Steeve Comeau

Global warming and recurrent mass bleaching of corals

During 2015–2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.

Response of the Arctic Pteropod Limacina helicina to Projected Future Environmental Conditions

Thecosome pteropods (pelagic mollusks) can play a key role in the food web of various marine ecosystems. They are a food source for zooplankton or higher predators such as fishes, whales and birds that is particularly important in high latitude areas. Since they harbor a highly soluble aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO2 emissions. The effect of changes in the seawater chemistry was investigated on Limacina helicina, a key species of Arctic pelagic ecosystems. Individuals were kept in the laboratory under controlled pCO2 levels of 280, 380, 550, 760 and 1020 µatm and at control (0°C) and elevated (4°C) temperatures. The respiration rate was unaffected by pCO2 at control temperature, but significantly increased as a function of the pCO2 level at elevated temperature. pCO2 had no effect on the gut clearance rate at either temperature. Precipitation of CaCO3, measured as the incorporation of 45Ca, significantly declined as a function of pCO2 at both temperatures. The decrease in calcium carbonate precipitation was highly correlated to the aragonite saturation state. Even though this study demonstrates that pteropods are able to precipitate calcium carbonate at low aragonite saturation state, the results support the current concern for the future of Arctic pteropods, as the production of their shell appears to be very sensitive to decreased pH. A decline of pteropod populations would likely cause dramatic changes to various pelagic ecosystems.

Coral reef calcifiers buffer their response to ocean acidification using both bicarbonate and carbonate.

Central to evaluating the effects of ocean acidification (OA) on coral reefs is understanding how calcification is affected by the dissolution of CO2 in sea water, which causes declines in carbonate ion concentration [CO32−] and increases in bicarbonate ion concentration [HCO3−]. To address this topic, we manipulated [CO32−] and [HCO3−] to test the effects on calcification of the coral Porites rus and the alga Hydrolithon onkodes, measured from the start to the end of a 15-day incubation, as well as in the day and night. [CO32−] played a significant role in light and dark calcification of P. rus, whereas [HCO3−] mainly affected calcification in the light. Both [CO32−] and [HCO3−] had a significant effect on the calcification of H. onkodes, but the strongest relationship was found with [CO32−]. Our results show that the negative effect of declining [CO32−] on the calcification of corals and algae can be partly mitigated by the use of HCO3− for calcification and perhaps photosynthesis. These results add empirical support to two conceptual models that can form a template for further research to account for the calcification response of corals and crustose coralline algae to OA.

Water flow modulates the response of coral reef communities to ocean acidification

By the end of the century coral reefs likely will be affected negatively by ocean acidification (OA), but both the effects of OA on coral communities and the crossed effects of OA with other physical environmental variables are lacking. One of the least considered physical parameters is water flow, which is surprising considering its strong role in modulating the physiology of reef organisms and communities. In the present study, the effects of flow were tested on coral reef communities maintained in outdoor flumes under ambient pCO2 and high pCO2 (1300 μatm). Net calcification of coral communities, including sediments, was affected by both flow and pCO2 with calcification correlated positively with flow under both pCO2 treatments. The effect of flow was less evident for sediments where dissolution exceeded precipitation of calcium carbonate under all flow speeds at high pCO2. For corals and calcifying algae there was a strong flow effect, particularly at high pCO2 where positive net calcification was maintained at night in the high flow treatment. Our results demonstrate the importance of water flow in modulating the coral reef community response to OA and highlight the need to consider this parameter when assessing the effects of OA on coral reefs.

Impact of aragonite saturation state changes on migratory pteropods

Thecosome pteropods play a key role in the food web of various marine ecosystems and they calcify, secreting the unstable CaCO3 mineral aragonite to form their shell material. Here, we have estimated the effect of ocean acidification on pteropod calcification by exploiting empirical relationships between their gross calcification rates (CaCO3 precipitation) and aragonite saturation state Ωa, combined with model projections of future Ωa. These were corrected for modern model-data bias and taken over the depth range where pteropods are observed to migrate vertically. Results indicate large reductions in gross calcification at temperate and high latitudes. Over much of the Arctic, the pteropod Limacina helicina will become unable to precipitate CaCO3 by the end of the century under the IPCC SRES A2 scenario. These results emphasize concerns over the future of shelled pteropods, particularly L. helicina in high latitudes. Shell-less L. helicina are not known to have ever existed nor would we expect them to survive. Declines of pteropod populations could drive dramatic ecological changes in the various pelagic ecosystems in which they play a critical role.

Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification.

Ocean acidification (OA) and its associated decline in calcium carbonate saturation states is one of the major threats that tropical coral reefs face this century. Previous studies of the effect of OA on coral reef calcifiers have described a wide variety of outcomes for studies using comparable partial pressure of CO2 (pCO2) ranges, suggesting that key questions remain unresolved. One unresolved hypothesis posits that heterogeneity in the response of reef calcifiers to high pCO2 is a result of regional-scale variation in the responses to OA. To test this hypothesis, we incubated two coral taxa (Pocillopora damicornis and massive Porites) and two calcified algae (Porolithon onkodes and Halimeda macroloba) under 400, 700 and 1000 μatm pCO2 levels in experiments in Moorea (French Polynesia), Hawaii (USA) and Okinawa (Japan), where environmental conditions differ. Both corals and H. macroloba were insensitive to OA at all three locations, while the effects of OA on P. onkodes were location-specific. In Moorea and Hawaii, calcification of P. onkodes was depressed by high pCO2, but for specimens in Okinawa, there was no effect of OA. Using a study of large geographical scale, we show that resistance to OA of some reef species is a constitutive character expressed across the Pacific.

Coral calcifying fluid pH is modulated by seawater carbonate chemistry not solely seawater pH

Reef coral calcification depends on regulation of pH in the internal calcifying fluid (CF) in which the coral skeleton forms. However, little is known about calcifying fluid pH (pHCF) regulation, despite its importance in determining the response of corals to ocean acidification. Here, we investigate pHCF in the coral Stylophora pistillata in seawater maintained at constant pH with manipulated carbonate chemistry to alter dissolved inorganic carbon (DIC) concentration, and therefore total alkalinity (AT). We also investigate the intracellular pH of calcifying cells, photosynthesis, respiration and calcification rates under the same conditions. Our results show that despite constant pH in the surrounding seawater, pHCF is sensitive to shifts in carbonate chemistry associated with changes in [DIC] and [AT], revealing that seawater pH is not the sole driver of pHCF. Notably, when we synthesize our results with published data, we identify linear relationships of pHCF with the seawater [DIC]/[H+] ratio, [AT]/ [H+] ratio and []. Our findings contribute new insights into the mechanisms determining the sensitivity of coral calcification to changes in seawater carbonate chemistry, which are needed for predicting effects of environmental change on coral reefs and for robust interpretations of isotopic palaeoenvironmental records in coral skeletons.

Response to coral reef calcification: carbonate, bicarbonate and proton flux under conditions of increasing ocean acidification.

We respond to the comment ‘Coral reef calcification: carbonate, bicarbonate and proton flux under conditions of increasing ocean acidification’ by Jokiel [[1][1]], who uses our data [[2][2]] to test his ‘proton flux hypothesis’, which accounts for the negative effect of ocean acidification (

Coralline algae elevate pH at the site of calcification under ocean acidification

Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species-specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pHcf ) using δ11 B as a proxy. Declines in δ11 B for all three species are consistent with shifts in δ11 B expected if B(OH)4- was incorporated during precipitation. In particular, the δ11 B ratio in Amphiroa anceps was too low to allow for reasonable pHcf values if B(OH)3 rather than B(OH)4- was directly incorporated from the calcifying fluid. This points towards δ11 B being a reliable proxy for pHcf for coralline algal calcite and that if B(OH)3 is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH)4- . We thus show that pHcf is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO2 , as did their pHcf . Neogoniolithon sp. had the highest pHcf , and most constant calcification rates, with the decrease in pHcf being ¼ that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pHcf under ocean acidification is physiologically important and should be included in future models involving calcification.

Effects of pCO2 on photosynthesis and respiration of tropical scleractinian corals and calcified algae

&NA; The effects of ocean acidification (OA) on coral reefs have been studied thoroughly with a focus on the response of calcification of corals and calcified algae. However, there are still large gaps in our knowledge of the effects of OA on photosynthesis and respiration of these organisms. Comparisons among species and determination of the functional relationships between pCO2 and either photosynthesis or respiration are difficult using previously published data, because experimental conditions typically vary widely between studies. Here, we tested the response of net photosynthesis, gross photosynthesis, dark respiration, and light‐enhanced dark respiration (LEDR) of eight coral taxa and seven calcified alga taxa to six different pCO2 levels (from 280 to 2000 &mgr;atm). Organisms were maintained during 7‐10 days incubations in identical conditions of light, temperature, and pCO2 to facilitate comparisons among species. Net photosynthesis was not affected by pCO2 in seven of eight corals or any of the algae; gross photosynthesis did not respond to pCO2 in six coral taxa and six algal taxa; dark respiration also was unaffected by pCO2 in six coral and six algae; and LEDR did not respond to pCO2 in any of the tested species. Overall, our results show that pCO2 levels up to 2000 &mgr;atm likely will not fertilize photosynthesis or modify respiration rates of most of the main calcifiers on the back reef of Moorea, French Polynesia.