Seasonal changes in macrophyte and macrozoobenthos assemblages in three coastal lagoons under varying degrees of eutrophication

The dynamics of macrophytic and macrozoobenthic communities were studied du ring two consecutive years in three French lagoons with differing degrees o f eutrophication: (1) Arcachon Bay, a macrotidal lagoon on the Atlantic coa st; (2) the fishponds of Certes, an almost enclosed system adjacent to Arca chon Bay; and (3) the Etang du Prevost, a highly eutrophic Mediterranean la goon experiencing summer dystrophic crises. Two stations were sampled seaso nally in each system. The intertidal area of Arcachon Bay was covered by a dense, stable seagrass (Zostera noltii) bed. In terms of abundance. macrofa una were dominated by oligochaetes, which could be related to the high belo w-ground plant biomass, including slow-decaying debris; faunal biomass rema ined relatively constant, throughout the study period. In the Certes lagoon s, which were intermediate between the other two systems in terms of eutrop hication, vegetation was dominated by another rooted phanerogam (Ruppia cir rhosa) with fairly constant biomass, while sporadic development of green ma croalgae occurred in spring; both biomass and species richness of macrofaun a were low. In the Prevost lagoon, macrophytes were opportunistic macroalga e that first proliferated and then disappeared over a short period in summe r; this seasonal crisis resulted in a marked decrease in both biomass and a bundance of macrozoobenthos. Macrobenthic dominance shifted after the first summer from suspension-feeding bivalves to subsurface deposit-feeding anne lids. The differences in structure and seasonal dynamics of benthos in the three systems may have significant effects on higher trophic levels. (C) 20 00 International Council for thr Exploration of the Sea.