IS SARGASSUM-MUTICUM A THREAT TO EELGRASS BEDS

Eelgrass, Zostera marina L., occurs mainly on sandy and muddy substrat a, and the recently introduced brown alga, Sargassum muticum (Yendo) F ensholt is a species of solid substrata; usually they are spatially se parated. However, when the bottom consists of a mixed substratum of sa nd, gravel and stones both species may occur together. S. muticum hard ly intrudes prosperous eelgrass beds, but in places where the beds are in decline as a consequence of the normal bed dynamics, S. muticum oc cupies almost immediately the empty spaces. There are no indications o f any direct competition between S. muticum and Z. marina; however, as S. muticum settles in all spots from where Z. marina has retreated, i t interferes with the regeneration of the bed. In S. muticum stands no germlings of Z. marina have been found, so in time one may expect tha t S. muticum will replace the eelgrass bed. In the area of Roscoff (no rthern Brittany) this process is in full progress. On the Ile of Re (G ulf of Biscay) the present distribution of S. muticum almost coincides with the distribution of the eelgrass beds in the 1920s, before they were struck by wasting disease. (C) 1997 Elsevier Science B.V.