Skeletal Extension Rates Research Articles

Coral growth along a natural gradient of seawater temperature, pH, and oxygen in a nearshore seagrass bed on Dongsha Atoll, Taiwan.

Coral reefs are facing threats from a variety of global change stressors, including ocean warming, acidification, and deoxygenation. It has been hypothesized that growing corals near primary producers such as macroalgae or seagrass may help to ameliorate acidification and deoxygenation stress, however few studies have explored this effect in situ. Here, we investigated differences in coral growth rates across a natural gradient in seawater temperature, pH, and dissolved oxygen (DO) variability in a nearshore seagrass bed on Dongsha Atoll, Taiwan, South China Sea. We observed strong spatial gradients in temperature (5°C), pH (0.29 pH units), and DO (129 μmol O2 kg-1) across the 1-kilometer wide seagrass bed. Similarly, diel variability recorded by an autonomous sensor in the shallow seagrass measured diel ranges in temperature, pH, and DO of up to 2.6°C, 0.55, and 204 μmol O2 kg-1, respectively. Skeletal cores collected from 15 massive Porites corals growing in the seagrass bed at 4 sites revealed no significant differences in coral calcification rates between sites along the gradients. However, significant differences in skeletal extension rate and density suggest that the dynamic temperature, pH, and/or DO variability may have influenced these properties. The lack of differences in coral growth between sites may be because favorable calcification conditions during the day (high temperature, pH, and DO) were proportionally balanced by unfavorable conditions during the night (low temperature, pH, and DO). Alternatively, other factors were simply more important in controlling coral calcification and/or corals were acclimated to the prevailing conditions at each site.

Trade-offs in a reef-building coral after six years of thermal acclimation

There is growing evidence that reef-building corals can acclimate to novel and challenging thermal conditions. However, potential trade-offs that accompany acclimation remain largely unexplored. We investigated physiological trade-offs in colonies of a globally abundant coral species (Pocillopora acuta) that were acclimated ex situ to an elevated temperature of 31 °C (i.e., 1 °C above their bleaching threshold) for six years. By comparing them to conspecifics maintained at a cooler temperature, we found that the energy storage of corals was prioritized over skeletal growth at the elevated temperature. This was associated with the formation of higher density skeletons, lower calcification rates and consequently lower skeletal extension rates, which entails ramifications for future reef-building processes, structural complexity and reef community composition. Furthermore, symbionts were physiologically compromised at 31 °C and had overall lower energy reserves, likely due to increased exploitation by their host, resulting in an overall lower stress resilience of the holobiont. Our study shows how biological trade-offs of thermal acclimation unfold, helping to refine our picture of future coral reef trajectories. Importantly, our observations in this six-year study do not align with observations of short-term studies, where elevated temperatures were often associated with the depletion of energy reserves, highlighting the importance of studying acclimation of organisms at relevant biological scales.

Coral extension rate analysis using computed axial tomography

Biology, geology, and paleoclimatology studies of coral reefs often rely on annual rates of corallite extension determined from cores of skeletal material recovered from scleractinian corals. To examine how corallite orientation within such cores may affect measured extension rates, we used high-resolution X-ray computed axial tomography (CT) to image a suite of short Orbicella faveolata cores from corals living in the northwest Gulf of Mexico. This nondestructive method enabled skeletal extension rates to be determined several ways: (1) along transects that cross individual slice/slab images, (2) along transects that cross both a 0.6-mm-thick CT slice and the corresponding conventional X-ray image from a 8.4-mm-thick slab of skeletal material, and (3) along transects that cross slices/slab images oriented at varied angles relative to a coral core’s longitudinal axis. Additionally, the true three-dimensional (3D) extension rates of individual corallites were determined and compared to the extension rates apparent on a two-dimensional (2D) slice/slab. Results suggest that differences between all methods are not statistically significant, confirming that the conventional extension rate methodology is suitable for studies if the core’s longitudinal axis is appropriately aligned with the growth axis of the corallites. Effects of core misalignment on extension rates measurement and the appearance of skeletal structures in 2D images are described, and the applicability of 3D imagery for investigations of coral polyp growth patterns and coral colony development is demonstrated.

Nearshore coral growth declining on the Mesoamerican Barrier Reef System

Anthropogenic global change and local stressors are impacting coral growth and survival worldwide, altering the structure and function of coral reef ecosystems. Here, we show that skeletal extension rates of nearshore colonies of two abundant and widespread Caribbean corals (Siderastrea siderea, Pseudodiploria strigosa) declined across the Belize Mesoamerican Barrier Reef System (MBRS) over the past century, while offshore coral conspecifics exhibited relatively stable extension rates over the same temporal interval. This decline has caused nearshore coral extension rates to converge with those of their historically slower growing offshore coral counterparts. For both species, individual mass coral bleaching events were correlated with low rates of skeletal extension within specific reef environments, but no single bleaching event was correlated with low skeletal extension rates across all reef environments. We postulate that the decline in skeletal extension rates for nearshore corals is driven primarily by the combined effects of long-term ocean warming and increasing exposure to higher levels of land-based anthropogenic stressors, with acute thermally induced bleaching events playing a lesser role. If these declining trends in skeletal growth of nearshore S. siderea and P. strigosa continue into the future, the structure and function of these critical nearshore MBRS coral reef systems is likely to be severely impaired.

Coral carbon isotope sensitivity to growth rate and water depth with paleo-sea level implications

Although reef coral skeletal carbon isotopes (δ13C) are routinely measured, interpretation remains controversial. Here we show results of a consistent inverse relationship between coral δ13C and skeletal extension rate over the last several centuries in Porites corals at Fiji, Tonga, Rarotonga and American Samoa in the southwest Pacific. Beginning in the 1950s, this relationship breaks down as the atmospheric 13C Suess effect shifts skeletal δ13C > 1.0‰ lower. We also compiled coral δ13C from a global array of sites and find that mean coral δ13C decreases by −1.4‰ for every 5 m increase in water depth (R = 0.68, p < 0.01). This highlights the fundamental sensitivity of coral δ13C to endosymbiotic photosynthesis. Collectively, these results suggest that photosynthetic rate largely determines mean coral δ13C while changes in extension rate and metabolic effects over time modulate skeletal δ13C around this mean value. The newly quantified coral δ13C-water depth relationship may be an effective tool for improving the precision of paleo-sea level reconstruction using corals.

Calibration of the pH-δ11B and temperature-Mg/Li proxies in the long-lived high-latitude crustose coralline red alga Clathromorphum compactum via controlled laboratory experiments

A solid understanding of global oceanic change throughout Holocene time is needed to contextualize and interpret recent observations of rapid warming (Moore, 2016), ocean acidification (Popova et al., 2014; Qi et al., 2017), increasing meltwater input (Halfar et al., 2013; Notz and Stroeve, 2016) and circulation changes (Liu et al., 2017; Rahmstorf et al., 2015; Yang et al., 2016) in the Arctic and subarctic Oceans. Precisely reconstructing acidification and temperature variations throughout the Holocene will provide a vital context for interpreting current environmental changes and future climate projections in the region. However, existing paleoenvironmental reconstructions are sparse and uncertain, largely owing to limited availability of high fidelity paleoceanographic archives, such as marine carbonates, in high latitude waters. Coralline algae of the genus Clathromorphum have emerged as key candidates for reconstructing high-latitude environmental variability at annual to sub-annual resolution. Here, we present the first empirical calibrations of boron isotope-pH and Mg/Li-temperature relationships within the long-lived, crustose coralline red alga Clathromorphum compactum. Calibration experiments were performed in triplicate, growing wild-collected specimens for four months at three controlled temperatures (6.4 – 12.4 oC) and four pCO2 conditions (352 - 3230 ppm), to test the effects of these environmental parameters on the isotopic and elemental composition of the algal skeleton. We find that boron isotopes within the skeleton of C. compactum (δ11Bcc) are well correlated with δ11B of seawater borate (δ11Βborate), defining the following equation: δ11Βcc (2σ) = 1.46 (0.06) δ11Βborate + 6.91 (0.72). This equation can be used to reconstruct δ11Βborate of the coralline alga’s ambient seawater, from which past seawater pH can be calculated. We also identified a strong correlation between skeletal Mg/Li ratio and seawater temperature, defined by the equation: Mg/Li (2σ) = 0.17 (0.02) temperature (oC) + 1.02 (0.16). Therefore, despite the strong biological control that this species appears to exert on calcification site pH (elevated 1.0-1.6 pH units above seawater pH, inferred from δ11Bcc > δ11Βborate), and the apparent relationship between skeletal extension rate and skeletal Li/Ca and Mg/Ca, the δ11Bcc and Mg/Li ratios of the coralline alga’s skeleton strongly and significantly respond to ambient seawater pH and temperature, respectively. These results support the use of δ11B and Mg/Li within C. compactum for pH and temperature reconstructions of northern high-latitude oceans.

Growth and survival of Pocillopora damicornis (Scleractinia: Pocilloporidae) coral fragments and their potential for coral reef restoration in the Tropical Eastern Pacific

ABSTRACTTransplantation of coral fragments is a common procedure for the restoration of degraded coral reefs. In the Tropical Eastern Pacific (TEP), the branching coral Pocillopora damicornis is the main reef-builder, and it frequently propagates through natural fragmentation; it is therefore a candidate for restoration efforts in this region. To evaluate the usefulness of P. damicornis for reef restoration in the TEP, fragments of three lengths were used: small (1–2 cm), medium (4 cm) and large (7 cm). Fragments of these three sizes were attached on the back-reef, reef flat and reef crest of La Azufrada reef (Gorgona Island, Colombian Pacific). Coral length, width, weight, and survival were followed during 119 days. Furthermore, skeletal density, linear extension and calcification rates of each coral fragment were obtained to evaluate the growth rates as a function of initial size and position on the reef. The overall fragment survival rate was 83.5%, but small fragments survived less, particularly on the back reef, than medium or large fragments. Fragment growth was greatest on the reef flat, whereas medium and large fragments grew faster than small ones. The overall mean of skeletal density, linear extension and calcification rates were 1.70 gCaCO3 cm−3, 1.89 cm yr−1 and 3.16 gCaCO3 cm−2 yr−1, respectively. These growth rates were comparable to those of other reef-building coral species in the world. Growth and survival rates of P. damicornis coral fragments found in this study, suggest that successful and low-cost restoration of TEP coral reefs is feasible using this species.

A high‐precision gamma densitometer for quantifying skeletal density in coral skeletons: Physical background and first results

AbstractMeasuring density in coral skeletons with high precision is challenging and forms the dominant source of experimental uncertainty in related studies. To reduce this uncertainty, we developed a precise and easy‐to‐handle γ‐densitometer. This instrument illuminates coral samples with a close‐to‐monochromatic γ‐ray beam and measures the attenuation in the material by means of single photon counting. Knowing the thickness of the sample, the density can be extracted from the attenuation. After calibration, we obtained a precision of 3.7% for absolute and 0.6% for relative density measurements. The spatial resolution is 0.5 mm. The detector system has been tested with the genus Porites, typically used in paleoclimate studies in the Indo‐Pacific, and with the genus Orbicella (formerly Montastraea), which has been commonly used in the Atlantic. A record (a.d. 1965–1999) from an Orbicella faveolata from the Belize Barrier Reef exhibits a decrease in skeletal density and in calcification rate; extension rates increase over time. A record from an O. faveolata from the offshore Glovers Reef Atoll, Belize, exhibits a density increase during a.d. 1900–2005 while skeletal extension rate decreased; calcification rate shows a decline over time. Skeletal density in a Porites lutea from the Maldives decreases from a.d. 1917–2007 whereas skeletal extension rate and calcification rate exhibit increasing trends. These first results along with those from other studies suggest that coral density data from numerous corals in a region are needed to establish robust trends in coral calcification over time, and, that susceptibility to ocean acidification apparently might vary among coral taxa.

Dissepiments, density bands and signatures of thermal stress in Porites skeletons

The skeletons of many reef-building corals are accreted with rhythmic structural patterns that serve as valuable sclerochronometers. Annual high- and low-density band couplets, visible in X-radiographs or computed tomography scans, are used to construct age models for paleoclimate reconstructions and to track variability in coral growth over time. In some corals, discrete, anomalously high-density bands, called “stress bands,” preserve information about coral bleaching. However, the mechanisms underlying the formation of coral skeletal density banding remain unclear. Dissepiments—thin, horizontal sheets of calcium carbonate accreted by the coral to support the living polyp—play a key role in the upward growth of the colony. Here, we first conducted a vital staining experiment to test whether dissepiments were accreted with lunar periodicity in Porites coral skeleton, as previously hypothesized. Over 6, 15, and 21 months, dissepiments consistently formed in a 1:1 ratio to the number of full moons elapsed over each study period. We measured dissepiment spacing to reconstruct multiple years of monthly skeletal extension rates in two Porites colonies from Palmyra Atoll and in another from Palau that bleached in 1998 under anomalously high sea temperatures. Spacing between successive dissepiments exhibited strong seasonality in corals containing annual density bands, with narrow (wide) spacing associated with high (low) density, respectively. A high-density “stress band” accreted during the 1998 bleaching event was associated with anomalously low dissepiment spacing and missed dissepiments, implying that thermal stress disrupts skeletal extension. Further, uranium/calcium ratios increased within stress bands, indicating a reduction in the carbonate ion concentration of the coral’s calcifying fluid under stress. Our study verifies the lunar periodicity of dissepiments, provides a mechanistic basis for the formation of annual density bands in Porites, and reveals the underlying cause of high-density stress bands.

Skeletal extension, density and calcification rates of massive free-living coral Porites lobata Dana, 1846

Hermatypic corals are an important sessile group in the benthic structure of tropical coral reef communities. Many organisms, in response to the environmental conditions, have developed different strategies of growth by modifying their morphologies and reproduction mode, as is the case of free-living colonies called coralliths. So far, important parameters such as calcification, growth rates and their relation with biotic and abiotic factors on coralliths have not been described. This study represents the first record of a corallith form of Porites lobata and provides information on its sclerochronology. Coral growth parameters of different coralliths were compared by measuring extension rate (cmy−1), skeletal density (gcm−3), calcification rate (gcm−2y−1), average age (yr) and sphericity (S). The influence of the environment on corallith growth was assessed using water temperature as an abiotic factor and bioturbation by fish as a biotic factor. Analysis of annual density bands using X-ray densitometry provided a mean extension rate of 0.47±0.23cmy−1, skeletal density of 1.08±0.14gcm−3, and calcification rate of 0.51±0.26gcm−2y−1. The results reveal differences in growth parameters between coralliths including a strong relationship of calcification rate with seawater temperature. In addition, direct and indirect bioturbations promoted the colony rotation resulting in a hemispherical form. Hence, the evidence suggests that scleractinian corals have developed an important growth strategy that allows the species to form new colonies and maintain successful coral reef communities through free-living corallith growth.

Spatial differences in the growth rates of Pocillopora damicornis associated with sedimentation rate and light intensity at Polhena patchy reef, southern Sri Lanka

Near shore patchy reef located at the leeward side of the fringing reef at Polhena was surveyed to evaluate spatial patterns of growth rates of Pocillopora damicornis . This study was carried out at three locations of the reef; front, side and back from June to November in 2004. Three sites were selected from each location considering the depth (surface, middle, and bottom). Forty five colonies of P. damicornis which were approximately of the same size were studied. Temperature, dissolved oxygen, BOD5, salinity, pH, soluble reactive phosphorous, turbidity and light intensity of each site were measured fortnightly and their mean values were 28.06 ± 0.24 °C, 8.78 ± 0.15 mg L -1 , 1.40 ± 0.32 mg L -1 , 31.91 ± 3.49 mg L -1 , 8.12 ± 0.04, 8.12 ± 0.01, 20.21 ± 2.05 μg L -1 , 5.78 ± 1.20 NTU, and 49.17 ± 1.97 Lux respectively. Sediments in each site were collected using traps and mean overall sedimentation rate for the experimental period was 45860.6 ± 3364.2 ml m -2 day -1 (2620.6 ± 192.2 g m -2 day -1 ) (n = 36). Growth of coral colonies, which was measured as linear skeletal extension rate (LSER), was 0.15 ± 0.01 mm day -1 . Significant differences in mean light intensity, mean sediment volume, and LSER of coral colonies were observed among different sites. The highest sedimentation rate 94818.2 ± 6981.5 ml m -2 day -1 (5418.2 ± 398.9 g m -2 day -1 ) and the lowest LSER of the colonies (0.05 ± 0.01 mm day -1 ) were observed at back bottom site. The highest LSER was observed at front surface (0.23 ± 0.01 mm day -1 ). LSER showed a significant negative correlation (R 2 = 0.59) with increased sedimentation rate and low light intensity. Back bottom site of the patchy reef was having comparatively unfavorable conditions for the growth of corals that were subjected to more environmental stress than the corals in other sites of the reefs. Sri Lanka J. Aquat. Sci. 20 (1) (2015): 47-58

Assessing coral health and resilience in a warming ocean: Why looks can be deceptive

In this paper I challenge the notion that a healthy and resilient coral is (in all cases) a fast-growing coral, and by inference, that a reef characterised by a fast trajectory toward high coral cover is necessarily a healthy and resilient reef. Instead, I explain how emerging evidence links fast skeletal extension rates with elevated coral-algae (symbiotic) respiration rates, most-often mediated by nutrient-enlarged symbiont populations and/or rising sea temperatures. Elevated respiration rates can act to reduce the autotrophic capacity (photosynthesis:respiration ratio) of the symbiosis. This restricts the capacity of the coral host to build and maintain sufficient energy reserves (e.g. lipids) needed to sustain essential homeostatic functions, including sexual reproduction and biophysical stress resistance. Moreover, it explains the somewhat paradoxical scenario, whereby at the ecological instant before the reef-building capacity of the symbiosis is lost, a reef can look visually at its best and be accreting CaCO(3) at its maximum.

Increasing temperature anomalies reduce coral growth in the Weizhou Island, northern South China Sea

The ways in which high-latitude corals respond to ongoing climate warming and the increasing incidence of extreme warm- and cold-water events remains unclear, probably due to a lack of field studies, especially in the northern South China Sea (NSCS). In this study, temporal variation in the skeletal growth rate in massive Porites lutea corals collected from the Weizhou Island, NSCS, was examined over a 27-year period (from 1984 to 2010). The coral skeletal extension rates were determined by using X-ray photography, and compared to the Degree Heating Weeks (DHWs) index (0–16.7 °C-weeks) and the Sea Surface Temperature (SST) Anomalies (0–0.6 °C for + SST_anomaly, and −3.2–0 °C for -SST_anomaly). Historical extreme high and low temperature events were calculated to estimate warm- (DHW > 6 °C-weeks, +SST_anomaly > 0.2 °C) and cold- (-SST_anomaly < −1.6 °C) water stresses on reef-building corals. The average skeletal growth rate for all coral colonies was 6.3 mm year−1, ranging from 3.1 to 9.9 mm year−1. Coral skeletal growth consistently showed a downward trajectory, especially after the year of 1997. The mean value was 7.3 mm year−1 from 1984 to 1996 and then declined by approximately 26% to 5.4 mm year−1 from 1997 to 2010. We found that an increasing frequency and intensity in warm- and cold-stresses can explain the observed decline in growth rate of Porites colonies in the Weizhou Island.

Gender-related differences in the apparent timing of skeletal density bands in the reef-building coral Siderastrea siderea

Density banding in skeletons of reef-building corals is a valuable source of proxy environmental data. However, skeletal growth strategy has a significant impact on the apparent timing of density-band formation. Some corals employ a strategy where the tissue occupies previously formed skeleton during as the new band forms, which leads to differences between the actual and apparent band timing. To investigate this effect, we collected cores from female and male colonies of Siderastrea siderea and report tissue thicknesses and density-related growth parameters over a 17-yr interval. Correlating these results with monthly sea surface temperature (SST) shows that maximum skeletal density in the female coincides with low winter SSTs, whereas in the male, it coincides with high summer SSTs. Furthermore, maximum skeletal densities in the female coincide with peak Sr/Ca values, whereas in the male, they coincide with low Sr/Ca values. Both results indicate a 6-month difference in the apparent timing of density-band formation between genders. Examination of skeletal extension rates also show that the male has thicker tissue and extends faster, whereas the female has thinner tissue and a denser skeleton—but both calcify at the same rate. The correlation between extension and calcification, combined with the fact that density banding arises from thickening of the skeleton throughout the depth reached by the tissue layer, implies that S. siderea has the same growth strategy as massive Porites, investing its calcification resources into linear extension. In addition, differences in tissue thicknesses suggest that females offset the greater energy requirements of gamete production by generating less tissue, resulting in differences in the apparent timing of density-band formation. Such gender-related offsets may be common in other corals and require that environmental reconstructions be made from sexed colonies and that, in fossil corals where sex cannot be determined, reconstructions must be duplicated in different colonies.

The Suess effect in Fiji coral δ13C and its potential as a tracer of anthropogenic CO2 uptake

In the context of increasing anthropogenic CO2 emissions, determining the rate of oceanic CO2 uptake is of high interest. Centennial-scale changes in δ13C of the surface water dissolved inorganic carbon (DIC) reservoir have been shown to be influenced by the carbon isotopic composition of atmospheric CO2. However, the availability of direct oceanic δ13C measurements is limited and methods for reconstructing past δ13C variability of the oceanic DIC are needed. Geochemical reconstructions of DIC variability can help in understanding how the ocean has reacted to historical changes in the carbon cycle. This study explores the potential of using temporal variations in δ13C measured in five Fijian Porites corals for reconstructing oceanic δ13C variability. A centennial-scale decreasing δ13C trend is observed in these Fiji corals. Other studies have linked similar decreasing δ13C trends to anthropogenic changes in the atmospheric carbon reservoir (the “13C Suess effect”). We conclude that solar irradiance is the factor influencing the δ13C cycle on a seasonal scale, however it is not responsible for the centennial-scale decreasing δ13C trend. In addition, variations in skeletal extension rate are not found to account for centennial-scale δ13C variability in these corals. Rather, we found that water depth at which a Fijian Porites colony calcifies influences both δ13C and extension rate mean values. The water depth-δ13C relationship induces a dampening effect on the centennial-scale decreasing δ13C trend. We removed this “water depth effect” from the δ13C composite, resulting in a truer representation of δ13C variability of the Fiji surface water DIC (δ13CFiji-DIC). The centennial-scale trend in this Fiji coral composite δ13CFiji-DIC time-series shares similarities with atmospheric δ13CCO2, implicating the 13C Suess effect as the source of the this coral δ13C trend. Additionally, our study finds that the δ13C variability between the atmosphere and the ocean in this region is not synchronous; the coral δ13C response is delayed by ~10years. This agrees with the previously established model of isotopic disequilibrium between atmospheric δ13CCO2 and oceanic surface water DIC.

Skeletal extension and calcification of reef-building corals in the central Indian Ocean

Skeletal extension, density and calcification rate of 12 scleractinian coral species comprising 7 morphological groups were examined on the reef crest of Vabbinfaru platform, Maldives. Growth rates of coral specimens were measured over the period of February 2010–March 2011 using alizarin staining and direct measurements. Skeletal extension rate was highly variable between coral species. Colony morphology was a major control on the skeletal extension and calcification of coral specimens. Growth rates of Acropora and Porites corals were comparable to existing data recorded for Caribbean and Indo-Pacific reef provinces. Skeletal density was less variable between species and was typically consistent among morphological groups. Findings represent the first estimates of coral growth in the central Indian Ocean region and add to the limited growth studies available that have examined a broad range of coral growth morphologies in other reef-building regions.

Skeletal extension rate of the reef building coral &amp;lt;i&amp;gt;Porites&amp;lt;/i&amp;gt; species from Aqaba and their environmental variables

Annual skeletal extension rates of the sclera-actinian corals Porites species were investigated in 32 colonies from the northern Gulf of Aqaba fringing reef at various depths (1 - 42 m). All corals reveal clear and regular skeletal density banding patterns. Results showed that the high-density annual growth bands were formed during winter and the low-density annual growth bands during summer. The mean annual extension rates of the studied corals reveal a large inter-colony variability with values ranged between 2.36 to 20.0 mm/year. While a general trend of decreasing coral extension rate with depth was observed and best explained by a simple exponential model, the rates clustered into two groups: 10.86 ± 2.54 mm/year in water depths less than 10 m, and 5.23 ± 1.99 mm/year below 12 m. Light intensity seems to be the primary environmental factor responsible for decreasing coral extension rate with depth since the effect of other environmental parameters could be neglected from the Gulf of Aqaba. Time series record of the mean annual coral extension rate showed a slight increasing linear trend which could be linked to increase seawater temperature over the period of time represented.

Coral skeletal extension rate: An environmental signal or a subject to inaccuracies?

Coral skeletal extension rate has been widely used to assess coral growth characteristics across environmental gradients and as an environmental proxy. Herein, the potential sources of errors when measuring skeletal extension rates are identified and discussed, and it is demonstrated that measurements of this growth parameter do not necessarily reflect an annual time scale and can result in substantial miscalculations. Instead, it is dissepiment spacing and number that best reflect the time scale. On the other hand, calcification rate is the growth parameter that appears to better reflect the environmental controls that regulate physiological processes in symbiotic corals. Nonetheless, any error associated with measuring extension rate would result in under- and overestimation of calcification rate values, which could lead to huge errors if this growth parameter is used as an environmental proxy. The aim is to promote further discussion, avoid future misuse of extension rate as an environmental proxy, and encourage future research about how to accurately measure extension rate and the use of calcification rate as an environmental proxy.

Growth characteristics of the reef-building coral Porites astreoides under different environmental conditions in the Western Atlantic

Skeletal extension (3.67 ± 0.65 mm year−1), density (1.49 ± 0.16 g cm−3), and calcification rate (0.55 ± 0.12 g cm−2 year−1) were determined using annual growth bands of Porites astreoides skeletons collected in three different reef systems in the Western Atlantic. The corals showed a low-density annual growth band at their apex, and seasonal timing of low and high-density band formation in P. astreoides appears to be similar at the three study sites in the Western Atlantic. The range of values presented here, for the three growth variables, spans the known range of skeletal-growth variability in P. astreoides for the Western Atlantic. The relationships between the growth parameters were similar to those previously described by other authors for massive Porites species from the Indo-Pacific, suggesting that P. astreoides has the same growth strategy, primarily investing calcification resources in extension rate. It is noteworthy that the P. astreoides population growing off the northwest coast of Cuba had similar growth characteristics as populations from the Caribbean region which were different from populations in the Gulf of Mexico, which seem to be isolated and adapted for growth at higher average sea-surface temperatures.

Differential physiological responses of two congeneric scleractinian corals to mineral accretion and an electric field

Despite increasing popularity of ‘electric’ reefs as a means for reef restoration, there is a distinct lack of quantitative evidence supporting the alleged benefits of this method. This study investigated the effects of an electric field versus an electric field in combination with a cathode on coral growth (skeletal extension) rates, coral survival, zooxanthella densities, chlorophyll a (chl a) concentrations, and chlorophyll fluorescence of Acropora pulchra and A. yongei. Coral transplants were grown for 4 months under three treatment conditions: (1) on an iron cathode, (2) on bamboo inside an electric field, or (3) on bamboo in the absence of an electric field. Contrary to predictions, coral growth rates of both species were highest inside the electric field and not on the cathode. Except for chl a concentrations, the cathode had a significant adverse effect on all measured variables for A. yongei but not for A. pulchra. Treatment had no effect on the survival of A. pulchra, while mortality rates of A. yongei were significantly higher in the presence of mineral accretion compared to the electric field and control. A. yongei on the cathode featured low zooxanthella densities, depressed electron transport rates (rETR) and maximum quantum yield (F v/F m), and reduced growth. By contrast, treatment had no effect on the fluorescence characteristics of A. pulchra, and zooxanthella densities were highest for corals on the cathode, coincident with high growth rates relative to the control. Overall, the data indicate that the proposed benefits of the mineral accretion technology to meet important objectives of reef rehabilitation with regard to colony growth and survival should be considered with caution.