Phytobenthic Intertidal Community Structure Along an Environmental Pollution Gradient

The intertidal vegetation of the “Abra de Bilbao” (Basque coast, N Spain) was studied following a pollution gradient. Under the effect of pollution, several signs of alteration are detected in the vegetation, which responds by simplifying its structure. Species which require high quality environmental characteristics such as Cystoseira tamariscifolia, Cystoseira baccata, Bifurcaria bifurcata, Gelidium sesquipedale, Gigartina pistillata and Laurencia obtusa disappear, whereas Corallina elongata, Chondria coerulescens, Caulacanthus ustulatus and Gelidium pusillum become dominant in successive steps as pollution increases. In general, large perennial algae are replaced by small turfing algae. In addition, an intensification of the herbivorous activity (mainly limpets) occurs on the soft caespitose vegetation. Species richness and algal abundance suffer a reduction. In the most degraded environmental conditions, vegetation is replaced by filter-feeders (mussels and barnacles), and cyanobacteria proliferate. Pollution also alters the zonation pattern reducing in the number of vegetation belts.