The dominant benthic algal species from two sites, located on opposite coasts of the island of São Miguel, Azores (São Roque in the south and São Vicente on the north coast) were studied over a two-year period (September 1993–September 1995). From both sites monthly collections were made in the intertidal zone and in the adjacent subtidal zone down to a depth of 15 m. The algae occur under different ecological conditions, a few species being restricted to the intertidal zone (Fucus spiralis, Gelidium microdon, Jania crassa, Caulacanthus ustulatus and Chondria coerulescens), while others (Codium elisabethae, Zonaria tournefortii, Jania verrucosa and Sphaerococcus coronopifolius) were exclusively present subtidally. Other species were present at both levels. Differences were observed between the two sites. A few species (Chaetomorpha linum, Corallina elongata and Caulacanthus ustulatus) were more abundant at São Roque, their presence on the north being only sporadic or occasional. On the other hand, Fucus spiralis, Gelidium microdon, Sphaerococcus coronopifolius and Centroceras clavulatum were statistically more abundant at São Vicente. Seasonal changes were seen in the growth and/or reproduction of certain species, with many (Ulva rigida, Bryopsis hypnoides, Stypocaulon scoparium, Dictyota dichotoma, Padina pavonica, Colpomenia sinuosa, Gelidium microdon, Asparagopsis armata, Chondracanthus acicularis and Plocamium cartilagineum) having larger plants and/or higher values of biomass in spring and/or summer. With the exception of Dictyota dichotoma that was statistically more abundant in the second sampling year, no differences were observed on the species abundance between the two years. Reproductive phenology varied according to species, with many (Fucus spiralis, Ulva rigida, Gelidium microdon, Jania rubens, Sphaerococcus coronopifolius, Caulacanthus ustulatus, and Plocamium cartilagineum) having prolonged fertile periods. Some species were more seasonal in their reproduction, being fertile only over two seasons: Codium elisabethae and the tetrasporophyte of Asparagopsis armata in autumn and winter; Cystoseira abies-marina and Pterocladiella capillacea in summer and autumn; the gametophyte of Asparagopsis armata in winter and early spring. A few had a more restricted fertile period: Chaetomorpha linum and Jania verrucosa in summer; Chondracanthus acicularis in autumn.
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