Changes in the composition and structure of Mediterranean rocky-shore communities following a gradient of nutrient enrichment: descriptive study and test of proposed methods to assess water quality regarding macroalgae.

Changes in the species composition and structure of Mediterranean macroalgal-dominated communities from the upper sublittoral zone are described along a gradient of nutrient enrichment coming from a urban sewage outfall. Ulva-dominated communities only appear close to the sewage outfall. Corallina-dominated communities replace ulvacean algae at intermediate levels of nutrient enrichment. Cystoseira-dominated communities thrive in the reference site but already appear at nutrient levels that are threefold higher than those reported from unpolluted sites. Assemblage variability of Cystoseira-dominated communities decreases along the gradient of nutrient enrichment. Methods based on the functional-form groups of macroalgae to assess the water quality provide equivocal results at intermediate levels of nutrient enrichment because species belonging to the same group can display a completely different response to pollution. Alternatively, methods based on indicator species showed correlated evidence among species abundances and pollution levels and seem to have better performances in water quality assessment.