Effects of in situ CO2 Enrichment on Structural Characteristics, Photosynthesis, and Growth of the Mediterranean Seagrass Posidonia oceanica

Chair: Sean Connell

T. Erin Cox (1), Frédéric Gazeau (1), Samir Alliouane (1), Iris E. Hendriks (2), Paul Mahacek (1), Arnaud Le Fur (1), and Jean-Pierre Gattuso (1,3)*

1 Sorbonne Universités, UPMC Univ Paris 06, CNRS-INSU, Laboratoire d’Océanographie de Villefranche, 181 chemin du Lazaret, F-06230 Villefranche-sur-mer, France
2 Global Change Department, IMEDEA (CSIC-UIB), Instituto Mediterraneo de Estudios Avanzados, C/Miquel Marques 21, 07190 Esporles, Mallorca, Spain
3 Institute for Sustainable Development and International Relations, Sciences Po, 27 rue Saint Guillaume, F-75007 Paris, France

Background
Seagrass are expected to benefit from increased carbon availability under future ocean acidification. This hypothesis has been little tested by in situ manipulation.

Methods
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 precise manipulation of pH as an offset from ambient. This system was deployed in a Posidonia oceanica meadow at 11 m depth. It consisted of two benthic enclosures, an experimental and a control unit both 1.7 m3, and an additional reference plot in the ambient (2 m2) to account for structural artifacts. The meadow was monitored from April to November. The pH of the experimental enclosure was lowered by 0.26 pH units for the second half of the eight-month study. The diel change in pHT for the ambient meadow was ~ 0.1 units, while the diel change in the experimental enclosure during the pertubation was two to three times greater.

Findings
Changes in P. oceanica leaf biometrics, photosynthesis, and leaf growth accompanied seasonal changes recorded in the environment but no differences were found between the two enclosures. Leaf thickness may change in response to lower pH but this requires further testing. Results suggest any benefit from ocean acidification, over the next century, on Posidonia physiology and growth may be minimal.

Conclusions
The limited stimulation and increased diel pH variation casts doubts on speculations that elevated CO2 would confer resistance to thermal stress and increase buffering capacity of meadows.