Effects of ocean acidification on Posidonia oceanica epiphytic community and shoot productivity

1.Biological interactions can alter predictions that are based on single species physiological response. It is known that leaf segments of the seagrass Posidonia oceanica will increase photosynthesis with lowered pH but, it is not clear whether the outcome will be altered when the whole plant and its epiphyte community, with different respiratory and photosynthetic demands, are included. In addition, the effects on the Posidonia epiphyte community has rarely been tested under controlled conditions, at near future pH levels.

2.In order to better evaluate the effects of pH levels as projected for the upcoming decades on seagrass meadows, shoots of P. oceanica with their associated epiphytes were exposed in the laboratory to three pH levels (ambient: 8.1, 7.7, and 7.3, on the total scale) for four weeks. Net productivity, respiration, net calcification, and leaf fluorescence were measured on several occasions. At the end of the study, epiphyte community abundance and composition, calcareous mass and crustose coralline algae growth were determined. Finally, photosynthesis versus irradiance curves (PE) were produced from segments of secondary leaves cleaned of epiphytes and pigments extracted.

3.Posidonia leaf fluorescence and chlorophyll concentrations did not differ between pH treatments. Net productivity of entire shoots and epiphyte-free secondary leaves increased significantly at the lowest pH level yet limited or no stimulation in productivity was observed at the intermediate pH treatment. Under both pH treatments, significant decreases in epiphytic cover were observed, mostly due to the reduction of crustose coralline algae. The loss of the dominant epiphyte producer yet similar photosynthetic response for epiphyte-free secondary leaves and shoots, suggests a minimal contribution of epiphytes to shoot productivity under experimental conditions.

4.Synthesis Observed responses indicate that, under future ocean acidification conditions foreseen in the next century, an increase in Posidonia productivity is not likely despite the partial loss of epiphytic coralline algae which are competitors for light. A decline in epiphytic cover could, however, reduce the feeding capacity of the meadow for invertebrates. In situ long-term experiments that consider both acidification and warming scenarios are needed to improve ecosystem-level predictions.

Cox T. E., Schenone S., Delille J., Díaz-Castañeda V., Alliouane S., Gattuso J.-P. & Gazeau F. in press. Effects of ocean acidification on Posidonia oceanica epiphytic community and shoot productivity. Journal of Ecology. Article (subscription required).

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