Posts Tagged 'corals'



Heterotrophy of oceanic particulate organic matter elevates net ecosystem calcification

Coral reef calcification is expected to decline due to climate change stressors such as ocean acidification and warming. Projections of future coral reef health are based on our understanding of the environmental drivers that affect calcification and dissolution. One such driver that may impact coral reef health is heterotrophy of oceanic‐sourced particulate organic matter, but its link to calcification has not been directly investigated in the field. In this study, we estimated net ecosystem calcification (NEC) and oceanic particulate organic carbon (POCoc) uptake across the Kāneʻohe Bay barrier reef in Hawai‘i. We show that higher rates of POCoc uptake correspond to greater NEC rates, even under low aragonite saturation states (Ωar). Hence, reductions in offshore productivity may negatively impact coral reefs by decreasing the food supply required to sustain calcification. Alternatively, coral reefs that receive ample inputs of POCoc may maintain higher calcification rates, despite a global decline in Ωar.

Continue reading ‘Heterotrophy of oceanic particulate organic matter elevates net ecosystem calcification’

Ocean acidification effects on in situ coral reef metabolism

The Anthropocene climate has largely been defined by a rapid increase in atmospheric CO2, causing global climate change (warming) and ocean acidification (OA, a reduction in oceanic pH). OA is of particular concern for coral reefs, as the associated reduction in carbonate ion availability impairs biogenic calcification and promotes dissolution of carbonate substrata. While these trends ultimately affect ecosystem calcification, scaling experimental analyses of the response of organisms to OA to consider the response of ecosystems to OA has proved difficult. The benchmark of ecosystem-level experiments to study the effects of OA is provided through Free Ocean CO2 Enrichment (FOCE), which we use in the present analyses for a 21-d experiment on the back reef of Mo’orea, French Polynesia. Two natural coral reef communities were incubated in situ, with one exposed to ambient pCO2 (393 µatm), and one to high pCO2 (949 µatm). Our results show a decrease in 24-h net community calcification (NCC) under high pCO2, and a reduction in nighttime NCC that attenuated and eventually reversed over 21-d. This effect was not observed in daytime NCC, and it occurred without any effect of high pCO2 on net community production (NCP). These results contribute to previous studies on ecosystem-level responses of coral reefs to the OA conditions projected for the end of the century, and they highlight potential attenuation of high pCO2 effects on nighttime net community calcification.

Continue reading ‘Ocean acidification effects on in situ coral reef metabolism’

Using B isotopes and B/Ca in corals from low saturation springs to constrain calcification mechanisms

Ocean acidification is expected to negatively impact calcifying organisms, yet we lack understanding of their acclimation potential in the natural environment. Here we measured geochemical proxies (δ11B and B/Ca) in Porites astreoides corals that have been growing for their entire life under low aragonite saturation (Ωsw: 0.77–1.85). This allowed us to assess the ability of these corals to manipulate the chemical conditions at the site of calcification (Ωcf), and hence their potential to acclimate to changing Ωsw. We show that lifelong exposure to low Ωsw did not enable the corals to acclimate and reach similar Ωcf as corals grown under ambient conditions. The lower Ωcf at the site of calcification can explain a large proportion of the decreasing P. astreoides calcification rates at low Ωsw. The naturally elevated seawater dissolved inorganic carbon concentration at this study site shed light on how different carbonate chemistry parameters affect calcification conditions in corals.

Continue reading ‘Using B isotopes and B/Ca in corals from low saturation springs to constrain calcification mechanisms’

Potential distribution under climate change scenarios of corals of the genus Pocillopora (Anthozoa: Scleractinia) in the tropical eastern Pacific (in Spanish)

In the tropical eastern Pacific (POT), the pociloporids represent a key component of the coral communities, which develop in limiting environmental conditions. The objective of this study is to understand how climate change, in particular the increase in temperature and acidification, could influence the distribution of these coral species. Ecological niche models were used to evaluate possible changes in the geographical distribution of 9 species of pociloporides, based on predictions of temperature increase and pH decrease under the “representative concentration pathways” (CPR) scenarios. 2.6, 4.5 and 8.5 for the year 2050. The projections made with Maxent show a tendency towards the conservation of the species distribution area in the CPR 2.6 scenario, while an increase in the favorable area for most of them was observed under the CPR 4.5 and 8.5 scenarios. By 2050, the optimal conditions for the presence of corals will be located at high latitudes and towards the equator. Finally, regions currently considered marginal for reef development are expected to persist and expand under future conditions. These results have important implications for the conservation of marginal reefs under a changing climate.

Continue reading ‘Potential distribution under climate change scenarios of corals of the genus Pocillopora (Anthozoa: Scleractinia) in the tropical eastern Pacific (in Spanish)’

High pCO2 promotes coral primary production

While research on ocean acidification (OA) impacts on coral reefs has focused on calcification, relatively little is known about effects on coral photosynthesis and respiration, despite these being among the most plastic metabolic processes corals may use to acclimatize to adverse conditions. Here, we present data collected between 2016 and 2018 at three natural CO2 seeps in Papua New Guinea where we measured the metabolic flexibility (i.e. in hospite photosynthesis and dark respiration) of 12 coral species. Despite some species-specific variability, metabolic rates as measured by net oxygen flux tended to be higher at high pCO2 (ca 1200 µatm), with increases in photosynthesis exceeding those of respiration, suggesting greater productivity of Symbiodiniaceae photosynthesis in hospite, and indicating the potential for metabolic flexibility that may enable these species to thrive in environments with high pCO2. However, laboratory and field observations of coral mortality under high CO2 conditions associated with coral bleaching suggests that this metabolic subsidy does not result in coral higher resistance to extreme thermal stress. Therefore, the combined effects of OA and global warming may lead to a strong decrease in coral diversity despite the stimulating effect on coral productivity of OA alone.

Continue reading ‘High pCO2 promotes coral primary production’

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.

Continue reading ‘Response of corals Acropora pharaonis and Porites lutea to changes in pH and temperature in the Gulf’

Mediterranean cold-water corals as paleoclimate archives

Scleractinian cold-water corals preserve in their aragonite skeleton information on the past changes of the physico-chemical properties of the seawater in which they grew. Such information is stored as geochemical signals, such as changes in trace elements concentration (B/Ca, Li/Mg, P/Ca, Sr/Ca, Ba/Ca, U/Ca) or stable and radiogenic isotopes composition (δ11B, δ13C, δ18O, 14C, εNd), that are usually converted into environmental parameters using empirical calibration equations. The aragonite skeleton of cold-water corals is sufficiently uranium-rich to be suitable for U-series dating, providing precise and accurate ages for the last 600–700 kyrs. This opens the possibility to obtain reconstructions of key oceanographic parameters for the intermediate and deep water masses at sub-decadal scale resolution for climatically-relevant time windows in the past. However, part of the geochemical signal incorporated into the coral skeleton is modulated by the physiology of the coral, which complicates the interpretation of the geochemical proxies. This “vital effect” needs to be taken into account and corrected for to obtain reliable reconstructions of past changes in seawater temperature, pH and nutrient content. On the other hand, these biologically-induced geochemical signals can be used to investigate the processes controlling coral biomineralisation and better understand the resilience of cold-water corals to environmental and climate changes.

In the recent years, Mediterranean cold-water corals have been targeted for geochemically-oriented studies and their trace elements and isotopes composition has contributed significantly to developing and understanding new and established coral proxies. Living in an environment characterised by relatively warm seawater temperatures (13–14 °C) and high pH (8.1), the Mediterranean cold-water corals provide the end-member geochemical composition useful to derive empirical calibration equations. In particular, the analysis of several specimens of the cold-water corals species Lophelia pertusa, Madrepora oculata and Desmophyllum dianthus live-collected in the western, central and eastern Mediterranean Sea, has contributed to the development of the Li/Mg thermometer, boron isotopes pH proxy and P/Ca nutrient proxy, as well as a better understanding of the neodymium isotopic composition of cold-water corals as a water mass tracer. A multi-proxy approach has been recently applied to precisely U/Th-dated cold-water corals fragments from coral-bearing sediment cores retrieved in the western and central Mediterranean Sea, showing large changes in the dynamics of the intermediate waters during the Holocene. Further investigations of fossil cold-water corals specimens from different Mediterranean locations will open new perspectives on the reconstruction of past changes in the physico-chemical properties of sub-surface waters and their potential role in modifying the Mediterranean climate.

Continue reading ‘Mediterranean cold-water corals as paleoclimate archives’


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Ocean acidification in the IPCC AR5 WG II

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