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Short sediment cores taken from (i) a recently regenerated salt marsh (Plentzia estuary), (ii) an incipient marsh and (iii) a pristine marsh (Urdaibai estuary), have been interpreted for evidence of environmental impacts and sea-level change on the basis of microfaunal and geochemical determinations and historical land management data. Under the current relative sea-level rise scenario, it might be expected that salt marsh ecosystems lose their ability to keep up with tidal flooding and drown following a transgressional pattern where marsh vegetation replaces woody plant species. With more frequent inundation from rising sea level, the change would progress from high marsh through a transition to the low marsh state. Ultimately this would lead to the development of a mud flat when sea level rises beyond low marsh accretion rates, with important coastal management implications. Geological observations from the southeastern Bay of Biscay, however, suggest that this model does not always apply. Evidence indicates that reclaimed areas rapidly gain elevation due to very high sedimentation rates and are colonized by salt marsh vegetation within a few decades, pristine salt marshes in this case have kept up with current sea-level rise, but more importantly, due to high sedimentation rates, even former intertidal mud flats have gained elevation over the last 50 years allowing salt marsh vegetation to colonize new areas. This recent evolution, however, has to be considered in a context of anthropogenic impacts in river catchments wherein sediment supply has been enhanced by human activity.
