Effect of explosive shallow hydrothermal vents on δ13C and growth performance in the seagrass Posidonia oceanica

1. Explosive volcanic submarine activity is expected to affect seagrass communities due to sudden and dramatic changes in the physical and chemical features of sea water and sediments, with possibly large ecosystem effects. However, seagrass response to the harsh environmental conditions that arise due to explosive volcanism is as yet unexplored as it is not easy to predict when and where an eruption will occur. Here, we investigate the uptake of hydrothermal carbon within the seagrass Posidonia oceanica by the analysis of d13C and growth rates in tissue laid down before and after an exceptional and massive hydrothermal gas release in the Aeolian Islands (Italy, Mediterranean Sea). 2. Hydrothermal submarine activity was recorded by P. oceanica, which showed a large and persistent 13C-depletion in both scales and rhizomes in the site close to the eruption. Both increased CO2 availability and reduced carbon demand, as a consequence of stressful environmental conditions (e.g. light limitation due to turbidity, high temperature), combined to give much lower d13C signatures. Our results suggest that the explosive volcanism caused physiological stress in the seagrass, leading to a reduction in productivity, whereas slower, more diffuse release of hydrothermal CO2 is known to enhance seagrass productivity. 3. Synthesis. We analysed the effect of a sudden and large hydrothermal event on d13C interannual variations and growth of the seagrass P. oceanica. Our results confirm that shallow submarine hydrothermal vents can be used as natural laboratories for exploring biological responses to acute and often extreme environmental conditions. P. oceanica can record geological events by capturing CO2 derived from vent systems, with implications for habitat management to mitigate against raising CO2 levels and ocean acidification. This leads the way to further studies to evaluate the effects of hydrothermal carbon on shallow ecosystems.