LARGE-SCALE DISTRIBUTION PATTERNS OF THE MUSSEL MYTILUS-EDULIS IN THEWADDEN SEA OF SCHLESWIG-HOLSTEIN - DO STORMS STRUCTURE THE ECOSYSTEM

The distribution of mussel beds in the Wadden Sea of Schleswig-Holstei n was mapped by aerial surveys from 1989 to 1991. The number of mussel beds decreased from 94 in 1989 to 49 in 1990, as a result of severe s torms in early 1990. Thereafter only small changes were observed. The mussel beds that remained in 1990 were found only in the shelter of is lands; all beds in exposed areas had disappeared between the surveys o f 1989 and 1990, leaving large areas without mussel beds. Storms are t hus identified as a major factor limiting the distribution of mussel b eds to the sheltered parts of the Wadden Sea. Beds in the exposed part s of the Wadden Sea are highly dynamic, whereas beds in sheltered area s may persist over long times. A comparison with distribution patterns of older surveys (from 1937,1968 and 1978) revealed great similaritie s with the results of recent investigations, indicating a constant dis tribution pattern over a long period. The results are discussed in rel ation to eutrophication and the structure of the benthic communities o f the Wadden Sea. It is concluded that any eutrophication-induced incr ease of the mussel population would be restricted to the sheltered par ts of the Wadden Sea. Storms will largely determine whether the commun ities of a given area have to compete with mussels, which are the most important filter feeders of the ecosystem. As competition for food is a major factor structuring the benthic communities of the Wadden Sea, it is assumed that storms indirectly affect all other communities, gi ving deeper-burying, storm-tolerant species a competitive advantage in exposed areas where epibenthic mussels are excluded. The impact of mu ssel fisheries will be different for persisting and dynamic beds: fish ing on persisting beds in sheltered areas may remove the crucial reser ve which mussel-feeding birds such as eiders or oystercatchers need in times of low mussel populations.