STRUCTURE OF A SUPRABENTHIC SHELF SUB-COMMUNITY OF GAMMARIDEAN AMPHIPODA IN THE BAY OF FUNDY COMPARED WITH SIMILAR SUBCOMMUNITIES IN THE GULF OF ST-LAWRENCE

A conceptual model of the adaptive structure of offshore bottom communities through seasonal patterns of pelagic-benthic coupling is tested by comparing the success of suprabenthic, shelf sub-communities of gammarideans selected in three areas differing in trophic status: the eutrophic Baie des Chaleurs and two oligotrophic ecosystems, the Bay of Fundy and the Lower St. Lawrence Estuary. The relative amount of primary production appears to affect both species richness and total density. It is higher in the very productive Baie des Chaleurs, implying tightly coupled pelagic-benthic links. In the Bay of Fundy, gammaridean density, although lower than in the Baie des Chaleurs, is higher than in the Lower St. Lawrence Estuary, but it appears to be characterized by smaller size at maturity or selection of smaller species of the same taxa. The Baie des Chaleurs community may be more biologically accommodated than those of the Lower St. Lawrence Estuary and the Bay of Fundy, which are physically less stable. In the latter, all trophic guilds appear less abundant, but scavengers-predators retain a similar relative success in all three ecosystems, presumably from being more remote from the seasonal pelagic food supply in the trophic chain. In both oligotrophic systems, slope species compete efficiently with shelf species in their preferred depth zone, presumably because of their natural adaptation to food scarcity. The amplitude of vertical migrations by suprabenthic gammarideans appears to be higher in the Baie des Chaleurs, possibly to favor horizontal redistribution to new patches of 'first-quality food' on the very stable and homogeneous mud basin of the Baie. Trends of seasonal abundance for ten gammaridean species in the Bay of Fundy confirm the reality of swimming guilds previously observed in the Baie des Chaleurs.