Net Calcification Rates Research Articles

Positive and negative responses of coral calcification to elevated pCO2: case studies of two coral species and the implications of their responses

Fragments of 2 coral species (Acropora nasuta and Pocillopora damicornis), collected from the South China Sea, were incubated for 94 d under controlled conditions of pCO(2) = 389, 700, and 1214 mu atm. Our incubation experiments showed that the net calcification rate of A. nasuta responded negatively to elevated pCO(2) in both short and mid-term incubations. In contrast, the net calcification rate of P. damicornis increased under elevated pCO(2) during the first 17 d, but then returned to background rates. Based on previous models, our modified models showed that the different responses of these 2 coral species depended on the dissolved inorganic carbon (DIC) and pH levels in coral calcifying fluid. In the studied models, the positive responses of coral calcification to higher pCO(2) could be explained by either low DIC due to a higher photosynthesis rate or a stronger H+ pump. If DIC in calcifying fluid is greatly reduced by photosynthesis, the de creased DIC in calcifying fluid may be benefited by enhanced external seawater CO2, which will partially offset the dissolution due to ocean acidification. Therefore, in addition to our experimental results, our model provides a theoretical basis showing how coral may respond negatively and positively to ocean acidification.

End of the Century pCO2 Levels Do Not Impact Calcification in Mediterranean Cold-Water Corals

Ocean acidification caused by anthropogenic uptake of CO2 is perceived to be a major threat to calcifying organisms. Cold-water corals were thought to be strongly affected by a decrease in ocean pH due to their abundance in deep and cold waters which, in contrast to tropical coral reef waters, will soon become corrosive to calcium carbonate. Calcification rates of two Mediterranean cold-water coral species, Lophelia pertusa and Madrepora oculata, were measured under variable partial pressure of CO2 (pCO2) that ranged between 380 µatm for present-day conditions and 930 µatm for the end of the century. The present study addressed both short- and long-term responses by repeatedly determining calcification rates on the same specimens over a period of 9 months. Besides studying the direct, short-term response to elevated pCO2 levels, the study aimed to elucidate the potential for acclimation of calcification of cold-water corals to ocean acidification. Net calcification of both species was unaffected by the levels of pCO2 investigated and revealed no short-term shock and, therefore, no long-term acclimation in calcification to changes in the carbonate chemistry. There was an effect of time during repeated experiments with increasing net calcification rates for both species, however, as this pattern was found in all treatments, there is no indication that acclimation of calcification to ocean acidification occurred. The use of controls (initial and ambient net calcification rates) indicated that this increase was not caused by acclimation in calcification response to higher pCO2. An extrapolation of these data suggests that calcification of these two cold-water corals will not be affected by the pCO2 level projected at the end of the century.

Carbon turnover rates in the One Tree Island reef: A 40‐year perspective

During November–December 2009 community rates of gross photosynthesis (Pg), respiration (R) and net calcification (Gnet) were estimated from low‐tide slack water measurements of dissolved oxygen, dissolved inorganic carbon and total alkalinity at the historical station DK13 One Tree Island reef, Great Barrier Reef, Australia. Compared to measurements made during the 1960s–1970s at DK13 in the same season,Pg increased from 833 to 914 mmol O2·m−2·d−1 and Pg:R increased from 1.14 to 1.30, indicating that the reef has become more autotrophic. In contrast, Gnet decreased from 133 mmol C·m−2·d−1 to 74 ± 24 mmol C·m−2·d−1. This decrease stems primarily from the threefold increase in nighttime CaCO3 dissolution from −2.5 mmol·m−2·h−1 to −7.5 mmol·m−2·h−1. Comparison of the benthic community survey results from DK13 and its vicinity conducted during this study and in studies from the 1970s, 1980s and 1990s suggest that there have been no significant changes in the live coral coverage during the past 40 years. The reduced Gnet most likely reflects the almost threefold increase in dissolution rates, possibly resulting from increased bioerosion due to changes in the biota (e.g., sea cucumbers, boring organisms) and/or from greater chemical dissolution produced by changing abiotic conditions over the past 40 years associated with climate change, such as increased temperatures and ocean acidification. However, at this stage of research on One Tree Island the effects of these changes are not entirely understood.

Seasonal coupling and de‐coupling of net calcification rates from coral reef metabolism and carbonate chemistry at Ningaloo Reef, Western Australia

Rates of net production, net calcification, and nutrient uptake were measured in a coral‐dominated reef flat community on Ningaloo Reef in northwestern Australia under seasonally minimum and maximum light levels. Daily integrated light decreased twofold while water temperatures remained relatively constant increasing by only 1°C on average from summer to winter. Rates of daily community gross primary production (GPP) were only 33% ± 9% higher in summer than in winter (1400 ± 70 versus 1050 ± 60 mmol C m−2 d−1), far less than the twofold seasonal changes reported for most shallow reef communities. Rates of daily community net calcification (Gnet) were not significantly different between seasons (190 ± 40 mmol CaCO3 m−2 d−1 in summer versus 200 ± 10 mmol CaCO3 m−2 d−1 in winter). The average rate of total nitrogen uptake (dissolved + particulate) was also not significantly different between summer and winter (8.3 ± 3.8 versus 6.6 ± 3.4 mmol N m−2 d−1, respectively), despite evidence of sporadically high nitrate uptake in both seasons. In summer, rates of hourly net calcification (gnet) were linearly correlated with diurnal changes in net production, pH, and aragonite saturation state (Ωar); and were mostly correlated with light except at mid‐day under heavy cloud cover. However, in winter,gnet was independent of diurnal changes in light, net production, pH, and Ωar indicating that the reef flat community had possibly reached a threshold above which rates of net calcification were insensitive to diurnal changes in their environment.

Effect of ocean acidification and temperature increase on the planktonic foraminifer Neogloboquadrina pachyderma (sinistral)

The present study investigated the effects of ocean acidification and temperature increase on Neogloboquadrina pachyderma (sinistral), the dominant planktonic foraminifer in the Arctic Ocean. Due to the naturally low concentration of CO 3 2− in the Arctic, this foraminifer could be particularly sensitive to the forecast changes in seawater carbonate chemistry. To assess potential responses to ocean acidification and climate change, perturbation experiments were performed on juvenile and adult specimens by manipulating seawater to mimic the present-day carbon dioxide level and a future ocean acidification scenario (end of the century) under controlled (in situ) and elevated temperatures (1 and 4 °C, respectively). Foraminifera mortality was unaffected under all the different experiment treatments. Under low pH, N. pachyderma (s) shell net calcification rates decreased. This decrease was higher (30 %) in the juvenile specimens than decrease observed in the adults (21 %) ones. However, decrease in net calcification was moderated when both, pH decreased and temperature increased simultaneously. When only temperature increased, a net calcification rate for both life stages was not affected. These results show that forecast changes in seawater chemistry would impact calcite production in N. pachyderma (s), possibly leading to a reduction of calcite flux contribution and consequently a decrease in biologic pump efficiency.

The combined influence of hydrodynamic forcing and calcification on the spatial distribution of alkalinity in a coral reef system

We investigated the influence of hydrodynamic forcing (waves, tides, alongshore currents and winds) and net calcification by coral reef organisms on the spatial distribution of total alkalinity (TA) in a fringing reef system through a combination of field measurements and numerical modeling. A field experiment was conducted over 10 days in Coral Bay (Ningaloo Reef, Western Australia) during which we measured wave heights, currents, and tides as well as the spatial distribution of TA across the fore reef, reef crest, and lagoon. We used observed changes in TA on the adjacent reef flat, along with synoptic measurements of cross‐reef transport, to estimate in situ rates of net calcification (gcv) using a control volume approach. Based on the gcv estimated, we simulated light‐driven, diurnal variations in benthic net calcification within a three‐dimensional ocean circulation model, ROMS (Regional Ocean Modeling System). By coupling ROMS with a spectral wave model (Simulating Waves Nearshore), we were able to simulate currents within Coral Bay reef‐lagoon system that were in good agreement with the field observations and demonstrate that circulation with the system was wave‐dominated. Both the field measurements and numerical model output confirmed that both residence time (τR) and TA varied primarily with offshore wave heights and location within the bay. However, variations in TA were also affected by the nonlinear interaction between rates of net calcification that varied as a function of diurnally changing light and water residence time that varied as a function of offshore wave heights.

Effects of ocean acidification and high temperatures on the bryozoan Myriapora truncata at natural CO2 vents

AbstractThere are serious concerns that ocean acidification will combine with the effects of global warming to cause major shifts in marine ecosystems, but there is a lack of field data on the combined ecological effects of these changes due to the difficulty of creating large‐scale, long‐term exposures to elevated CO2 and temperature. Here we report the first coastal transplant experiment designed to investigate the effects of naturally acidified seawater on the rates of net calcification and dissolution of the branched calcitic bryozoan Myriapora truncata (Pallas, 1766). Colonies were transplanted to normal (pH 8.1), high (mean pH 7.66, minimum value 7.33) and extremely high CO2 conditions (mean pH 7.43, minimum value 6.83) at gas vents off Ischia Island (Tyrrhenian Sea, Italy). The net calcification rates of live colonies and the dissolution rates of dead colonies were estimated by weighing after 45 days (May–June 2008) and after 128 days (July–October) to examine the hypothesis that high CO2 levels affect bryozoan growth and survival differently during moderate and warm water conditions. In the first observation period, seawater temperatures ranged from 19 to 24 °C; dead M. truncata colonies dissolved at high CO2 levels (pH 7.66), whereas live specimens maintained the same net calcification rate as those growing at normal pH. In extremely high CO2 conditions (mean pH 7.43), the live bryozoans calcified significantly less than those at normal pH. Therefore, established colonies of M. truncata seem well able to withstand the levels of ocean acidification predicted in the next 200 years, possibly because the soft tissues protect the skeleton from an external decrease in pH. However, during the second period of observation a prolonged period of high seawater temperatures (25–28 °C) halted calcification both in controls and at high CO2, and all transplants died when high temperatures were combined with extremely high CO2 levels. Clearly, attempts to predict the future response of organisms to ocean acidification need to consider the effects of concurrent changes such as the Mediterranean trend for increased summer temperatures in surface waters. Although M. truncata was resilient to short‐term exposure to high levels of ocean acidification at normal temperatures, our field transplants showed that its ability to calcify at higher temperatures was compromised, adding it to the growing list of species now potentially threatened by global warming.

Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification

Anthropogenic elevation of atmospheric carbon dioxide ( p CO2) is making the oceans more acidic, thereby reducing their degree of saturation with respect to calcium carbonate (CaCO3). There is mounting concern over the impact that future CO2-induced reductions in the CaCO3 saturation state of seawater will have on marine organisms that construct their shells and skeletons from this mineral. Here, we present the results of 60 d laboratory experiments in which we investigated the effects of CO2-induced ocean acidification on calcification in 18 benthic marine organisms. Species were selected to span a broad taxonomic range (crustacea, cnidaria, echinoidea, rhodophyta, chlorophyta, gastropoda, bivalvia, annelida) and included organisms producing aragonite, low-Mg calcite, and high-Mg calcite forms of CaCO3. We show that 10 of the 18 species studied exhibited reduced rates of net calcification and, in some cases, net dissolution under elevated p CO2. However, in seven species, net calcification increased under the intermediate and/or highest levels of p CO2, and one species showed no response at all. These varied responses may reflect differences amongst organisms in their ability to regulate pH at the site of calcification, in the extent to which their outer shell layer is protected by an organic covering, in the solubility of their shell or skeletal mineral, and in the extent to which they utilize photosynthesis. Whatever the specific mechanism(s) involved, our results suggest that the impact of elevated atmospheric p CO2 on marine calcification is more varied than previously thought.

Effects of carbon dioxide on the coccolithophorid Pleurochrysis carterae in incubation experiments

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 7:59-70 (2009) - DOI: https://doi.org/10.3354/ab00182 Effects of carbon dioxide on the coccolithophorid Pleurochrysis carterae in incubation experiments Beatriz E. Casareto1,2,*, Mohan P. Niraula2,4, Hiroyuki Fujimura3, Yoshimi Suzuki2 1Laboratory of Aquatic Science and Consultant Co., Ltd., Meishin Bldg. Kamiikedai 1-14-1, Ota-ku, Tokyo 145-0064, Japan 2Graduate School of Science and Technology, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan 3Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa 903-0213, Japan 4Institute of Science and Technology, Tribhuvan University Trichandra Campus, Kathmandu, Nepal *Email: casaretobe@aol.com ABSTRACT: Production (abundance and biomass) and net calcification rates of the coccolithophorid Pleurochrysis carterae under different partial pressures of CO2 (pCO2) were examined using short (15, 24 and 39 h), long (7 d) and dark (7 d) incubation experiments. Short incubations were conducted at ambient, 500 and 820 ppm pCO2 levels in natural seawater that was enriched with nutrients and inoculated with P. carterae. Long incubations were conducted at ambient and 1200 ppm pCO2 levels in natural seawater (0.2 µm filtered as well as unfiltered) that was enriched with nutrients and inoculated with P. carterae. Dark incubations were conducted at ambient and 1200 ppm pCO2 in unfiltered seawater that was inoculated with P. carterae. The abundance and biomass of coccolithophorids increased with pCO2 and time. The abundance and biomass of most noncalcifying phytoplankton also increased, and were hardly affected by CO2 inputs. Net calcification rates were negative in short incubations during the pre-bloom phase regardless of pCO2 levels, indicating dissolution of calcium carbonate. Further, the negative values of net calcification in short incubations became less negative with time. Net calcification rates were positive in long incubations during blooms regardless of pCO2 level, and the rate of calcification increased with pCO2. Our results show that P. carterae may adapt to increased (~1200 ppm) pCO2 level with time, and such increase has little effect on the ecology of noncalcifying groups and hence in ecosystem dynamics. In dark incubations, net calcification rates were negative, with the magnitude being dependent on pCO2 levels. KEY WORDS: pCO2 · Pleurochrysis carterae · Noncalcifying phytoplankton · Biomass · Net calcification rates · Adaptation Full text in pdf format PreviousNextCite this article as: Casareto BE, Niraula MP, Fujimura H, Suzuki Y (2009) Effects of carbon dioxide on the coccolithophorid Pleurochrysis carterae in incubation experiments. Aquat Biol 7:59-70. https://doi.org/10.3354/ab00182 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 7, No. 1-2. Online publication date: September 24, 2009 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2009 Inter-Research.

Estimation of photosynthesis and calcification rates at a fringing reef by accounting for diurnal variations and the zonation of coral reef communities on reef flat and slope: a case study for the Shiraho reef, Ishigaki Island, southwest Japan

Seven coral reef communities were defined on Shiraho fringing reef, Ishigaki Island, Japan. Net photosynthesis and calcification rates were measured by in situ incubations at 10 sites that included six of the defined communities, and which occupied most of the area on the reef flat and slope. Net photosynthesis on the reef flat was positive overall, but the reef flat acts as a source for atmospheric CO2, because the measured calcification/photosynthesis ratio of 2.5 is greater than the critical ratio of 1.67. Net photosynthesis on the reef slope was negative. Almost all excess organic production from the reef flat is expected to be effused to the outer reef and consumed by the communities there. Therefore, the total net organic production of the whole reef system is probably almost zero and the whole reef system also acts as a source for atmospheric CO2. Net calcification rates of the reef slope corals were much lower than those of the branching corals. The accumulation rate of the former was approximately 0.5 m kyr−1 and of the latter was ~0.7–5 m kyr−1. Consequently, reef slope corals could not grow fast enough to keep up with or catch up to rising sea levels during the Holocene. On the other hand, the branching corals grow fast enough to keep up with this rising sea level. Therefore, a transition between early Holocene and present-day reef communities is expected. Branching coral communities would have dominated while reef growth kept pace with sea level rise, and the reef was constructed with a branching coral framework. Then, the outside of this framework was covered and built up by reef slope corals and present-day reefs were constructed.

Alkalinity of the Mediterranean Sea

Total alkalinity (AT) was measured during the Meteor 51/2 cruise, crossing the Mediterranean Sea from west to east. AT concentrations were high (∼2600 μmol kg−1) and alkalinity‐salinity‐correlations had negative intercepts. These results are explained by evaporation coupled with high freshwater AT inputs into coastal areas. Salinity adjustment of AT revealed excess alkalinity throughout the water column compared to mid‐basin surface waters. Since Mediterranean waters are supersaturated with respect to calcite and aragonite, the excess alkalinity likely reflects alkalinity inputs to coastal areas close to regions of deep and intermediate water formation. An alkalinity budget shows that main alkalinity inputs come from the Black Sea and from rivers, whereas the Strait of Gibraltar is a net sink. The major sink appears to be carbonate sedimentation. The basin‐average net calcification rate and CaCO3 sedimentation was estimated to be 0.38 mol m−2 yr−1. The estimated residence time of AT is ∼160 yr.

CO<sub>3</sub><sup>2−</sup> concentration and pCO<sub>2</sub> thresholds for calcification and dissolution on the Molokai reef flat, Hawaii

Abstract. The severity of the impact of elevated atmospheric pCO2 to coral reef ecosystems depends, in part, on how seawater pCO2 affects the balance between calcification and dissolution of carbonate sediments. Presently, there are insufficient published data that relate concentrations of pCO2 and CO32− to in situ rates of reef calcification in natural settings to accurately predict the impact of elevated atmospheric pCO2 on calcification and dissolution processes. Rates of net calcification and dissolution, CO32− concentrations, and pCO2 were measured, in situ, on patch reefs, bare sand, and coral rubble on the Molokai reef flat in Hawaii. Rates of calcification ranged from 0.03 to 2.30 mmol CaCO3 m−2 h−1 and dissolution ranged from –0.05 to –3.3 mmol CaCO3 m−2 h−1. Calcification and dissolution varied diurnally with net calcification primarily occurring during the day and net dissolution occurring at night. These data were used to calculate threshold values for pCO2 and CO32− at which rates of calcification and dissolution are equivalent. Results indicate that calcification and dissolution are linearly correlated with both CO32− and pCO2. Threshold pCO2 and CO32− values for individual substrate types showed considerable variation. The average pCO2 threshold value for all substrate types was 654±195 μatm and ranged from 467 to 1003 μatm. The average CO32− threshold value was 152±24 μmol kg−1, ranging from 113 to 184 μmol kg−1. Ambient seawater measurements of pCO2 and CO32− indicate that CO32− and pCO2 threshold values for all substrate types were both exceeded, simultaneously, 13% of the time at present day atmospheric pCO2 concentrations. It is predicted that atmospheric pCO2 will exceed the average pCO2 threshold value for calcification and dissolution on the Molokai reef flat by the year 2100.

Diurnal variation in rates of calcification and carbonate sediment dissolution in Florida Bay

Water quality and criculation in Florida Bay (a shallow, subtropical estuary in south Florida) are highly dependent upon the development and evolution of carbonate mud banks distributed throughout the Bay. Predicting the effect of natural and anthropogenic perturbations on carbonate sedimentation requires an understanding of annual, seasonal, and daily variations in the biogenic and inorganic processes affecting carbonate sediment precipitation and dissolution. In this study, net calcification rates were measured over diurnal cycles on 27 d during summer and winter from 1999 to 2003 on mud banks and four representative substrate types located within basins between mud banks. Substrate types that were measured in basins include seagrass beds of sparse and intermediate densityThalassia sp., mud bottom, and hard bottom communities. Changes in total alkalinity were used as a proxy for calcification and dissolution. On 22 d (81%), diurnal variation in rates of net calcification was observed. The highest rates of net carbonate sediment production (or lowest rates of net dissolution) generally occurred during daylight hours and ranged from 2.900 to −0.410 g CaCO3 m−2d−1. The lowest rates of carbonate sediment production (or net sediment dissolution) occurred at night and ranged from 0.210 to −1.900 g CaCO3 m−2 night−1. During typical diurnal cycles, dissolution during the night consumed an average of 29% of sediment produced during the day on banks and 68% of sediment produced during the day in basins. Net sediment dissolution also occurred during daylight, but only when there was total cloud cover, high turbidity, or hypersalinity. Diurnal variation in calcification and dissolution in surface waters and surface sediments of Florida Bay is linked to cycling of carbon dioxide through photosynthesis and respiration. Estimation of long-term sediment accumulation rates from diurnal rates of carbonate sediment production measured in this study indicates an overall average accumulation rate for Florida Bay of 8.7 cm 1000 yr−1 and suggests that sediment dissolution plays a more important role than sediment transport in loss of sediment from Florida Bay.

Response of coccolithophorid Emiliania huxleyi to elevated partial pressure of CO2 under nitrogen limitation

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 261:111-122 (2003) - doi:10.3354/meps261111 Response of coccolithophorid Emiliania huxleyi to elevated partial pressure of CO2 under nitrogen limitation Antoine Sciandra1,*, Jérome Harlay2, Dominique Lefèvre3, Rodolphe Lemée1, Peguy Rimmelin3, Michel Denis3, Jean-Pierre Gattuso1 1Laboratoire d¹Océanographie, Université Mixte de Recherche (UMR) 7093, Centre National de la Recherche Scientifique- Université Pierre et Marie Curie (CNRS-UPMC), BP 28, 06234 Villefranche-sur-mer Cedex, France 2Service d¹Océanographie Chimique et Géochimie des Eaux, Université Libre de Bruxelles, Campus Plaine, CP 208, Boulevard du Triomphe, 1050 Brussels, Belgium 3Laboratoire d¹Océanographie et de Biogéochimie, Centre d'Océanologie de Marseille, Université Mixte de Recherche (UMR) 6535, Centre National de la Recherche Scientifique (CNRS)-Université de la Méditerranée, Case 901, 13288 Marseille Cedex 9, France *Email: sciandra@obs-vlfr.fr ABSTRACT: Precipitation of calcium carbonate by phytoplankton in the photic oceanic layer is an important process regulating the carbon cycling and the exchange of CO2 at the ocean-atmosphere interface. Previous experiments have demonstrated that, under nutrient-sufficient conditions, doubling the partial pressure of CO2 (pCO2) in seawater‹a likely scenario for the end of the century‹can significantly decrease both the rate of calcification by coccolithophorids and the ratio of inorganic to organic carbon production. The present work investigates the effects of high pCO2 on calcification by the coccolithophore Emiliania huxleyi (Strain TW1) grown under nitrogen-limiting conditions, a situation that can also prevail in the ocean. Nitrogen limitation was achieved in NO3-limited continuous cultures renewed at the rate of 0.5 d-1 and exposed to a saturating light level. pCO2 was increased from 400 to 700 ppm and controlled by bubbling CO2-rich or CO2-free air into the cultures. The pCO2 shift has a rapid effect on cell physiology that occurs within 2 cell divisions subsequent to the perturbation. Net calcification rate (C) decreased by 25% and, in contrast to previous studies with N-replete cultures, gross community production (GCP) and dark community respiration (DCR) also decreased. These results suggest that increasing pCO2 has no noticeable effect on the calcification/photosynthesis ratio (C/P) when cells of E. huxleyi are NO3-limited. KEY WORDS: Calcification · Carbon fixation · Coccolith · Emiliania huxleyi · Nitrate · Alkalinity · CO2 Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 261. Online publication date: October 17, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Calcium uptake and net calcification rates in the octocoral Eunicella papillosa

This paper presents the results of a preliminary study on the uptake of 45Ca in the mesoskeleton of the gorgonian Eunicella papillosa (Octocorallia). Hourly seawater aliquots over 12 h periods displayed considerable fluctuations of activity with time, reflecting a disappearance and reappearance of tracer during the experiment. Analysis of total coenenchyme mass showed an initial rapid but fluctuating uptake of tracer and a progressive decrease with time. Distinct differences in the calcification rate of specific growth regions of the coral were detected, the rate being higher in branch tips than in lower branch regions. Chase experiments on colonies previously incubated for 24 h revealed that up to 45% of the tracer taken up during labelling were returned to the seawater. A comparison of the tracer content of the coenenchyme and the calcite spicules revealed that up to 70% of the calcium taken up remained in the coral tissue. Quantification of isotopic exchange phenomena with dead corals and isolated spicules emphasized the importance of the live coral tissue as a barrier and regulator of Ca uptake. The results are discussed in the light of the limitations of radioisotope techniques for the determination of calcification rates, and an attempt to compare rates with data in the literature is made.