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Sublittoral communities of macroalgae from the Abra d e Bilbao and its surrounding area, which are affected by pollution, were studied in 1984 and 1992. Statistical cluster analysis applied to a network of 230 quadrats distributed along 7 transects resulted in 4 structural levels of community, depending on the pollution gradient. These levels are defined by algal cover, vertical layering and the predominant morphology of the species forming the communlty. Sedimentation, turbidity and water toxicity are the principal destabilizing factors among the phytobenthic communities Species with simple morphology, mainly ceramiaceous algae (Ceramium diaphanum, Calllthamnion neglecturn, Antithamnionella spirographidis. Pterotharnnion plumula, Anotrichium furcellaturn, Compsothamnion thuyoides), sciaphilous (Mesophyllum lichenoides, Zanardjnia prototypus, Rhodymenia coespitosella, Drachiella minuta, Erythroglossum laciniatum, Hypoglossurn hypoglossoides) and those resistant to sedimentation (Pterosiphonia pennata and Champia parvula) are the most widely found species in severely polluted settings. Species such as Gelidium sesquipedale, Heterosiphonia plumosa, Calliblephans ciliata. Halopitys incurvus, Djctyopterjs polypodioides and Halurus equisetifolius are species typically found at the reference site and are good indicators of unchanged environments. Pterosiphonia complanata, abundant at the reference site, took the place of G. sesquipedale in those sites farthest from the source of pollut~on Ord~nation and classification analyses detected changes in flora after 8 yr, pointing toward a recuperating process. The improvement consisted of an increase in algal cover along with a decrease in ceramiaceous algae and a greater complexity in communlty stratification. This recovery was directly related to a decrease in industrial activity and closure of the mineral washeries, as well as the establishment of urban wastewater-treatment plants for various municipalities. KEY L\'ORDS: Pollution. Sublittoral. Benthic macroalgae-Community parameters. Bioindicators Recuperating process

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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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