Il est généralement admis, sur la base d’expériences de laboratoire, que les herbiers de phanérogames (plantes marines) vont bénéficier de l’augmentation du gaz carbonique. Cela serait l’un des rares effets positifs de l’acidification des océans. Nous avons, pour la première fois, simulé sur plusieurs mois la concentration de CO2 attendue pour 2100 autour d’un herbier naturel de posidonies en Méditerranée nord-occidentale. Contrairement aux études précédentes menées en laboratoire sur de courtes périodes et sur des plantes isolées, nous n’avons pas observé de stimulation significative de la croissance ni de la photosynthèse de ces plantes. Ces résultats mettent en doute l’idée qu’une stimulation de la photosynthèse par l’augmentation de CO2 permettrait à ces plantes d’augmenter leur résilience à d’autres perturbations, par exemple l’augmentation de température.
Références :
Cox, T. E. et al. (2016). Effects of in situ CO2 enrichment on structural characteristics, photosynthesis, and growth of the Mediterranean seagrass Posidonia oceanica, Biogeosciences 13 (2179-2194, 2016), doi:10.5194/bg-13-2179-2016
Résumé [en anglais] :
"Seagrass is expected to benefit from increased carbon availability under future ocean acidification. This hypothesis has been little tested by in situ manipulation. To test for ocean acidification effects on seagrass meadows under controlled CO2/pH conditions, we used a Free Ocean Carbon Dioxide Enrichment (FOCE) system which allows for the manipulation of pH as continuous offset from ambient. It was deployed in a Posidonia oceanica meadow at 11 m depth in the Northwestern Mediterranean Sea. It consisted of two benthic enclosures, an experimental and a control unit both 1.7 m3, and an additional reference plot in the ambient environment (2 m2) to account for structural artifacts. The meadow was monitored from April to November 2014. The pH of the experimental enclosure was lowered by 0.26 pH units for the second half of the 8-month study. The greatest magnitude of change in P. oceanica leaf biometrics, photosynthesis, and leaf growth accompanied seasonal changes recorded in the environment and values were similar between the two enclosures. Leaf thickness may change in response to lower pH but this requires further testing. Results are congruent with other short-term and natural studies that have investigated the response of P. oceanica over a wide range of pH. They suggest any benefit from ocean acidification, over the next century (at a pH of ∼ 7.7 on the total scale), on Posidonia physiology and growth may be minimal and difficult to detect without increased replication or longer experimental duration. The limited stimulation, which did not surpass any enclosure or seasonal effect, casts doubts on speculations that elevated CO2 would confer resistance to thermal stress and increase the buffering capacity of meadows."
