Light availability and temperature, not increased CO2, will structure future meadows of Posidonia oceanica

Abstract We evaluated the photosynthetic performance of Posidonia oceanica during short-term laboratory exposures to ambient and elevated temperatures (24–25 °C and 29–30 °C) warming and p CO 2 (380, 750 and 1000 ppm p CO 2 ) under normal and low light conditions (200 and 40 μmol photons m −2  s −1 respectively). Plant growth was measured at the low light regime and showed a negative response to warming. Light was a critical factor for photosynthetic performance, although we found no evidence of compensation of photosynthetic quantum efficiency in high light. Relative Electron Rate Transport (rETR max ) was higher in plants incubated in high light, but not affected by p CO 2 or temperature. The saturation irradiance (I k ) was negatively affected by temperature. We conclude that elevated CO 2 does not enhance photosynthetic activity and growth, in the short term for P. oceanica , while temperature has a direct negative effect on growth. Low light availability also negatively affected photosynthetic performance during the short experimental period examined here. Therefore increasing concentrations of CO 2 may not compensate for predicted future conditions of warmer water and higher turbidity for seagrass meadows.