We experimentally investigated the effect of Mediterranean seawater summer warming projected for the 21st century under Intergovernmental Panel on Climate Change (IPCC) scenarios of greenhouse gas emissions on Posidonia oceanica sulphide stress. The results reveal that projected warming would enhance sulphide toxicity in P. oceanica and, hence, the risk of seagrass loss. We assessed sulphide stress of seagrasses by estimating the sulphide intrusion (F sulphide) into leaf tissue from the sulphur isotopic signature in sediments (ο 34S sulphide), in the water column (ο 34S sulphate) and in P. oceanica leaves (ο 34S tissue). Seagrasses were grown at 6 seawater temperature treatments, ranging from 26 °C to 32 °C, in 3 replicated experimental mesocosms for 50 days. At the end of the experiment, the seagrass shoots in all of the treatments exhibited sulphide intrusion (F sulphide > 0%), which had been negligible when the experiment started. Similarly, the leaf ο 34S tissue was lighter at the end of the experiment than at the beginning, when seagrasses in the field were experiencing a seawater temperature of 23 °C. F sulphide linearly increased with experimental warming, while leaf ο 34S tissue declined with increasing temperature at a rate of 0.3‰ for each centigrade degree of warming. The daily production of new leaves per shoot decreased as the sediment became more reduced. Leaf ο 34S tissue below 20.5‰, indicative of 1% sulphide intrusion, is expected to compromise P. oceanica performance. © 2012 Elsevier Ltd.

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