Grazer-controlled recruitment of the introduced Sargassum muticum (Phaeophyceae, Fucales) in northern Europe

Abstract

Sargassum muticum (Yendo) Fensholt was introduced to European coasts during the 1970s. We studied survival and growth of seeded S. muticum germlings in field experiments on the southwestern coast of Norway, close to its present northernmost limit of distribution in Europe. A cage experiment was conducted twice (using different starting densities of germlings) at 2 localities during the summer and autumn of 2002, each experiment lasting 1 mo. Germling survival was significantly higher within grazer-excluding cages than on unprotected substrate. Germlings within cage controls (cages accessible to mesograzers) experienced intermediate mortality, which was nevertheless closer to that of unprotected than to cage-protected germlings. At a third locality, survival of unprotected germlings was recorded from September 2000 to February 2001, and only 0.6% of the germlings had survived by February. When relating proportion of mortality to initial density of germlings, we found no tendency of positive density-dependent mortality; on the contrary, on the substrate where grazers had access to the germlings, there was a tendency for high mortality when initial density was low and lower mortality when initial density was high. This suggests a 'swamping' effect of high germling densities on grazers. The growth of germlings during autumn and winter was very low compared with earlier reported measurements from Spain, and we suggest that relatively slow growth rates may make the germlings vulnerable to grazing impact. The results indicate that herbivorous grazing on early post-settlement stage germlings may be a limiting factor for the spread of S. muticum in southwestern Norway. © Inter-Research 2007.