RESISTANCE OF A COASTAL ECOSYSTEM TO INCREASING EUTROPHIC CONDITIONS - THE BAY OF BREST (FRANCE), A SEMIENCLOSED ZONE OF WESTERN-EUROPE

The Bay of Brest is a semi-enclosed coastal ecosystem receiving high n utrients loading from freshwater inputs. In order to analyse the respo nse of phytoplankton stocks to increasing eutrophic conditions, a surv ey of the annual cycle of hydrographic properties, nutrients and chlor ophyll a concentrations, and carbon uptake rates was performed at four stations in 1993. This database has been compared to earlier measurem ents performed during several comparable surveys within the last 20 ye ars. As compared to the seventies, a doubled nitrate loading is now en tering this ecosystem, which is related to increased agricultural acti vities on the drainage basins, while the geographical origin of the ni trate input has been modified. As a result of these anthropogenic modi fications, summer averaged Si/N stoichiometric balance has decreased d uring the two last decades but, contrary to what has been observed in other coastal ecosystems, phytoplankton stocks have not increased. Sev eral ecological factors have hindered eutrophication: the high hydrody namic mixing with adjacent marine waters, caused by the macrotidal reg ime, induces important nutrients losses, temperature and mostly light limit primary production while Si and P high recycling maintain nitrog en limitation in this ecosystem. Conjunction of these nonanthropogenic factors explains the global stability of phytoplankton stocks. Copyri ght (C) 1996 Elsevier Science Ltd