Articles | Volume 13, issue 7
Biogeosciences, 13, 2179–2194, 2016
Biogeosciences, 13, 2179–2194, 2016

Research article 13 Apr 2016

Research article | 13 Apr 2016

Effects of in situ CO2 enrichment on structural characteristics, photosynthesis, and growth of the Mediterranean seagrass Posidonia oceanica

T. Erin Cox1, Frédéric Gazeau1, Samir Alliouane1, Iris E. Hendriks2, Paul Mahacek1, Arnaud Le Fur1, and Jean-Pierre Gattuso1,3 T. Erin Cox et al.
  • 1Sorbonne Universités, UPMC Univ. Paris 06, CNRS-INSU, Laboratoire d'Océanographie de Villefranche, 181 chemin du Lazaret, 06230 Villefranche-sur-mer, France
  • 2Global Change Department, IMEDEA (CSIC-UIB), Instituto Mediterraneo de Estudios Avanzados, C/Miquel Marques 21, 07190 Esporles, Mallorca, Spain
  • 3Institute for Sustainable Development and International Relations, Sciences Po, 27 rue Saint Guillaume, 75007 Paris, France

Abstract. 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.

Short summary
The ocean absorbs atmospheric carbon dioxide (CO2) which increases the concentrations of CO2 and decreases pH in a process called ocean acidification. Because seagrass rely on carbon for photosynthesis they are expected to benefit under future ocean acidification. We manipulated pH in a Posidonia oceanica seagrass meadow. Seagrass traits, photosynthesis, and growth were not affected. Any benefit from ocean acidification over the next century on Posidonia physiology and growth may be minimal.
Final-revised paper