Net Calcification Rates Research Articles

Ocean acidification reduces net calcification and wound healing in the tropical crustose coralline alga, Porolithon onkodes (Corallinales, Rhodophyta)

Reef dwelling algae employ a variety of physical and chemical defenses against herbivory, and the response to wounding is extremely important in algal communities. Wound healing mechanisms in crustose coralline algae (CCA) are related to skeletal growth and net calcification rate. Ocean acidification (OA) is known to affect rates of net calcification in a number of calcifying organisms, including CCA. Reduced rates of net calcification in CCA are likely to alter wound healing, and thus affect the consequences of herbivore-CCA interactions on coral reefs. The response of the tropical CCA Porolithon onkodes to OA and artificial wounding was quantified in a 51-day laboratory experiment. Eight artificially wounded (cut to a mean depth of 182 μm) and eight non-wounded samples of P. onkodes were randomly placed into each of four treatments (n = 64 samples total). Each treatment was maintained at a different pCO2 level representative of either ambient conditions or end-of-the-century, predicted conditions (IPCC, 2014); 429.31 ± 20.84 (ambient), 636.54 ± 27.29 (RCP4.5), 827.33 ± 38.51 (RCP6.0), and 1179.39 ± 88.85 μatm (RCP8.5; mean ± standard error). Elevated pCO2 significantly reduced rates of net calcification in both wounded and non-wounded samples of P. onkodes (slopes = −6.4 × 10−4 and −5.5 × 10−4 mg cm−2 d−1 per μatm pCO2, respectively over 51 days). There also was a significant reduction in the rate of vertical regeneration of thallus tissue within the wounds as pCO2 increased (slope = −1.5 × 10−3 μm d−1 per μatm pCO2 over 51 days). This study provides evidence that elevated pCO2 could reduce the ability of this important alga to recover from wounding. Because wounding by herbivores plays an important role in determining CCA community structure, we propose reduced wound healing as a mechanism by which OA might affect the structure and functional roles of CCA communities on coral reefs.

Environmental influence on calcification of the bivalve Chamelea gallina along a latitudinal gradient in the Adriatic Sea

Environmental factors are encoded in shells of marine bivalves in the form of geochemical properties, shell microstructure and shell growth rate. Few studies have investigated how shell growth is affected by habitat conditions in natural populations of the commercial clam Chamelea gallina. Here, skeletal parameters (micro-density and apparent porosity) and growth parameters (bulk density, linear extension and net calcification rates) were investigated in relation to shell sizes and environmental parameters along a latitudinal gradient in the Adriatic Sea (400 km). Net calcification rates increased with increasing solar radiation, sea surface temperature and salinity and decreasing Chlorophyll concentration in immature and mature shells. In immature shells, which are generally more porous than mature shells, enhanced calcification was due to an increase in bulk density, while in mature shells was due to an increase in linear extension rates. The presence of the Po river in the Northern Adriatic Sea was likely the main driver of the fluctuations observed in environmental parameters, especially salinity and Chlorophyll concentration, and seemed to negatively affect the growth of C. gallina.

Light-dependent calcification in Red Sea giant clam <i>Tridacna maxima</i>

Abstract. Tropical giant clams of the subfamily Tridacninae, including the species Tridacna maxima, are unique among bivalves as they live in a symbiotic relationship with unicellular algae and generally function as net photoautotrophs. Light is therefore crucial for these species to thrive. Here we examine the light dependency of calcification rates of T. maxima in the central Red Sea as well as the patterns of its abundance with depth in the field. Red Sea T. maxima show the highest densities at a depth of 3 m with 0.82±0.21 and 0.11±0.03 individuals m−2 (mean ± SE) at sheltered and exposed sites, respectively. Experimental assessment of net calcification (µmol CaCO3 cm−2 h−1) and gross primary production (µmol O2 cm−2 h−1) under seven light levels (1061, 959, 561, 530, 358, 244, and 197 µmol quanta m−2 s−1) showed net calcification rates to be significantly enhanced under light intensities corresponding to a water depth of 4 m (0.65±0.03 µmol CaCO3 cm−2 h−1; mean ± SE), while gross primary production was 2.06±0.24 µmol O2 cm−2 h−1 (mean ± SE). We found a quadratic relationship between net calcification and tissue dry mass (DM in gram), with clams of an intermediate size (about 15 g DM) showing the highest calcification. Our results show that the Red Sea giant clam T. maxima stands out among bivalves as a remarkable calcifier, displaying calcification rates comparable to other tropical photosymbiotic reef organisms such as corals.

Response of Corals Acropora pharaonis and Porites lutea to Changes in pH and Temperature in the Gulf

Coral reefs are harboring a large part of the marine biodiversity and are important ecosystems for the equilibrium of the oceans. As a consequence of anthropogenic CO2 emission, a drop in pH and an increase in seawater temperature is observed in the Gulf coastal waters that potentially threaten coral assemblages. An experimental study was conducted on two species of corals to assess the effect of ocean warming and ocean acidification on the net calcification rate. Two pH conditions 8.2 and 7.5 and three temperatures, 22.5, 27.5 and 32.5 °C, were considered. Net calcification rates were measured using 45Ca radiotracer. Both temperature and pH had a significant effect on net calcification rates following a similar pattern for both species. The highest calcification rate was observed at low temperature and high pH. Increased temperature and decreased pH led to a decrease in net calcification rates. An interactive effect was observed as the effect of pH decreased with increasing temperature. However, the two species of coral were able to calcify in all the tested combination of temperature and pH suggesting that they are adapted to short term changes in temperature and pH. Ability to calcify even at a high temperature of 32.5 °C that is identical to the summertime Gulf seawater temperature under both the ambient and low pH condition with no mortalities, raises a question: are these corals adapted to high seawater temperatures and low pH? More in-depth assessments will be required to confirm if this is an adaptation to higher temperatures in Persian Gulf corals.

Latitudinal Variation of Planktonic Foraminifera Shell Masses During Termination I

The oceans’ surface layer holds large amounts of dissolved inorganic carbon that is exchanged rapidly with the atmosphere. Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide (CO2), and a part of it is converted into carbonate by marine organisms. Calcifying marine organisms include planktonic foraminifers that contribute to the marine carbon turnover by generating inorganic carbon production (CaCO3 shells). Anthropogenic CO2 acidifies the surface ocean, changes the carbonate chemistry and decreases the saturation state of carbonate minerals in sea water, thus affecting the biological precipitation of carbonate shells. Relative changes in average foraminiferal shell mass can be interpreted as variations in test thickness and the extent of calcification that subsequently impacts the global carbonate budgets. The response of calcifying marine organisms to elevated atmospheric pCO2 is diverse and complex with studies reporting from reduced rates of net calcification to neutral receptivity or even increased calcification intensities. This diverse behavior implies that the concentration of dissolved inorganic carbon may not be the dominant factor controlling the amount of carbonate shell masses. Here we provide further evidence that glacial/interglacial variations of planktonic foraminifera shell masses are invariant to atmospheric pCO2. We identify that differences in shell weights of several planktonic foraminiferal species from narrow size intervals, over the most recent deglaciation (Termination I) vary systematically as a function of latitude. Past intervals of abruptly changing pCO2 and temperatures, such as the terminations, can offer a glimpse into the response of marine calcifying plankton to changes in surface oceans. We have compiled all the available bibliographic data of planktonic foraminifera shell weights from restricted sieve fractions of different species from the Atlantic, Pacific and Indian Oceans and we find that for the same pCO2 conditions planktonic foraminifera from equatorial regions may alter their shell mass only as little as 8.2%, while towards higher latitudes changes in shell mass reach up to 54% during the transition from the last glacial to interglacial conditions. We attribute this low variability in the shell mass of planktonic foraminifera from the equator to the stability of the physical oceanographic conditions that characterize the equatorial zone.

Decomposing monthly declines in subsurface-water pH and aragonite saturation state from spring to autumn in the North Yellow Sea

The North Yellow Sea (NYS) is a western North Pacific marginal sea of major ecological and economic importance, where seasonal thermocline and subsurface cold water mass are well developed from spring to autumn. Earlier researchers have shown monthly/bimonthly declines of pH (ΔpH) and aragonite saturation state (ΔΩarag) in the NYS cold water mass. To quantify the biogeochemical processes controlling these variations, we applied a systematic decomposition methodology to ΔΩarag and ΔpH values, using the Redfield ratio and published data sets of NYS carbonate and ancillary parameters for 2011 and 2013. Results show that the contributions of the NYS cold water mass community respiration (ΔΩResp), calcification (ΔΩCalc) and temperature changes (ΔΩΔT) to the monthly ΔΩarag from spring to summer were 84 ± 9%, 30 ± 16%, and − 13 ± 10%, while the ΔΩResp, ΔΩΔT and contributions of CaCO3 dissolution (ΔΩDiss) to the bimonthly ΔΩarag from late summer to autumn were 103%, 1% and − 5%. Meanwhile, the contributions of the NYS cold water mass community respiration (ΔpHResp), temperature changes (ΔpHΔT) and calcification (ΔpHCalc) to the monthly ΔpH from spring to summer were 50 ± 23%, 39 ± 23%, and 13 ± 4%, while the ΔpHResp, ΔpHΔT and contributions of CaCO3 dissolution (ΔpHDiss) to the bimonthly ΔpH from late summer to autumn were 110%, − 5%, and − 5%. Our results suggest that the NYS cold water mass net calcification rate declines to nearly zero when Ωarag reaches a critical level of 1.5 − 1.6. Using a future scenario to predict the impacts of rising atmospheric CO2 levels on seasonal variations of Ωarag and pH values, we show that very low Ωarag values of < 1.5 may exist all year round in the NYS subsurface waters by the 2050 s, while aragonite undersaturated water (Ωarag <1) may appear in autumn months, causing great stress to its benthic faunal community.

Generality in multispecies responses to ocean acidification revealed through multiple hypothesis testing.

Decades of research have demonstrated that many calcifying species are negatively affected by ocean acidification, a major anthropogenic threat in marine ecosystems. However, even closely related species may exhibit different responses to ocean acidification and less is known about the drivers that shape such variation in different species. Here, we examine the drivers of physiological performance under ocean acidification in a group of five species of turf-forming coralline algae. Specifically, quantitating the relative weight of evidence for each of ten hypotheses, we show that variation in coralline calcification and photosynthesis was best explained by allometric traits. Across ocean acidification conditions, larger individuals (measured as noncalcified mass) had higher net calcification and photosynthesis rates. Importantly, our approach was able to not only identify the aspect of size that drove the performance of coralline algae, but also determined that responses to ocean acidification were not dependent on species identity, evolutionary relatedness, habitat, shape, or structural composition. In fact, we found that failure to test multiple, alternative hypotheses would underestimate the generality of physiological performances, leading to the conclusion that each species had different baseline performance under ocean acidification. Testing among alternative hypotheses is an essential step toward determining the generalizability of experiments across taxa and identifying common drivers of species responses to global change.

Ocean acidification alters early successional coral reef communities and their rates of community metabolism.

Ocean acidification is expected to alter community composition on coral reefs, but its effects on reef community metabolism are poorly understood. Here we document how early successional benthic coral reef communities change in situ along gradients of carbon dioxide (CO2), and the consequences of these changes on rates of community photosynthesis, respiration, and light and dark calcification. Ninety standardised benthic communities were grown on PVC tiles deployed at two shallow-water volcanic CO2 seeps and two adjacent control sites in Papua New Guinea. Along the CO2 gradient, both the upward facing phototrophic and the downward facing cryptic communities changed in their composition. Under ambient CO2, both communities were dominated by calcifying algae, but with increasing CO2 they were gradually replaced by non-calcifying algae (predominantly green filamentous algae, cyanobacteria and macroalgae, which increased from ~30% to ~80% cover). Responses were weaker in the invertebrate communities, however ascidians and tube-forming polychaetes declined with increasing CO2. Differences in the carbonate chemistry explained a far greater amount of change in communities than differences between the two reefs and successional changes from five to 13 months, suggesting community successions are established early and are under strong chemical control. As pH declined from 8.0 to 7.8, rates of gross photosynthesis and dark respiration of the 13-month old reef communities (upper and cryptic surfaces combined) significantly increased by 10% and 20%, respectively, in response to altered community composition. As a consequence, net production remained constant. Light and dark calcification rates both gradually declined by 20%, and low or negative daily net calcification rates were observed at an aragonite saturation state of <2.3. The study demonstrates that ocean acidification as predicted for the end of this century will strongly alter reef communities, and will significantly change rates of community metabolism.

Skeletal growth, morphology and skeletal parameters of a temperate, solitary and zooxanthellate coral along a depth gradient in the Dardanelles (Turkey)

Studying coral populations along depth gradients is important to find out if deeper habitats can act as microrefugia from bleaching and/or mortality events associated with high temperature and irradiance. Skeletal biometry, growth, and parameters of the solitary, zooxanthellate, Mediterranean endemic scleractinian Balanophyllia europaea were determined at depths of 1, 11, and 21 m in the Dardanelles (Turkey), to (1) compare the population parameters in the Eastern Mediterranean Sea with the well-characterized populations in the NW Mediterranean Sea, that are threatened by ocean warming and acidification; and (2) assess the variation of studied parameters along a depth gradient in the Dardanelles. Biometric relationships were similar to those previously described on Italian coasts. At shallow depths, average coral height decreased, likely because of (1) the higher current velocity and wave action that is expected to limit vertical skeletal growth and/or (2) the need to increase photoprotection in high light conditions. Only a very slight increase of skeletal bulk density with depth (and consequent slight decrease of porosity) was detected, while age–length relationship, linear extension rate and net calcification rate were homogeneous among depths. The homogeneous net calcification rate with depth may depend on the balance between the response of zooxanthellae photosynthesis to temperature and light. In fact, shallow depths are likely to experience higher temperatures that negatively affect photosynthetic efficiency of B. europaea, thus reducing available energy for calcification. On the other hand, lower light availability with depth is expected to decrease photosynthesis and these two effects may compensate each other. Unexpectedly, the observed net calcification rate in the Dardanelles was almost double than the higher value reported in the northwestern Mediterranean Sea. Further analyses are required to analyse if energetic trade-offs between skeletal growth and reproduction differ in the two regions.

Linking genotype to phenotype in a changing ocean: inferring the genomic architecture of a blue mussel stress response with genome-wide association.

A key component to understanding the evolutionary response to a changing climate is linking underlying genetic variation to phenotypic variation in stress response. Here, we use a genome-wide association approach (GWAS) to understand the genetic architecture of calcification rates under simulated climate stress. We take advantage of the genomic gradient across the blue mussel hybrid zone (Mytilus edulis and Mytilus trossulus) in the Gulf of Maine (GOM) to link genetic variation with variance in calcification rates in response to simulated climate change. Falling calcium carbonate saturation states are predicted to negatively impact many marine organisms that build calcium carbonate shells - like blue mussels. We sampled wild mussels and measured net calcification phenotypes after exposing mussels to a 'climate change' common garden, where we raised temperature by 3°C, decreased pH by 0.2 units and limited food supply by filtering out planktonic particles >5μm, compared to ambient GOM conditions in the summer. This climate change exposure greatly increased phenotypic variation in net calcification rates compared to ambient conditions. We then used regression models to link the phenotypic variation with over 170000 single nucleotide polymorphism loci (SNPs) generated by genotype by sequencing to identify genomic locations associated with calcification phenotype, and estimate heritability and architecture of the trait. We identified at least one of potentially 2-10 genomic regions responsible for 30% of the phenotypic variation in calcification rates that are potential targets of natural selection by climate change. Our simulations suggest a power of 13.7% with our study's average effective sample size of 118 individuals and rare alleles, but a power of >90% when effective sample size is 900.

Recovery of reef‐scale calcification following a bleaching event in Kāne'ohe Bay, Hawai'i

AbstractIncreasing anthropogenic disturbances have driven declines of many coral‐dominated reef states, threatening critical ecosystem functions such as reef‐scale calcification and accretion. Few studies have investigated the effect of coral bleaching on reef‐scale calcification. In this study, we monitored bay‐wide alkalinity anomalies in Kāne'ohe Bay, Hawai'i along an inshore‐offshore transect as a proxy for net calcification during the 2015 coral bleaching event and following recovery over a full seasonal cycle. We observed no net calcification in October 2015 during the bleaching event followed by a recovery to significant, positive net calcification rates in June 2016, November 2016, and February 2017 across a range of seawater temperatures and hydrodynamic conditions. Post‐bleaching net calcification rates were not significantly different between survey dates and agreed with the range of pre‐bleaching net calcification rates from a previous study suggesting that net calcification in Kāne'ohe Bay had fully recovered following the 2015 bleaching event.

The short-term combined effects of temperature and organic matter enrichment on permeable coral reef carbonate sediment metabolism and dissolution

Abstract. Rates of gross primary production (GPP), respiration (R), and net calcification (Gnet) in coral reef sediments are expected to change in response to global warming (and the consequent increase in sea surface temperature) and coastal eutrophication (and the subsequent increase in the concentration of organic matter, OM, being filtered by permeable coral reef carbonate sediments). To date, no studies have examined the combined effect of seawater warming and OM enrichment on coral reef carbonate sediment metabolism and dissolution. This study used 22 h in situ benthic chamber incubations to examine the combined effect of temperature (T) and OM, in the form of coral mucus and phytodetritus, on GPP, R, and Gnet in the permeable coral reef carbonate sediments of Heron Island lagoon, Australia. Compared to control incubations, both warming (+2.4 °C) and OM increased R and GPP. Under warmed conditions, R (Q10 = 10.7) was enhanced to a greater extent than GPP (Q10 = 7.3), resulting in a shift to net heterotrophy and net dissolution. Under both phytodetritus and coral mucus treatments, GPP was enhanced to a greater extent than R, resulting in a net increase in GPP / R and Gnet. The combined effect of warming and OM enhanced R and GPP, but the net effect on GPP / R and Gnet was not significantly different from control incubations. These findings show that a shift to net heterotrophy and dissolution due to short-term increases in seawater warming may be countered by a net increase GPP / R and Gnet due to short-term increases in nutrient release from OM.

Effects of CO2-driven acidification of seawater on the calcification process in the calcareous hydrozoan Millepora alcicornis (Linnaeus, 1758)

Ocean acidification is expected to intensify due to increasing levels in the partial pressure of atmospheric CO2 (pCO2). This could negatively affect major calcifying reef organisms. In this study, the effects of different levels of CO2-driven acidification of seawater (control: pH 8.1; moderate: pH 7.8; intermediate: pH 7.5; and severe: pH 7.2) on the net calcification rate and activity of enzymes related to the calcification process (Ca-ATPase and carbonic anhydrase) were evaluated in the calcareous hydrozoan Millepora alcicornis. The experiment was run for 30 d using a marine mesocosm system. Net calcification ratio was significantly reduced in hydrocorals exposed to intermediate seawater acidification for 16 d and to severe seawater acidification for 16 d or 30 d, compared to animals at control conditions. However, only hydrocorals exposed to severe seawater acidification showed lower net calcification rates than those exposed to control conditions for 30 d. In accordance, the activities of enzymes involved in the calcification process markedly increased in hydrocorals exposed to reduced pH. Ca-ATPase seemed to be more sensitive to seawater acidification than carbonic anhydrase as it increased in hydrocorals exposed to intermediate and severe seawater acidification for 30 d, while carbonic anhydrase activity was only stimulated under severe seawater acidification. Therefore, our findings clearly show that the hydrocoral M. alcicornis is able to cope, to some extent, with long-term CO2-driven acidification of seawater (pH ≥ 7.5). In addition, they show that Ca-ATPase plays a key role in the maintenance of calcification rate under scenarios of moderate and intermediate levels of seawater acidification. However, the observed increase in Ca-ATPase and carbonic anhydrase activity was not enough to compensate for the effects of CO2-driven reduction in seawater pH on the net calcification rate of the hydrocoral M. alcicornis under a scenario of severe ocean acidification (pH 7.2).

New rates of Indian Ocean carbonate production by encrusting coral reef calcifiers: Periodic expansions following disturbance influence reef-building and recovery

Encrusting marine calcifying organisms, including coralline algae, bryozoans, calcareous worms, foraminifera and oysters, contribute to the geological development of reefs by precipitating calcium carbonate that binds detrital sediment and coral framework, and may produce new sediment after their death. However, it is difficult to quantify the relative importance of encrusting communities within contemporary reef carbonate production budgets because of a lack of regional datasets of either encrusting calcification or diversity, and the factors that control their settlement and growth. Here, we report rates of encrusting carbonate production for the Maldives in the Indian Ocean, determined by deploying artificial settlement substrates in shallow-water reef sites for 13months. Eight taxonomic groups are identified with high average rates of net calcification (mean=0.056±0.02gcm−2yr−1). Encrusting communities vary between exposed (upwards-facing) and cryptic (downwards-facing) reef habitats (p<0.05), driving inter-habitat differences in carbonate production rates (median cryptic=0.066gcm−2yr−1; median exposed=0.042gcm−2yr−1). These changes are a function of differences in light availability, grazing pressure, and wave exposure reflecting reversing monsoon conditions. Non-geniculate coralline algae are the dominant encrusting group (~50% cover) and major contributors to net calcification. The rapid expansion of coralline algae on post-mortem coral frameworks following the 2016 mass bleaching event in the Maldives has important implications for short-term carbonate cycling and reef budgetary states (pre-bleaching encrusting production=0.24kgm−2yr−1; post-bleaching=0.40kgm−2yr−1). Our findings suggest that encrusting communities add meaningful quantities of carbonate to reefs, and that their increased expression following declines in coral health could offset negative budgets on reefs where bioerosion rates are low, and aid future reef recovery by facilitating coral settlement.

Intra-Specific Variation Reveals Potential for Adaptation to Ocean Acidification in a Cold-Water Coral from the Gulf of Mexico

Ocean acidification, the decrease in seawater pH due to the absorption of atmospheric CO2, profoundly threatens the survival of a large number of marine species. Cold-water corals are considered to be among the most vulnerable organisms to ocean acidification because they are already exposed to relatively low pH and corresponding low calcium carbonate saturation states (Ω). Lophelia pertusa is a globally distributed cold-water scleractinian coral that provides critical three-dimensional habitat for many ecologically and economically significant species. In this study, four different genotypes of L. pertusa were exposed to three pH treatments (pH=7.60, 7.75, and 7.90) over a short (two-week) experimental period, and six genotypes were exposed to two pH treatments (pH=7.60, and 7.90) over a long (six-month) experimental period. Their physiological response was measured as net calcification rate and the activity of carbonic anhydrase, a key enzyme in the calcification pathway. In the short-term experiment, net calcification rates did not significantly change with pH, although they were highly variable in the low pH treatment, including some genotypes that maintained positive net calcification in undersaturated conditions. In the six-month experiment, average net calcification was significantly reduced at low pH, with corals exhibiting net dissolution of skeleton. However, one of the same genotypes that maintained positive net calcification (+0.04% day-1) under the low pH treatment in the short-term experiment also maintained positive net calcification longer than the other genotypes in the long-term experiment, although none of the corals maintained positive calcification for the entire 6 months. Average carbonic anhydrase activity was not affected by pH, although some genotypes exhibited small, insignificant, increases in activity after the sixth month. Our results suggest that while net calcification in L. pertusa is adversely affected by ocean acidification in the long term, it is possible that some genotypes may prove to be more resilient than others, particularly to short perturbations of the carbonate system. These results provide evidence that populations of L. pertusa in the Gulf of Mexico may contain the genetic variability necessary to support an adaptive response to future ocean acidification.

Twenty Years of Marine Carbon Cycle Observations at Devils Hole Bermuda Provide Insights into Seasonal Hypoxia, Coral Reef Calcification, and Ocean Acidification

Open–ocean observations have revealed gradual changes in seawater carbon dioxide (CO2) chemistry resulting from uptake of atmospheric CO2 and ocean acidification (OA), but, with few long–term records (>five years) of the coastal ocean that can reveal the pace and direction of environmental change. In this paper, observations collected from 1996 to 2016 at Harrington Sound, Bermuda, constitute one of the longest time–series of coastal ocean inorganic carbon chemistry. Uniquely, such changes can be placed into the context of contemporaneous offshore changes observed at the nearby Bermuda Atlantic Time-series Study (BATS) site. Onshore, surface dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2; >10% change per decade) have increased and OA indicators such as pH and calcium carbonate (CaCO3) saturation state (Ω) decreased from 1996–2016 at a rate of two to three times that observed offshore at BATS. Such changes, combined with reduction of total alkalinity over time, reveal a complex interplay of biogeochemical processes influencing Bermuda reef metabolism, including net ecosystem production (NEP=gross primary production–autotrophic and heterotrophic respiration) and net ecosystem calcification (NEC=gross calcification–gross CaCO3 dissolution). These long–term data show ae seasonal shift between wintertime net heterotrophy and summertime net autotrophy for the entire Bermuda reef system. Over annual time-scales, the Bermuda reef system does not appear to be in trophic balance, but rather slightly net heterotrophic. In addition, the reef system is net accretive (i.e., gross calcification > gross CaCO3 dissolution), but there were occasional periods when the entire reef system appears to transiently shift to net dissolution. A previous five–year study of the Bermuda reef suggested that net calcification and net heterotrophy have both increased. Over the past twenty years, rates of net calcification and net heterotrophy determined for the Bermuda reef system have increased by ~30%, most likely due to increased coral nutrition occurring in concert with increased offshore productivity in the surrounding subtropical North Atlantic Ocean. Importantly, this long–term study reveals that other environmental factors can mitigate against the effects of ocean acidification on coral reef calcification, at least over the past couple of decades.

Ocean warming and acidification synergistically increase coral mortality

Organisms that accumulate calcium carbonate structures are particularly vulnerable to ocean warming (OW) and ocean acidification (OA), potentially reducing the socioeconomic benefits of ecosystems reliant on these taxa. Since rising atmospheric CO2 is responsible for global warming and increasing ocean acidity, to correctly predict how OW and OA will affect marine organisms, their possible interactive effects must be assessed. Here we investigate, in the field, the combined temperature (range: 16–26 °C) and acidification (range: pHTS 8.1–7.4) effects on mortality and growth of Mediterranean coral species transplanted, in different seasonal periods, along a natural pH gradient generated by a CO2 vent. We show a synergistic adverse effect on mortality rates (up to 60%), for solitary and colonial, symbiotic and asymbiotic corals, suggesting that high seawater temperatures may have increased their metabolic rates which, in conjunction with decreasing pH, could have led to rapid deterioration of cellular processes and performance. The net calcification rate of the symbiotic species was not affected by decreasing pH, regardless of temperature, while in the two asymbiotic species it was negatively affected by increasing acidification and temperature, suggesting that symbiotic corals may be more tolerant to increasing warming and acidifying conditions compared to asymbiotic ones.

Studying interactions between excavating sponges and massive corals by the use of hybrid cores

Excavating sponges often compete with reef-building corals. To study sponge–coral interactions, we devised a design of hybrid cores that allows sponges and corals to be arranged side by side with similar size and shape, mimicking the situation of neighbouring organisms. Compared to earlier methods that attached sponge cores onto coral surfaces, hybrid cores provide an opportunity to study organism interactions under conditions more equal to the interacting partners. The use of hybrid cores was demonstrated for the excavating sponge Cliona orientalis and the massive coral Porites, which commonly interact on the Great Barrier Reef. Cliona orientalis and massive Porites were cut into half-moon shaped explants and combined as hybrid cores under replicate conditions. After 90 days in an aquarium setting, positive growth of Cl. orientalis along with net bioerosion were observed in sponge control cores that combined Cl. orientalis with blank substrate. However, when Cl. orientalis and massive Porites were in contact in interaction cores, the sponge displayed negative growth and undetectable bioerosion, and was slightly overgrown by the coral. Cliona orientalis may have developed tissue extension beneath the living coral tissue, but growth and net calcification rates of massive Porites were apparently not affected by Cl. orientalis when comparing the interaction cores to coral control cores that combined massive Porites with blank substrate. Overall, the present work demonstrated that hybrid cores can be used to generate conditions suitable for studying sponge–coral interactions in the laboratory, which can also be applied in the field.

Calcification and photophysiology responses to elevated pCO2 in six Halimeda species from contrasting irradiance environments on Little Cayman Island reefs

While elevated pCO2 associated with ocean acidification has been shown to lower calcification rates in some marine calcifiers, the modulating potential of irradiance is not well studied. Therefore, we examined the interactive effects of irradiance (high and low) and elevated pCO2 on six species of the macroalgal genus, Halimeda, an ecologically important tropical calcifier. Calcification, growth, and aragonite crystal formation were studied in a 42d aquaria experiment. Species-specific photophysiology and inorganic C saturation levels were ascertained by establishing photosynthesis to irradiance and inorganic C (Ci) relationships potentially linking photophysiology to growth and calcification responses to elevated pCO2. Because of Halimeda's role as a sediment producer on tropical reefs, the effect of elevated pCO2 on dissolution of non-living segments was also examined. Net calcification rates varied among species, with no significant pCO2 or irradiance effects. However, low irradiance enhanced new apical segment growth in all six species. Crystals formed in new apical segments under elevated pCO2. Non-living segment mineral:organic content and internal calcium carbonate (CaCO3) crystal microstructure remained stable over 21d at elevated pCO2. Species-specific photosynthetic responses corresponded to irradiance conditions at collection sites. For example, high irradiance-adapted species photosynthetic rates were enhanced under increased availability of Ci. In contrast, low irradiance-adapted species photosynthetic rates were saturated under current Ci levels. While species-specific photosynthetic responses to elevated pCO2 occurred, these photophysiology shifts did not result in reduced calcification or microstructural alteration of carbonate crystals at pCO2 levels predicted for the year 2100 in any of the six Halimeda species examined.

Coral calcification under environmental change: a direct comparison of the alkalinity anomaly and buoyant weight techniques

Two primary methods—the buoyant weight (BW) and alkalinity anomaly (AA) techniques—are currently used to quantify net calcification rates (G) in scleractinian corals. However, it remains unclear whether they are directly comparable since the few method comparisons conducted to date have produced inconsistent results. Further, such a comparison has not been made for tropical corals. We directly compared GBW and GAA in four tropical and one temperate coral species cultured under various pCO2, temperature, and nutrient conditions. A range of protocols for conducting alkalinity depletion incubations was assessed. For the tropical corals, open-top incubations with manual stirring produced GAA that were highly correlated with and not significantly different from GBW. Similarly, GAA of the temperate coral was not significantly different from GBW when incubations provided water motion using a pump, but were significantly lower than GBW by 16% when water motion was primarily created by aeration. This shows that the two techniques can produce comparable calcification rates in corals but only when alkalinity depletion incubations are conducted under specific conditions. General recommendations for incubation protocols are made, especially regarding adequate water motion and incubation times. Further, the re-analysis of published data highlights the importance of using appropriate regression statistics when both variables are random and measured with error. Overall, we recommend the AA technique for investigations of community and short-term day versus night calcification, and the BW technique to measure organism calcification rates integrated over longer timescales due to practical limitations of both methods. Our findings will facilitate the direct comparison of studies measuring coral calcification using either method and thus have important implications for the fields of ocean acidification research and coral biology in general.