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

Seagrass are 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 precise manipulation of pH as an offset from the ambient. This system 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 (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 eight-month study. Changes 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 suggest any benefit from ocean acidification, over the next century, on Posidonia physiology and growth may be minimal. The limited stimulation casts doubts on speculations that elevated CO2 would confer resistance to thermal stress and increase buffering capacity of meadows.

Cox T. E., Gazeau F., Alliouane S., Hendriks I. E., Mahacek P., Le Fur A. & Gattuso J.-P., 2016. Effects of in situ CO2 enrichment on structural characteristics, photosynthesis, and growth of the Mediterranean seagrass Posidonia oceanica. Biogeosciences Discussions. Article.


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