suggest that forereef S. siderea colonies are more susceptible to environmental stress than backreef and nearshore counterparts, which may have historically been exposed to higher natural baseline stressors. Alternatively, sediment plumes, nutrients, and pollution originating from watersheds of Guatemala and Honduras may disproportionately impact the forereef environment of the MBRS. We are presently reconstructing the history of environmental stressors that have impacted the MBRS to constrain the cause(s) of the observed reductions in coral skeletal growth. This should improve our ability to predict and potentially mitigate the effects of future environmental stressors on coral reef ecosystems.     

Influence of size and seasonal factors on the growth of the deep-sea coral Flabellum alabastrum in mesocosm

Growth rate (linear skeleton extension) was determined in live specimens of the deep-sea cup coral Flabellum alabastrum (Anthozoa: Flabellidae) collected between 600 and 1,200 m off insular Newfoundland (eastern Canada) and kept under laboratory conditions for over 2 years. Smaller individuals grew faster (~5 mm year⁻¹) than larger ones (~1 mm year⁻¹). Seasonal variations in extension rates and qualitative appearance of the growth bands were recorded, with maximum extension occurring in late summer and early fall during maxima in seawater temperature, zooplankton levels, and deposition of suspended detritus. Estimates from a growth model indicate that the largest individuals of F. alabastrum (~43 mm calyx height) are at least 45 years old.     

Calibration of x-ray densitometry for the measurement of coral skeletal density

A new method is proposed for the measurement of coral skeletal density by x-radiography. X-radiographs were made of sections cut from skeletons of massive corals of the genus Porites. Included on the x-ray film with each specimen were an aluminium step-wedge, a set of aragonite standards and several aluminium bars, all of measured thickness and density. The images on the developed x-ray film were scanned with a microdensitometer. Semilogarithmic plots of microdensitometer output voltage vs. thickness of the aluminium and aragonite standards provided characteristic curves, with initial linear slopes which were defined as relative linear absorption coefficients. These coefficients varied with the x-ray exposure and microdensitometer measurement conditions; however, they were consistent within one set of conditions. The relative linear absorption coefficients and densities of the standards, together with data for thickness of standard vs. film exposure, can be used to determine the density of coral specimens at points along microdensitometer traverses of their x-ray images. The bars of aluminium can be used to correct for the non-uniform irradiation of the x-ray film which is characteristic of x-ray machines.Contribution No. 299 from the Australian Institute of Marine Science     

Demographic aspects of the soft coral Sinularia flexibilis leading to local dominance on coral reefs

We evaluated the role that demography may play in the formation of local aggregations of Sinularia flexibilis (Quoy & Gaimard, 1833), a soft coral that commonly dominates inshore coral reefs of the Great Barrier Reef (GBR), Australia. Two populations on inshore reefs of the Palm Islands were censused once a year for 3 years, starting 10 mo after the extensive bleaching mortality in early 1998. Larger colonies became more prevalent over time; mean colony size increasing by 35%, from 276 cm² in 1998 to 373 cm² in 2000. Growth rates were size dependent, with smaller colonies growing proportionally faster than larger colonies. Change in size relative to initial size indicated an expected mean annual growth of 128 cm² for a 50-cm² colony. Zero growth was predicted at 532±21cm², with colonies larger than this likely to undergo fission or shrink. Forty-three percent of colonies were undergoing fission at any time at both localities. Most new colonies were produced by fission (70%, n=285), with the remainder produced by the recruitment of sexually produced larvae (19%) or by colony translocation (11%). The sexual and asexual recruitment rates were 0.24 and 1.0 recruits m- ² year−¹, respectively. Opportunistic recruitment and rapid growth following disturbances are commonly assumed to be the mechanisms leading soft corals to dominate locally. In this study, these mechanisms operated more slowly than expected, with no net change in population size.     

Coral growth rates and environmental control of density banding

Linear and mass growth rates are compared for the massive coral species Favia pallida (Dana), Goniastrea retiformis (Lamarck), and Porites lutea Milne Edwards & Haime at Enewetak Atoll. Marshall Islands. Goniastrea retiformis is the densest of the three species and has an intermediate growth rate; Porites lutea grows more rapidly. All three grow indeterminately at a declining rate with increasing depth.The high-density portion of annual band couplets is produced during the late summer and fall when water temperatures are highest and possibly the availability of light is reduced, and the low density portion is formed during periods of seasonally lower water temperatures and possibly higher availability of light. A similar pattern is found in three massive coral species from Belize.The high density portions of annual bands account for a greater proportion of linear and mass growth in deeper water and in corals with relatively slow growth rates. I predict that linear growth rates will be highest where conditions are most favorable for deposition of the low density portion.Geographical patterns of coral density banding based on the literature are discussed and a model is proposed relating the interplay of light availability and water temperature to the production of high and low density skeletal bands.     

Genetic control of septal numbers and the species problem in a fossil solitary scleractinian coral

Intraspecific morphological variations of a Pleistocene solitary scleractinian coral, Cylindrophyllia orientatis (Yabe & Eguchi), have been examined based on 792 specimens. The specimens are discoidal to short cylindrical in shape, with no significant change in their diameter during skeletal growth. Septal arrangements of the coralla are observed on upper and basal surfaces. Septal numbers do not change through the ontogeny of each corallum, even when the last cycle of septa is incomplete. Septal arrangements and numbers are controlled by intrinsic genetic factors. Heights of the coralla are controlled by environmental factors where they lived If growth rates are presumed to be constant, heights can be regarded as indicating age of specimens. Assuming that this is the case, the survivorship curve shows that this fossil population had a constant death rate. Two varieties exist in this population: one has 20–28 septa, the other 30–48 septa, showing a dimorphic feature. Scleractinian coral, intraspecific variation, population, septa, species problem ,     

Use of Bayesian analysis with individual‐based modeling to project outcomes of coral restoration

Various approaches to coral restoration have been developed to help increase rate of reef recovery from perturbations, among the most common of which is coral transplantation. Success is often evaluated based on short‐term observations that capture only the initial phase of space colonization by coral transplants. Here, an individual‐based model is developed to quantify uncertainty in future trajectories in experimental plots given past observations. Empirical data were used to estimate probabilistic growth, survival, and fission rates of Acropora pulchra and A. intermedia (order Scleractinia) in a sandy reef flat (Bolinao, Philippines). Simulations were initialized with different densities (25 or 50 transplants per species per 16 m²) to forecast possible coral cover trajectories over a 5‐year period. Given current conditions, there is risk of local extinction which is higher in low‐density plots for both species, and higher for A. intermedia compared to A. pulchra regardless of density. While total coral cover is projected to increase, species composition in the future is more likely to be highly uneven. The model was used to quantify effect on recovery rate of protection from pulse anthropogenic disturbances, given different initial transplantation densities. When monitoring data are limited in time, stochastic models may be used to assess whether the restoration trajectory is heading toward the desired state and at what rate, and foresee system response to various adaptive interventions.     

Computerized tomography and skeletal density of coral skeletons

In this paper I describe and discuss the use of medical X-ray computerized tomography (CT) in the study of coral skeletons. CT generates X-ray images along freely chosen sections through the skeleton and offers, as well, the possibility of density measurements based on X-ray attenuation. This method has been applied to measure the skeletal density of the Caribbean reef-building coral Montastrea annularis, from Curaçao, Netherlands Antilles. The observed, non-linear increase of skeletal density with depth can be attributed to decreasing photo-synthetic rates with increasing water depth. A comparison with extension rate measurements shows the inverse relationship between extension rate and skeletal density. CT proves to be aquick and non-destructive method to reveal growth structures (density banding) since it measures skeletal density.     

The impact of seawater temperature on coral growth parameters of the colonial coral Cladocora caespitosa (Anthozoa, Scleractinia) in the eastern Adriatic Sea

The scleractinian coral Cladocora caespitosa deserves a special place among the major carbonate bioconstructors of the Mediterranean Sea. Annual coral skeleton growth, coral calcification, and skeleton density of the colonial coral C. caespitosa taken from 25 locations in the eastern Adriatic Sea were analyzed and compared with annual sea surface temperatures (SST). The growth rates of the coral C. caespitosa from the 25 stations in the Adriatic Sea ranged from 1.92 to 4.19?mm per year, with higher growth rates of the investigated corallites in the southern part of the Adriatic Sea. These growth rates are similar to those measured in other areas of the Mediterranean Sea. The correlation between coral growth and sea temperatures in the Adriatic Sea is seen as follows: An X-radiograph analysis of coral growth in C. caespitosa colonies that are over 60?years old showed that higher growth rates of this coral coincided with a warmer period in the Mediterranean Sea. A positive significant correlation exists between corallite growth rates and SST and coral calcification and SST. A negative correlation exists between coral density and SST. Coral growth rates also showed a correlation with higher eutrophication caused by nearby fish farms, along with a greater depth of the investigated colonies and high bottom currents.     

Coral density and predation affect growth of a reef-building coral

The influence of predation on the growth of stony corals has gained increased attention, although the degree to which coral conspecific density can modify the effects of corallivores remains poorly studied. Here, a field experiment was used to quantify the independent and combined effects of coral colony density and coral predators on the skeletal growth of massive Porites. Predator exclusion increased coral growth by 20%. Increasing coral density increased growth by 30%. However, the effect of predators was independent of coral density. Possible alternative mechanisms for increased skeletal growth at higher colony density include changes in near-field flow, resulting in increases in photosynthetic activity, nutrient uptake, or the increased accessibility of coral mutualists.     

Growth rhythms recorded in stable isotopes and density bands in the reef coral Porites lobata (Cebu,Philippines)

Growth rhythms in the reef coral Porites lobata are revealed by X-radiography and stable carbon and oxygen isotopic analysis. High density increments were deposited during warm temperatures in summer and low density increments during winter. The seasonal temperature variations are reflected in the oxygen isotope ratios. The coral carbonate shows a constant depletion in ¹⁸O of –2.7%₀ relative to calcite in equilibrium with the ambient seawater. The mean annual growth rate of the specimen studied was 1.3±0.3 cm/year.