The role of pelagic-benthic coupling in structuring littoral benthic communities at Terra Nova Bay (Ross Sea) and in the Straits of Magellan

In Antarctic and peri-Antarctic regions, benthic communities are persistent in time and show high biomass and large numbers of individuals, mainly con sisting of suspension and deposit feeders. In fact, apart from recruitment, the major factor structuring these communities is the high flow of organic matter from the pelagic domain to the bottom, representing an important en ergy source for the benthic organisms. The aim of this paper is to review, compile and compare the data from earlier investigations in Terra Nova Bay (Ross Sea) and the Straits of Magellan, in order to come to a more general conclusion about the role of the pelagic-benthic coupling in structuring li ttoral benthic communities in southern coastal areas. Few measurements of f lux rates and the biochemical composition of the sinking particles occurrin g in Antarctic and peri-Antarctic shallow waters are available, but a compi lation of our own data and others allows a comparison of these two systems. The different environmental conditions between Antarctica and the Straits of Magellan lead to differences in the origin of the particulate organic ma tter and in its biochemical composition, and consequently in the coupling b etween pelagic and benthic domains. At Terra Nova Bay the summer particulat e matter shows a high labile fraction of a good food value: its flux has be en evaluated at about 0.67 g m(-2)d(-1). Conversely, the Straits of Magella n show multi-structured ecosystems where the quality and quantity of the or ganic matter flux towards the bottom change according to the local geomorph ology and current dynamics. Moreover, the three-dimensional assemblages of suspension-feeders, so common in Antarctic shallow waters, seem to be absen t in the Magellan area. In particular sponges, gorgonarians and bryozoans p lay a secondary role inside the Straits of Magellan, where polychaetes (60% ) and molluscs (9-10%) are dominant on soft bottoms, and where they reach h igh values in density and biomass. Bivalves seem to play an important role in both regions: for instance, at Terra Nova Bay, the scallop Adamussium co lbecki processes about 14% of the total carbon flux, with an assimilation e fficiency of 36%. This scallop seems to be able to adapt its reproductive p eriod and its trophic behaviour to the changes in the quality and quantity of the pelagic events. The pulsing trend of the vertical flux, which in a f ew weeks can reach the total annual input, produces significant changes in the physiology (growth, reproduction, spawning) and trophic behaviour of ma ny benthic species, such as sponges and polychaetes. The study of the pelag ic-benthic coupling could be essential in the evaluation of the trophic cap acity and the environmental response around sites of sea-fanning, which are an ever-growing activity in the Magellan area.