Geographical morphological variations in Fucus vesiculosus populations were studied along the coast of Iceland. Principal component analysis (PCA) based on 11 morphological characters clustered the 26 sampling sites into four different morphological types as follows: (1) a morphological type found along the S-SW-W coast; (2) an intermediate form comprising only two populations in the NW; (3) a type found along the N-NE-E-SE coast; and (4) a type found independent of geographical area, in estuaries and at sites influenced by large freshwater outflows. Populations influenced by low salinity had significantly (P less than or equal to 0.001) shorter thalli, a shorter distance from the holdfast to the oldest dichotomy, smaller fronds, narrower stipes and midrib width compared to the morphology of all other more saline populations. No significant difference in frond width was found between the S-SW-W and the N-NE-E-SE populations. However, significant (P less than or equal to 0.001) morphological differences between them were observed, the former having shorter thalli, a greater distance from the holdfast to the oldest dichotomy, narrower stipes and smaller midrib width compared to the latter. These differences between populations along the S-SW-W and the N-NE-E-SE coasts are suggested to be due to combined effects of rare, long distance dispersal of reproductive fronds by the warm-temperate Irminger Current and the cold, Arctic East Icelandic Current respectively and the short dispersal range of Fucus gametes. Thus, over the years, the East Icelandic Current may have transported reproductive fronds from the northernmost F. vesiculosus populations on the Greenland coast and the Barents Sea to the eastern coast of Iceland while the Irminger Current has carried thalli from the more southerly distribution of Fucus on the North American east coast to the southwestern part of Iceland. Excluding the populations affected by reduced salinity, the comparatively similar morphology within these two areas, may be explained by the restricted dispersal of the short lived F, vesiculosus gametes.
