This proposal focuses on investigating carbon fluxes whithin coastal pelagic ecosystems under the hypothesis that fluxes can be modulated by both community structure and seston quality. The study considers estimation of selected carbon fluxes, mesurement of seston chemical composition, and determination of planktonic community structure at seasonal and sub-seasonal scales in two contrasting coastal sites: the Gullmarsfjord (Sweden, Incoming Phase of the project) and the Río de la Plata (Uruguay, Returning Phase). The fluxes to be estimated by a combination of field observations and field/laboratory experiments include microzooplankton grazing, mesoplankton grazing on phytoplankton and predation on heterotrophs, and carbon downward export due to sinking of phytoplankton, of mesozooplankton faecal pellets, and of other biogenic structures (carcasses, marine snow, etc.). Expected results are that the planktonic community structure will modulate certain carbon fluxes at a seasonal scale, but seston quality will be most important at sub-seasonal scales. Results will contribute to the understanding of pelagic ecosystem functioning and to the development of better management practices. Furthermore, training and experience acquired during the developement of this proposal will imply a transfer of knowledge with a very high potential impact for the developement of this line of research in Uruguay. The development of this proposal will also contribute to the establishment of effective scientific cooperation between the European Community and the third country.