As transition zones between land and sea, estuaries can play an important role in storing, transforming, consuming and producing organic matter and nutrients. The most active, and most peculiar site of these transformations is the maximum turbidity zone in the low salinity range of tidal estuaries. The role of biological processes in the formation and subsequent utilization of particles in the maximum turbidity zone will be studied in three European estuaries : Gironde, Schelde and Elbe. Special attention will be given to organic matter and biological processes acting on it. Nutrients and trace elements will be considered insofar as they are regulating biological processes in the estuary. Numerical modelling of water and suspended matter transport is an integrated part of the project. In general, research will be focused around six joint field campaigns. Each estuary will be sampled during one week in 1993 and in 1994 during which two longitudinal transects along the river and estuary will be made, as well as frequent measurements at one anchor station. Field enclosures will be adapted to, and deployed in estuarine conditions. Transport of water, dissolved substances and particles will be described by numerical models for the three estuaries. The workprogramme is organised around four themes. Theme 1 - Organic matter, suspended matter : Measurements of in situ floc size, particle concentration, POC, PON etc. Development of a camera for use in high turbidity environments. Theme 2 - Hydrodynamic modelling : Adaptation of a three-dimensional water movement model for the Schelde and Gironde. Use of the existing model of the Elbe estuary. Theme 3 - Microbiology : Measurement and localisation of bacterial biomass and activity during the joint field campaigns. Description of primary production along the estuaries. Determination of the spectral composition of the underwater light. Estimation of leaking and lysis from (salinity-)stressed phytoplankton cells. Study of the microbial loop. Study of the role of microzooplankton. Theme 4 - Higher trophic levels : Study of the distribution and condition of mesozooplankton during the joint sampling campaigns. Measurement of zooplankton grazing rates on natural particulate matter etc. Mesocosm experiments.