Before and after wasting disease in common eelgrass Zostera marina along the French Atlantic coasts: a general overview and rst accurate mapping

We examined the original manuscripts of a French national survey conducted in 1933 onthe state of common eelgrass Zostera marina beds along the French Atlantic coasts during the periodwhen wasting disease struck the entire North Atlantic population in the 1930s. Based on GIS relatedtechniques and old sets of aerial photographs, we present the first accurate mapping of the Z. marina beds before wasting disease occurred and assess their spatial recolonization since the 1950s in theChausey Archipelago (France), which contains large Z. marina beds. The national survey confirmedthat the Z. marina beds almost totally disappeared from the French coasts during the 1930s. However,the disease symptoms seem to have begun locally a few years before. On the study site, we found thatthe Z. marina beds were more than twice as extended than as they are today, and covered both sub-tidal and intertidal areas. By the 1950s, 20 yr after the onset of the disease, the beds had hardly recol-onized, and contrary to the recolonization patterns reported elsewhere in Europe, they were mainlyrestricted to subtidal areas. The subtidal and intertidal Z. marina beds on the site are now rapidlyexpanding