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Ecological impact of sea level rise on coastal ecosystems of Mont-Saint-Michel Bay (France)
Lefeuvre, J.-C. (1990). Ecological impact of sea level rise on coastal ecosystems of Mont-Saint-Michel Bay (France), in: Beukema, J.J. et al. (Ed.) Expected effects of climatic change on marine coastal ecosystems. Developments in Hydrobiology, 57: pp. 139-153
In: Beukema, J.J. et al. (Ed.) (1990). Expected effects of climatic change on marine coastal ecosystems. Developments in Hydrobiology, 57. Kluwer Academic: Dordrecht. ISBN 0-7923-0697-X. 221 pp., more
In: Dumont, H.J. (Ed.) Developments in Hydrobiology. Kluwer Academic/Springer: The Hague; London; Boston; Dordrecht. ISSN 0167-8418, more

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    Marine

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  • Lefeuvre, J.-C.

Abstract
    Over the next few centuries a warming up of the lower atmosphere of several degrees is expected as a result of the greenhouse effect. A rise in temperature would cause a rise in sea level, so the transition area between land and ocean is particularlyat risk. This paper puts forth a number of hypotheses concerning the future of a well-studied littoral system: Mont-Saint-Michel Bay. These hypotheses are based on the past history and knowledge of current organization, structure and functioning of this system, considered by sedimentologists as one of the best models of sedimentation in the world. The different steps of the bay's sedimentation, linked to sea-level changes, are well-known. Since 7500 BP, the sea rose in a serie of successive oscillations sometimes leading to sedimentation of the intertidal area and sometimes of the supratidal area. In the historical period, the patterns of sedimentation have been greatly influenced by human activities, especially reclamation, damming and channelling. They mainly concern the immediate surroundings of the Mont-Saint-Michel. The sedimentary balance is presently largely positive in this part of the bay. The annual average input is estimated at 2 cm per year and the average extension of the salt-marsh area is 30 ha per year. As the estimation of the average increase in sea level over the next century has a large margin of uncertainty, two scenarios of the future of this bay can be foreseen. One follows current trends (20 cm per century), the other considers the possibility of a maximal rise in sea level (1 to 2 m, or even 3.5 m per century). In the hypothesis of weak and constant rise in sea level, only the general distribution of organisms could be subject to important changes, partly due to a large extension of salt marshes and modifications in the courses of the rivers in the estuarine part of the Bay. In the hypothesis of a significant rise in sea level, salt marshes and polders risk undergoing important fragmentation. Such a transformation would forbid sheep breeding and could cause some of the ducks and geese to abandon this wintering site. Nevertheless, the marine environment could benefit from this structural change which would enhance production of salt-marsh tidal-flat interfaces. Because of the difficulties in prediction, it is urgent to establish on a national, European and scale a coherent network of permanent observatories such as Mont-Saint-Michel Bay to develop long-term research on the evolution of environments, variations in their ecological functioning and consequent economic and social changes as an aid in decision-making.

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