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Few studies have documented the recovery of phytobenthic marine assemblages following the reduction of long-standing sewage discharges. This paper reports on the main changes in intertidal macroalgal communities after the gradual application from 1984 to 2006 of a sewerage plan for the metropolitan area of Bilbao. Sampling sites along a pollution gradient were surveyed eight times during the course of the above period. Improvements in water quality were followed by noticeable changes in species composition and vegetation structure. Species richness significantly increased throughout the study area, while algal cover only increased at the most degraded sites. Pollution removal promoted the development of morphologically more complex species. Intertidal vegetation at the degraded sites became progressively more similar to that at the reference site. Five recovery stages discriminated by different species (SIMPER routine) were characterized from ordination (MDS) analyses: (i) extremely degraded -Gelidium pusillum is the most abundant species which is accompanied by Bachelotia antillarum at the low intertidal level (0.75 m); (ii) heavily degraded -Gelidium pusillum remains dominant and accompanied by Caulacanthus ustulatus at the high intertidal level (1.4 m); (iii) moderately degraded -Corallina elongata becomes dominant, C. ustulatus remains abundant at the high level; (iv) slightly degraded -C. elongata remains dominant in both tidal levels, Chondracanthus acicularis and Lithophyllum incrustans are abundant at the high level, whereas the latter, Pterosiphonia complanata and Stypocaulon scoparium become abundant at the low level; (v) reference stage -Lithophyllum incrustans and Laurencia obtusa are abundant together with C. elongata at the high level, whereas Stypocaulon scoparium dominates the low level, with Bifurcaria bifurcata, Jania rubens and Cystoseira tamariscifolia as abundant species. Thus, this study reveals that phytobenthic communities are useful indicators of water quality and provide real data that contribute to the assessment of the ecological status of rocky open shores on the Basque coast.

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Following the Prestige oil spill, six-monthly samplings (spring and autumn) of intertidal macroalgal assemblages were carried out from 2004 to 2006 in twelve locations along the Basque coast. Macroalgal species appeared to be little modified, but species richness, diversity, and algal cover were significantly lower in the first year of the study, so it cannot be ruled out that the arrival of oil on the Basque coast might have had a damaging effect on intertidal communities. On the other hand, no significant differences were detected between locations slightly and moderately affected by oil in any of the structural parameters considered. By contrast, significant differences were detected between locations within each oiling level that indicate that other natural environmental factors play a greater role in the differences between locations than the oiling level. Likewise, significant differences were detected between areas within each location which were not consistent with time, indicating that communities are highly heterogeneous in species richness, diversity and algal cover at the scale of tens of meters. The dominant Corallina elongata and most of the accompanying species did not show significant year-on-year differences in terms of cover. Also, there were no differences between the two oiling levels. Analyses revealed a high spatial variability at the scale of kilometers (locations) and tens of meters (areas) for most of the taxa. The results obtained provide detailed quantitative data on intertidal phytobenthic assemblages of the Basque coast at different spatial and temporal scales that were hitherto unavailable for the region.