Effects of sewage on bryozoan diversity in Mediterranean rocky bottoms

Impacts of sewage from Marseilles (France, Mediterranean Sea) on sublittoral bryozoans were assessed in the rocky area including the sewer outlet, which is predominantly submitted to westward flow. The assemblage features were assessed on both sides of the sewer outlet at short (0.4 km), intermediate (0.7-1.3 km) and long (2.4-5 km) distances from it. Three assemblage categories were considered: the cryptic fauna of block undersides, the large, erect bryozoans, and the 5 and 12 month-old assemblages colonising artificial cryptic habitats. Each of these fauna categories showed abrupt drop in diversity and abundance at the nearest sites and intermediate west site, A set of species highly frequent in remote sites including a large proportion of cyclostomes were excluded from the most polluted areas, where a few species (Schizoporella dunkeri, Tubulipora aperta) were particularly successful. Large, erect cheilostome species proved to be highly sensitive to pollution, except Myriapora francata , which was relatively tolerant.