The withdrawal of the lower limit of the Posidonia oceanica seagrass meadow in the Bay of Hyères (NW Mediterranean) : a combination of natural and human-induced recent and ancient phenomena ?

The Posidonia oceanicaseagrass meadows are very sensitive to disturbance of human or natural origin. Here we consider the losses of the seagrass localizednear the lower limit of the meadowinthe Bay of Hyeres(France). Samples of dead mattewere taken nearand beyond the current lower limit of the meadow, between 26 and 37 m depth. Radiocarbon 14C analyses were used for dating the withdrawal. The death of P. oceanicapossibly occurred between 160±100 (-29 m) and 1050±110 years BP (-37 m). The deeper and the further from the shore the sampled dead mattewas, the more ancient it was. This withdrawal appears to be a general feature, which concerns both highly human-impacted areas (Bay of Hyeres)and pristine areas, such as Port-Cros Island (National Park). Several non-mutually exclusive hypotheses might explain this recent vs. ancient and actual vsapparent withdrawal: (i)trawling;(ii)post mortem erosion of the roof ofthe dead matte, by hydrodynamismor trawling; (iii)compaction of dead matteover time; (iv)current andancient rise in sea level; (v)current and past climate changeand/or (vi)human induced changes (e.g. turbidity).The lower limit withdrawal is more markedin highly human-impacted areas than in pristine areas, whereitmayreflect global forcings such as sea level rise, warmingandclimate episodes)