Intertidal ecology of northern Portuguese rocky shores

Abstract

Abundance and zonation of species were described for northern Portuguese rocky shores. All shores displayed exposed or moderately exposed zonation patterns, with mussels and barnacles dominating the eulittoral zone. Differences between the northern and southern coast were mainly observed at the lower shore level. Several southern limits of cold-water species of brown macroalgae were observed in this region. Northern shores formed a distinct group within the Portuguese coast and were similar to those found on the northern coast of Spain, French Brittany and southwest coast of the United Kingdom. The dynamics of midshore mussel assemblages was investigated at several spatial and temporal scales. A model was put forward, using wave action as the main factor driving the variation of mussel occupancy through time. Percentage cover of mussels, barnacles and free space were highly variable through time. Mussels did not display any seasonal patterns at the shore or site scales. Conversely, the percentage cover of barnacles decreased significantly during the winter, whilst free space increased after it. The proposed model as rejected, and a new model was developed based on the evidence gathered. The new model predicted that gains and losses in area would be similar within each season but higher during the winter. Moreover, losses in the winter would be compensated by growth of mussels on top of barnacles. The effects of an oil spill on mussel percentage cover and limpet density were assessed through the use of beyond BACI analyses. No effects were detected, either on mussels or limpets, most probably because the variability caused by the oil spill was within the natural range of variability of these populations.

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