Changes in the structure and dynamics of marine assemblages dominated by Bifurcaria bifurcata and Cystoseira species over three decades (1977–2007)

• Two low intertidal rocky shore assemblages dominated by brown seaweeds studied in 1977 were re-surveyed in 2007.
• Assemblages from 2007 were significant different and more variable in time regarding 1977.
• Canopy-forming species underwent changes in patterns of biomass and primary production affecting understory species.
• Species diversity increased and specially species richness that has almost doubled.

Two low intertidal assemblages dominated in 1977 by Bifurcaria bifurcata and Cystoseira baccata-Saccorhiza polyschides in the North coast of Spain show changes in the structure and dynamics after 30 years. A re-survey in 2007 detected phenological changes affecting the annual cycle of dominant canopy species. B. bifurcata has shortened its growth period and undergone a decrease in biomass, while C. baccata lengthened its period of growth and increased its biomass. Also important were the disappearance of Saccorhiza polyschides and the increase of Cystoseira tamariscifolia. These changes affect the rest of the species of the assemblages, with a shift in the main understory species and an increase in crustose coralline algae although the overall biomass of the subcanopy was similar. The species richness shows a sharp increase, at the expense of increasing epiphytes and simpler functional and morphological groups. These biological changes agree with the general trends of increasing sea surface temperature and the relaxation of the summer upwelling affecting the North coast of Spain, but the results were unexpected in the case of Bifurcaria bifurcata.

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