Changes of macroalgal biodiversity in sublittoral sites in southwest Norway: impact of an introduced species or higher temperature?

The Pacific red alga Heterosiphonia japonica has dispersed rapidly along European shores. Due to the species’ high abundance in many habitats, an impact on species richness and composition of local macroalgal communities might be expected. Higher sea temperatures may also influence local macroalgal composition, by providing more favourable conditions for species requiring higher temperatures. Macroalgal composition at 22 sublittoral sites along the south-western coast of Norway investigated prior to the establishment of H. japonica (1994–1995) were reinvestigated in 2003–2004, using similar methods. The total number of species collected in the area was approximately the same in the present investigation as in the previous study. With regard to number and composition of species at each site, there were no significant differences between sites with high abundance of H. japonica and sites with low or no abundance. Similarity percentage analysis (SIMPER) showed that there were temporal changes in com...