Macroalgal species diversity and biomass of subtidal communities of Sao Miguel (Azores)

The benthic algal communities of two subtidal sites, located on opposite co asts of Sao Miguel Island (Sao Roque in the south and Sao Vicente on the no rth coast), were studied over a 2 year period (September 1993-September 199 5). At both sites the sublittoral region was surveyed from low-tide level d own to a depth of 15 m. Qualitative and quantitative changes are described. A depth-related gradient in species diversity and biomass was found at bot h localities. In general, red algae such as corallines and Pterocladiella c apillacea predominated in the shallow sublittoral (5 m) while brown algae s uch as Zonaria and Stypocaulon were more abundant at 15 in. Multivariate an alyses emphasized the existence at each study site of two communities (5 an d 15 m depth), separated by a large transition zone. The 15 m community on the south coast site showed the largest number of species (52), whereas the lowest diversity (30 species) also occurred at this site in the 5 m commun ity. A clear seasonal pattern of biomass change could be discerned only at Sao Vicente where the highest biomass was recorded in spring/summer. No maj or inter-annual variations could be detected, indicating relatively stable communities at least on a short-term basis.