Les peuplements macrozoobenthiques d'un écosystème côtier sous-saturé en oxygène la baie de Vilaine (sud-Bretagne)

Oxygen depletion occurs today in many marine coastal ecosystems of industrialized countries. This phenomenon has been observed in the Bay of Vilaine, a sheltered and shallow ecosystem of Southern Brittany subjected to eutrophication. The soft bottom macrofauna of this site has been studied in order to highlight the influence of the oxygen under-saturation 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 six macrozoobenthic community units distributed from offshore to the estuary: Haploops tubicola muds; Sternaspis scutata - Nucula turgida muds; Ampelisca spinimana sandy muds; Lanice conchilega muddy heterogeneous sands; Nephtys hombergii - Glycera convoluta muddy sands; and Nephtys hombergii muds. The faunal poverty of the communities in the vicinity of the estuary is related to the estuarine influence (salinity decrease, turbidity...). Structural and faunal characteristics of the mud communities of the bay are related to the oxidized conditions of the sediment, indicating a low oxygen consumption despite the organic content and the weak bottom water renewal. This low consumption could be linked to the refractory nature of the organic matter within the sediments. Therefore, the periodic hypoxia, mainly due to phytoplanktonic biomass degradation, may be insufficiently severe or too temporally limited to have an effect on the communities' health. Only the long-term (20 years) spreading of the Haploops tubicola muds community might indicate a growing eutrophication of the site, linked to an increasing nutrient discharge.