The anthropogenic perturbation of the nitrogen cycle has the potential to change the coupling of fundamental element cycles in aquatic ecosystems. Coastal areas as transition zones between land and sea are directly affected by this due to heavy industrialization, dense human colonization, and massive riverine inputs of artificial reactive nitrogen (Nr). In order to better predict the potential impact of increasing Nr supply on the biogeochemistry and ecology of coastal areas and to evaluate the consequences for the adjacent ocean one need to understand the flux of inorganic and organic nitrogen through the lower food web (bacteria to zooplankton). Biological research on a micrometer scale, however, is often limited by methodological drawbacks like e.g. size independent separation of cells and instant rate measurements. MicroTRANCE wants to overcome these constraints by stable isotope tracer experiments, state-of-the-art cell identification and sorting by Flow Cytometry, and highly sensitive rate measurements using Membrane Induced Mass Spectrometry. The proposed interdisciplinary research will cover eutrophicated as well as pristine coastal habitats and includes advanced lab work. This approach will lead to a better insight into nitrogen transformation processes due to a high functional and cellular resolution. The scientific aim of MicroTRANCE is to study the cascading effects of increasing inorganic nitrogen inputs on the linkage between primary production and the microbial loop with respect to the coupling of the nitrogen cycle with carbon and phosphorous in coastal environments. Thus, MicroTRANCE will help to close gaps in our knowledge regarding the flux of reactive nitrogen as urgently recommended e.g. by the International Nitrogen Initiative (INI). Moreover the proposed research contributes to an integrated coastal policy as recently called by the European Commission (Impact Assessment Summary, Brussels, Oct. 2007).