MACROZOOBENTHIC COMMUNITIES OF AN OXYGEN UNDER-SATURATED COASTAL ECOSYSTEM - THE BAY-OF-VILAINE (SOUTHERN BRITTANY)

Oxygen depletion occurs today in many marine coastal ecosystems of ind ustrialized countries. This phenomenon has been observed in the Bay of Vilaine, a sheltered and shallow ecosystem of Southern Brittany subje cted to eutrophication. The soft bottom macrofauna of this site has be en studied in order to highlight the influence of the oxygen under-sat uration of bottom waters on the faunal and structural characteristics of benthic communities. Concurrently, the sediment has been analysed. A clustering classification analysis permitted the identification of s ix macrozoobenthic community units distributed from offshore to the es tuary: Haploops tubicola muds; Sternaspis scutata - Nucula turgida mud s; Ampelisca spinimana sandy muds; Lanice conchilega muddy heterogeneo us sands; Nephtys hombergii - Glycera convoluta muddy sands; and Nepht ys hombergii muds. The faunal poverty of the communities in the vicini ty of the estuary is related to the estuarine influence (salinity decr ease, turbidity...). Structural and faunal characteristics of the mud communities of the bay are related to the oxidized conditions of the s ediment, indicating a low oxygen consumption despite the organic conte nt and the weak bottom water renewal. This low consumption could be li nked to the refractory nature of the organic matter within the sedimen ts. Therefore, the periodic hypoxia, mainly due to phytoplanktonic bio mass degradation, may be insufficiently severe or too temporally limit ed to have an effect on the communities' health. Only the long-term (2 0 years) spreading of the Haploops tubicola muds community might indic ate a growing eutrophication of the site, linked to an increasing nutr ient discharge.