The invasion of the intertidal canopy-forming alga Fucus serratus L. to southwestern Iceland: Possible community effects

The intertidal serrated wrack, Fucus serratus L. (Fucales: Phaeophyceae), has become an abundant canopy-forming alga along a ca. 100 km stretch of shore in southwestern Iceland in the last century. Its distribution has not changed noticeably since a survey in 1975/1976. Many experimental studies have shown that canopy-forming algae can have profound effects on the community structure. Although such experiments involving F. serratus are few it was decided to test predictions from these experiments on a geographical scale by comparing community compositions within the area where F. serratus is a dominant algae (since at least 1975/1976) with an adjacent area of similar size where F. serratus is absent (with a single exception without consequences). The work is based on measurements on some 372 stations in the F. serratus area, termed region A, and 227 stations from the F. serratus-free area, termed region B. Percentage cover of algae and sessile invertebrates was estimated on 2 m² on each station and animals collected from 800 cm² from each station. The vertical distribution of F. serratus was most similar to that of Fucus distichus and the two species often grew intermingled on the lower part of the shore. F. serratus appeared to have reduced the cover of F. distichus in the lowermost part of the shore, while having little or no effect on other canopy-forming species. The low abundance of Semibalanus balanoides in the F. serratus area (region A) is in line with experiments showing detrimental effect on this barnacle by whiplash of F. serratus. In general algal grazers were more abundant in region A, consistent with the greater attractiveness of F. serratus than other canopy-forming species to grazers, although this may be partly explained by the abundance of understorey algae in region A.