Coastal eutrophication is expanding in the southern Portugal due to man-induced activities e.g. coastal management, tourism and aquaculture. Microbial foodweb, bacteria and protozoa act as a sink of organic matter and nutrients produced in the process of eutrophication. The efficiency of removal of organic and inorganic nutrients from water is dependent on the "activity" (process rates) of the microbial foodweb and growth efficiencies of the organisms in the foodweb. The research cruises made during the present TMR programme to the river and estuary of the river Guadiana, at the border between Spain and Portugal, has revealed high bacterioplankton standing stocks and production rates suggesting of importance of bacterioplankton in the food web processes. The current literature documents a wide variety of growth efficiencies for bacterioplankton, which further affects on the estimates of carbon and nutrient sinks in the environment. To promote research on bacterioplankton growth efficiencies in the gradient of different carbon sources (riverine-estuarine) and to continue the established research activity on the river Guadiana and to widen the scientific contacts in the project, the study of "Growth efficiency of estuarine bacterioplankton" was formulated. The study focuses on bacterioplankton growth efficiency in microcosm experiments. A Portuguese chemist is involved in chemical measurements and a German researcher to perform dissolved organic carbon (DOC) analysis. The proposed TMR programme thus deepens the microbial ecology research in the University of Algarve and widens the international research contacts. The background research activity in the river Guadiana is supported by bilateral 3-year research project INTERREG (Inter-Regional Research between Portugal and Spain)starting in January 1997.