Changes in macroalgal communities in the vicinity of a Mediterranean sewage outfall after the setting up of a treatment plant

Benthic macroalgal communities of the upper rocky sublittoral were studied in 1995-1996 in the vicinity of the Marseille (Mediterranean, France) sewag e outfall, 8 years after the setting up of a wastewater treatment plant and compared to a previous study carried out in 1972-1974. The number of taxa has increased, a clear stational and seasonal gradient of differentiation o f the vegetation appeared, and a turf of ephemeral species is taking place of Corallina elongata at sites close to the outfall. These changes may be d ue to a decrease in pollutant load, the discharge of ferric chlorates used in the treatment process. However, the overall change is much less conspicu ous than that described for deeper soft bottom communities, in particular t he Cystoseira amantacea community is not still restored. Biological traits of this species (short distance dispersal) and the nature of most pollutant s removed from the effluent (solids and organic matter) may explain this ph enomenon. (C) 2001 Elsevier Science Ltd. All rights reserved.