Comparing Chemistry and Census-Based Estimates of Net Ecosystem Calcification on a Rim Reef in Bermuda

Coral reef net ecosystem calcification (NEC) has decreased for many Caribbean reefs over recent decades primarily due to changes in benthic community composition. Chemistry-based approaches to calculate NEC utilize the drawdown of seawater total alkalinity (TA) combined with residence time to calculate an instantaneous measurement of NEC. Census-based approaches combine annual growth rates with benthic cover and reef structural complexity to estimate NEC occurring over annual timescales. Here, NEC was calculated for Hog Reef in Bermuda using both chemistry and census-based NEC techniques to compare the mass-balance generated by the two methods and identify the dominant biocalcifiers at Hog Reef. Our findings indicate close agreement between the annual 2011 census-based NEC 2.35 ± 1.01 kg CaCO3•m−2•y−1 and chemistry-based NEC 2.23 ± 1.02 kg CaCO3•m−2•y−1 at Hog Reef. An additional record of Hog Reef TA data calculated from an autonomous CO2 mooring measuring pCO2 and modeled pHtotal every 3-h highlights the dynamic temporal variability in coral reef NEC. This ability for chemistry-based NEC techniques to capture higher frequency variability in coral reef NEC allows the mechanisms driving NEC variability to be explored and tested. Just four coral species, Diploria labyrinthiformis, Pseudodiploria strigosa, Millepora alcicornis, and Orbicella franksi, were identified by the census-based NEC as contributing to 94 ± 19% of the total calcium carbonate production at Hog Reef suggesting these species should be highlighted for conservation to preserve current calcium carbonate production rates at Hog Reef. As coral cover continues to decline globally, the agreement between these NEC estimates suggest that either method, but ideally both methods, may serve as a useful tool for coral reef managers and conservation scientists to monitor the maintenance of coral reef structure and ecosystem services

Les récifs coralliens malades ... mais de quoi ?

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La microflore perforante et son impact sur les récifs coralliens

International audienceDepuis les dernières décennies et sous l'effet du changement climatique et du développement des activités humaines en milieu côtier tropical, 20 % des écosystèmes coralliens ont disparu. En outre, 15 % sont menacés de disparition imminente et environ 75 % sont en voie de dégradation. Ces éco-systèmes extrêmement fragiles, sont des lieux majeurs de biodiversité, per-mettent la subsistance d'1/15ème de la population mondiale, protègent les côtes et les îles des catastrophes telles que les cyclones et les tsunamis, et sont une source de biomolécules encore peu explorée. Il est donc impératif de mieux comprendre leur fonctionnement dans le contexte du changement global afin de mieux les préserver et de pérenniser les ressources écosystémiques qu'ils fournissent

Carbonate dissolution by reef microbial borers : a biogeological process producing alkalinity under different pCO(2) conditions

Rising atmospheric CO2 is acidifying the world's oceans, affecting both calcification and dissolution processes in coral reefs. Among processes, carbonate dissolution by bioeroding microflora has been overlooked, and especially its impact on seawater alkalinity. To date, this biogeological process has only been studied using microscopy or buoyant weight techniques. To better understand its possible effect on seawater alkalinity, and thus on reef carbonate budget, an experiment was conducted under various seawater chemistry conditions (2(arag)3.5 corresponding to 440pCO(2) (mu atm)940) at 25 degrees C under night and daylight (200 mu mol photons m(-2)s(-1)) with natural microboring communities colonizing dead coral blocks (New Caledonia). Both the alkalinity anomaly technique and microscopy methods were used to study the activity of those communities dominated by the chlorophyte Ostreobium sp. Results show that (1) the amount of alkalinity released in seawater by such communities is significant and varies between 12.8 +/- 0.7 at (Arag)similar to 2 and 5.6 +/- 0.4mmol CaCO3 m(-2) day(-1) at (Arag)similar to 3-3.5 considering a 12:12 photoperiod; (2) although dissolution is higher at night (similar to 80 vs. 20% during daylight), the process can occur under significant photosynthetic activity; and (3) the process is greatly stimulated when an acidity threshold is reached (pCO(2)920 mu atm vs. current conditions at constant light intensity). We show that carbonate dissolution by microborers is a major biogeochemical process that could dissolve a large part of the carbonates deposited by calcifying organisms under ocean acidification

The three steps of the carbonate biogenic dissolution process by microborers in coral reefs (New Caledonia)

Article en "Online First"International audienceBiogenic dissolution of carbonates by microborers is one of the main destructive forces in coral reefs and is predicted to be enhanced by eutrophication and ocean acidification by 2100. The chlorophyte Ostreobium sp., the main agent of this process, has been reported to be one of the most responsive of all microboring species to those environmental factors. However, very little is known about its recruitment, how it develops over successions of microboring communities, and how that influences rates of biogenic dissolution. Thus, an experiment with dead coral blocks exposed to colonization by microborers was carried out on a reef in New Caledonia over a year period. Each month, a few blocks were collected to study microboring communities and the associated rates of biogenic dissolution. Our results showed a drastic shift in community species composition between the 4th and 5th months of exposure, i.e., pioneer communities dominated by large chlorophytes such as Phaeophila sp. were replaced by mature communities dominated by Ostreobium sp. Prior the 4th month of exposure, large chlorophytes were responsible for low rates of biogenic dissolution while during the community shift, rates increased exponentially (×10). After 6 months of exposure, rates slowed down and reached a "plateau" with a mean of 0.93 kg of CaCO3 dissolved per m2 of reef after 12 months of exposure. Here, we show that (a) Ostreobium sp. settled down in new dead substrates as soon as the 3rd month of exposure but dominated communities only after 5 months of exposure and (b) microbioerosion dynamics comprise three distinct steps which fully depend on community development stage and grazing pressure