The aim of this PhD-thesis was to contribute to the existing knowledge on the role of diazotrophic cyanobacteria in marine environments. In the first part dealt with N2 and C fixations as well as the release of N and C compounds, which were tightly regulated. The second part aimed at the quantification of diazotrophic N transferred to mesozooplankton species. Because filamentous cyanobacteria are grazed upon only by a limited number of zooplankter the profound transfer of newly fixed N in the Baltic Sea was mediated by microbial loop constituents (67%).