Long-term changes of seagrass beds in the Glenan Archipelago (South Brittany)

Aerial photographs and in situ data of the Glenan archipelago permit the establishment of a cartography of its Zostera marina seagrass beds. Due to the exceptionally clear water, it was possible to distinguish submerged structures, such as rocks, sand dunes, maerl beds and seagrass meadows on the photographs. The distribution of Zostera meadows was incorporated into a geographical information database through scanning, and then compared with historical data. Ten aerial photographic surveys, made over a sixty-year period from 1932 to 1992 were available. The earliest of these surveys showed the seagrass beds to be in goad condition. Low cover in 1952 suggests that the Zostera meadows within the studied area were subject to severe destructions, presumably due to the ''wasting disease'', which caused a general breakdown of the North-Atlantic populations during the 1930s. During the 1970s, the distribution of Zostera beds increased; this was followed by a gradual decline during the 1980s and early 1990s. For the investigation of the environmental circumstances under which Zostera beds are fluctuating, the Glenan site is unique. This site being relatively remote from direct anthropogenic disturbances (light irradiance decline, sewage inputs), the causes of such fluctuations during this 60-year period can be more easily identified. Z. marina is a boreal species naturally affected by climate changes and in particular by global warming, which was at a maximum during the 1940s and 1950s. Various human activities, such as scallop dredging, maerl exploitation, yachting and anchoring, should also be considered. However, these anthropogenic disturbances were of limited importance in comparison with the dramatic decline and recovery of the seagrass beds as a result of climate fluctuations.