[en] Long-term experiments at ecosystem scale are needed in order to predict effects of ocean acidification (OA) on marine organisms, including interactions and acclimation processes.
In this context, the effects of OA on a simplified reef ecosystem was studied using artificial mesocosms. The community consisted of hermatypic corals (two Acroporidae and two Pocilloporidae), grazer and bioeroder sea urchins (Echinometra mathaei) and field reef calcareous substrate with algae and micro organisms. pH in acidified mesocosms was gradually decreased down to a mean pH of 7.65 and then maintained during 7 months. Control mesocosms were maintained at mean pH 8.10. Daily field variations of pH were reproduced. Physiological parameters of corals and sea urchins and the bioerosion rate of coral rubble were measured during the experiment.
Results highlighted that corals were able to acclimate to lower pH: calcification, net photosynthesis and respiration remained similar between treatments. A significantly higher calcification rate was even measured for Pocilloporidae in acidified mesocosms at the end. Sea urchins were able to regulate the acid-base balance of the coelomic fluid. Growth, metabolism and mechanical properties of the skeleton were not affected showing that adult E. mathaei would resist to a moderate acidosis at long term. Bioerosion rate increased at higher pCO2. This could impact future coral reefs, particularly where E. mathaei is a major bioeroder. It appears that the subtle and dynamic balance between bioaccretion and bioerosion may be affected in a complex way by OA in coral reef.
