Estuaries critically control how much of allochthonous nutrients from rivers reach the open ocean and how much carbon dioxide can be sequestered in the river plume. The Amazon River contributes nearly one fifth of the global riverine freshwater input to the ocean and is fed by the largest drainage basin in the world. The outflow of water and nutrients into the tropical Atlantic and Caribbean Sea has recently been linked to massive blooms of Sargassum in these areas. However, the role of allochthonous nutrients for the production along the plume is still unclear. Discordant lines of evidence suggest that all or none of the nutrients reach the open ocean. Light availability and inorganic and organic nitrogen compounds seem to control the production of photo- and heterotrophs - here summarized as trophic functional groups (TFGs). From recent studies it is possible to identify three different habitats along the plume; the estuary, the mesohaline and oceanic region. The MeNARP project wants to identify and quantify the metabolism of inorganic and organic nitrogen compounds and their role in shaping the TFGs. During an approved Meteor cruise the plume structure and mixing will be studied by a combination of measurements with standard CTD casts, microstructure profiler, current meters and drifter buoy. These data and satellite images will identify the habitat types where additional sampling and experiments will be carried out. Different station types will be studied: 1) Process stations where only basic variables are measured will deliver the hydrographic context, at 2)  biogeochemical stations samples for numerous stable isotope data will be gathered and 3) at four long-term (up to 48 h) experimental stations microbial rates will also be measured. The ultimate goal is to generate a data based characterization of the habitats along the plume to improve estimations of carbon sequestration and N export by means of a biogeochemical model developed in close cooperation with partners of the project.