Récif corallien | EPFL Graph Search

thumb|upright=1.5|Le corail, dont les colonies forment les récifs. thumb|upright=1.5|Colonie d’Acropora pulchra. Un récif corallien ou barrière de corail est une structure naturelle bioconstruite à l'origine de laquelle sont essentiellement les coraux. La plus grande de ces formations, la Grande Barrière de corail, au large des côtes australiennes, s'étend sur quelque et est visible depuis l'espace. La Nouvelle-Calédonie quant à elle, abrite dans ses lagons le deuxième ensemble corallien de la planète et la plus longue barrière récifale continue avec ses . La barrière de corail du Belize est la plus longue de l'hémisphère nord ; ce récif est moins important que celui de la Grande Barrière de corail, mais son écosystème est tout aussi foisonnant : 500 espèces de poissons, 65 sortes de coraux... Un récif corallien résulte de la construction d'un substrat minéral durable (formé de carbonate de calcium) sécrété par des êtres vivants, principalement des coraux. Il existe de très nombreus

Photic stress on coral reefs in the Maldives: The Amphistegina bleaching index

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

The Amphistegina Bleaching Index (ABI) was applied to three Maldivian reefs in the Rasdhoo and North Ari Atolls in 2018, during normal sea surface temperature conditions. This dataset was then compared with a 2015, pre-coral bleaching study. The results provide a context for the verification and application of the ABI in outlining the photoinhibitory stress status of coral reefs outside of the Florida Reef Tract where it was originally developed. The sampling periods encompass different seasons and temperature regimes. The 2015 field sampling preceeded the El Nino induced, mass coral-bleaching events of 2015 and 2016. It was carried out in late April and early May, during the dry season, when temperatures exceeded 31.5 degrees C and photosynthetically active radiation (PAR) was high. The 2018 sampling took place near the September equinox, towards the end of the monsoon, when PAR was again high, though water temperatures were similar to 30 degrees C. Although there were slightly higher percentages of bleached Amphistegina in 2018, there were also higher percentages of juveniles, indicating either that (1) the chronic stress was insufficient to impact asexual reproduction or (2) the onset of stress was within the past few weeks; the latter hypothesis was supported by an increase in PAR and temperature coinciding with the time of sampling. From the ABI plots it is possible to distinguish between the 2015 (high data scatter), highly stressed pre-bleaching conditions with elevated photo-oxidative stress levels, and the near-baseline conditions represented by the 2018 dataset (tight data clustering). Overall, this study thus shows the potential of Amphistegina populations and the ABI in forecasting bleaching events, and contributing to the question of the resilience potential of the coral reefs as a whole. It also highlights the usefulness and suitability of the ABI, within Maldivian coral reefs, as an indicator of photo-inhibition through photo-oxidative stress that can increase susceptibility to coral bleaching as water temperatures approach or exceed the bleaching threshold.

ELSEVIER2020

High rates of carbon and dinitrogen fixation suggest a critical role of benthic pioneer communities in the energy and nutrient dynamics of coral reefs

Nils Rädecker

Following coral mortality in tropical reefs, pioneer communities dominated by filamentous and crustose algae efficiently colonize substrates previously occupied by coral tissue. This phenomenon is particularly common after mass coral mortality following prolonged bleaching events associated with marine heatwaves. 2. Pioneer communities play an important role for the biological succession and reorganization of reefs after disturbance. However, their significance for critical ecosystem functions previously mediated by corals, such as the efficient cycling of carbon (C) and nitrogen (N) within the reef, remains uncertain. 3. We used 96 carbonate tiles to simulate the occurrence of bare substrates after disturbance in a coral reef of the central Red Sea. We measured rates of C and dinitrogen (N2) fixation of pioneer communities on these tiles monthly over an entire year. Coupled with elemental and stable isotope analyses, these measurements provide insights into macronutrient acquisition, export and the influence of seasonality. 4. Pioneer communities exhibited high rates of C and N2 fixation within 4–8 weeks after the introduction of experimental bare substrates. Ranging from 13 to 25 μmol C cm−2 day−1 and 8 to 54 nmol N cm−2 day−1, respectively, C and N2 fixation rates were comparable to reported values for established Red Sea coral reefs. This similarity indicates that pioneer communities may quickly compensate for the loss of benthic productivity by corals. Notably, between 40% and 85% of fixed organic C was exported into the environment, constituting a vital source of energy for the coral reef food web. 5. Our findings suggest that benthic pioneer communities may play a crucial, yet overlooked role in the C and N dynamics of oligotrophic coral reefs by contributing to the input of new C and N after coral mortality. While not substituting other critical ecosystem functions provided by corals (e.g. structural habitat complexity and coastal protection), pioneer communities likely contribute to maintaining coral reef nutrient cycling through the accumulation of biomass and import of macronutrients following coral loss.

2020

Standardized short‐term acute heat stress assays resolve historical differences in coral thermotolerance across microhabitat reef sites

Nils Rädecker

coral thermotolerance differences reflective of in situ reef temperature thresholds. Using a suite of physiological parameters (photosynthetic efficiency, coral whitening, chlorophyll a , host protein, algal symbiont counts, and algal type association), we as- sessed bleaching susceptibility of Stylophora pistillata colonies from the windward/ exposed and leeward/protected sites of a nearshore coral reef in the central Red Sea, which had previously shown differential mortality during a natural bleaching event. Photosynthetic efficiency was most indicative of the expected higher thermal tolerance in corals from the protected reef site, denoted by an increased retention of dark-adapted maximum quantum yields at higher temperatures. These differ- ences were resolved using both experimental setups, as corroborated by a positive linear relationship, not observed for the other parameters. Notably, short-term acute heat stress assays resolved per-colony (genotype) differences that may have been masked by acclimation effects in the long-term experiment. Using our newly devel- oped portable experimental system termed the Coral Bleaching Automated Stress System (CBASS), we thus highlight the potential of mobile, standardized short-term acute heat stress assays to resolve fine-scale differences in coral thermotolerance. Accordingly, such a system may be suitable for large-scale determination and com- plement existing approaches to identify resilient genotypes/reefs for downstream ex- perimental examination and prioritization of reef sites for conservation/restoration. Development of such a framework is consistent with the recommendations of the National Academy of Sciences and the Reef Restoration and Adaptation Program committees for new intervention and restoration strategies.

2020

Photic stress on coral reefs in the Maldives: The Amphistegina bleaching index

Arthur Adams, Aurélie Angéloz, Anaël Lehmann

ELSEVIER2020

High rates of carbon and dinitrogen fixation suggest a critical role of benthic pioneer communities in the energy and nutrient dynamics of coral reefs

Nils Rädecker

Following coral mortality in tropical reefs, pioneer communities dominated by filamentous and crustose algae efficiently colonize substrates previously occupied by coral tissue. This phenomenon is particularly common after mass coral mortality following prolonged bleaching events associated with marine heatwaves. 2. Pioneer communities play an important role for the biological succession and reorganization of reefs after disturbance. However, their significance for critical ecosystem functions previously mediated by corals, such as the efficient cycling of carbon (C) and nitrogen (N) within the reef, remains uncertain. 3. We used 96 carbonate tiles to simulate the occurrence of bare substrates after disturbance in a coral reef of the central Red Sea. We measured rates of C and dinitrogen (N2) fixation of pioneer communities on these tiles monthly over an entire year. Coupled with elemental and stable isotope analyses, these measurements provide insights into macronutrient acquisition, export and the influence of seasonality. 4. Pioneer communities exhibited high rates of C and N2 fixation within 4–8 weeks after the introduction of experimental bare substrates. Ranging from 13 to 25 μmol C cm−2 day−1 and 8 to 54 nmol N cm−2 day−1, respectively, C and N2 fixation rates were comparable to reported values for established Red Sea coral reefs. This similarity indicates that pioneer communities may quickly compensate for the loss of benthic productivity by corals. Notably, between 40% and 85% of fixed organic C was exported into the environment, constituting a vital source of energy for the coral reef food web. 5. Our findings suggest that benthic pioneer communities may play a crucial, yet overlooked role in the C and N dynamics of oligotrophic coral reefs by contributing to the input of new C and N after coral mortality. While not substituting other critical ecosystem functions provided by corals (e.g. structural habitat complexity and coastal protection), pioneer communities likely contribute to maintaining coral reef nutrient cycling through the accumulation of biomass and import of macronutrients following coral loss.

2020