Recovery stages over long-term monitoring of the intertidal vegetation in the ‘Abra de Bilbao’ area and on the adjacent coast (N. Spain)

Abstract

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.