The dynamics of macrophytic and macrozoobenthic communities were studied during two consecutive years in three French lagoons with differing degrees of eutrophication: (1) Arcachon Bay, a macrotidal lagoon on the Atlantic coast; (2) the fishponds of Certes, an almost enclosed system adjacent to Arcachon Bay; and (3) the Etang du Prevost, a highly eutrophic Mediterranean lagoon experiencing summer dystrophic crises. Two stations were sampled seasonally in each system. The intertidal area of Arcachon Bay was covered by a dense, stable seagrass (Zostera noltii) bed. In terms of abundance. macrofauna were dominated by oligochaetes, which could be related to the high below-ground plant biomass, including slow-decaying debris; faunal biomass remained relatively constant, throughout the study period. In the Certes lagoons, which were intermediate between the other two systems in terms of eutrophication, vegetation was dominated by another rooted phanerogam (Ruppia cirrhosa) with fairly constant biomass, while sporadic development of green macroalgae occurred in spring; both biomass and species richness of macrofauna were low. In the Prevost lagoon, macrophytes were opportunistic macroalgae that first proliferated and then disappeared over a short period in summer; this seasonal crisis resulted in a marked decrease in both biomass and abundance of macrozoobenthos. Macrobenthic dominance shifted after the first summer from suspension-feeding bivalves to subsurface deposit-feeding annelids. The differences in structure and seasonal dynamics of benthos in the three systems may have significant effects on higher trophic levels. (C) 2000 International Council for thr Exploration of the Sea.