Invasion of Sargassum muticum in Limfjorden (Denmark) and its possible impact on the indigenous macroalgal community

Attached Sargassum muticum (Yendo) Fensholt was first observed in the weste rnmost part of Limfjorden (Denmark) in 1984, and it has since colonized Lim fjorden from west to east at a rate of 15 to 17 km yr(-1). By 1997, S. muti cum had become the most dominant macroalga, covering more than 6 % of the b ottom between 0 and 6 m depth, corresponding to ca 35 % cover of the hard s ubstrate. In 1990, the cover of S, muticum was strongly correlated with dis tance from the original source area in the westernmost part of the estuary, suggesting that colonization was still in progress. By 1997, the cover of S, muticum was less correlated with distance from the original source area and more strongly correlated with the amount of hard substrate, indicating that colonization was reaching its climax. The absence of S, muticum from c ertain parts of Limfjorden by 1997 is therefore best explained by lack of h ard substrate in these areas, and not by insufficient colonization time. Th e increased abundance of S, muticum between 1990 and 1997 affected species richness and diversity of the macroalgal community only marginally. However , multivariate community analysis revealed significant changes in the macro algal community structure that were closely related to the increased abunda nce of S. muticum. Not only did the dominance of S. muticum increase signif icantly from 1990 to 1997, but the cover of several indigenous species belo nging to the genera Laminaria, Fucus, and Codium decreased during the same period, indicating competitive displacement through competition for hard su bstrate and light.