The objectives of this project are to study the biological processes that influence the fate of organic matter in estuaries by: a) comparison of the ecological structure of major European tidal estuaries. MAST funding will be used for co ordinating simultaneous field surveys in different estuaries; b) specific co-ordinated studies on the rates of major processes on short and medium time scales in the field in Schelde and Gironde and in laboratory experiments; c) building mathematical ecosystem models describing the major biological processes in the estuaries studied, which are based on appropriate hydrodynamical models, and incorporate the field data and results from process studies specified in (a) and (b).