Responses of coastal sedimentary ecosystems of Southern Brittany to extreme meteorological events at different time and space scales. In : Symposium Coastal ecosystems vulnerability to global change and extreme events ", Biarritz, 18-21 octobre 2011"

Three shallow lagoons and littoral pond systems from Southern Brittany: the Lannénec pond and Grand Loc'h lagoon in Morbihan, and the Trunvel pond in Finistère, have been chosen for a multi-temporal scale (millennial, secular, and factual) study of sedimentary processes involved in shoreline dynamics. This study has so far been conducted on the Lannénec and Grand Loc'h sites, situated in an area that is particularly well exposed to western and southern oriented storms. Sedimentological and micropaleontological studies have been conducted on two cores, one from the Lannénec pond (5.93 m long, dated 5600 years BP), and the other from the Grand Loc'h (3.74 m long, dated 5500 years). This study demonstrates that in 5500 yrs, these two proximal ecosystems have registered two distinct evolutionary cycles. The Grand Loc'h lagoon shows 4 cycles of widening and constriction that at times results in full closure. For the same period, the Lannénec pond demonstrates six cycles, some of which evolve into fresh water continental lake. The differences between these two systems can be explained by a differential orientation of the respective shorelines with respect to the swell. In the case of Lannénec, major sediment transfers characterize the local dynamics during violent storms, while the Grand Loc'h shows a stable shoreline that is protected from these extreme events. Data obtained at the secular and factual scales, from in-situ measurements during storms and observation of aerial photographs and old maps, confirm the timelessness of these processes on this part of the Morbihan littoral.