Algal Provinces in the North Atlantic — Do They Exist?

Abstract

Biogeographical studies of the benthic algal flora of the North Atlantic Ocean were initially based on subjective assessments of patterning in species distribution. Subsequently, computer based (clustering and ordination) approaches have been used to attempt more objective appraisals. The present paper commences from a null hypothesis (that algal biogeographic provinces are not proven to exist), and analyses a larger number of data sets than hitherto used (1116 species from 33 floras). Two methods of analysis were used: an ecocladistic (Wagner tree) method (PAUP program), a phenetic ordination method (DECORANA program). Previous biogeographic analyses utilising cluster-methods identified apparently distinct algal provinces in the North Atlantic Ocean. The present ecocladistic analyses produced ecocladograms with no clear basal division of branches likely to indicate major biogeographical provinces, the main exception being the flora of the mid-Atlantic Azores island group, which was well separated from the continental European flora and showed affinities with the American flora. Branching at higher levels in the ecocladograms suggested a degree of floristic dissimilarity between east and west Atlantic, which can be related to the species poor nature of the latter. Ordination confirmed these results, clearly indicating a continuum of floristic relationships in the North Atlantic. A second series of analyses performed on a comparatively well known regional flora (comprising 33 local floras in the British Isles and adjacent areas, Faroes to the Channel Islands and northern France), traversing propounded biogeographical boundaries, revealed no clear biogeographical separation of the Faroes or Shetland Islands, but a continuum of floristic relationships. It is suggested therefore that the North Atlantic Ocean is a single biogeographical province, with perhaps a southern boundary excluding the Azorean flora.