In aquatic ecosystems, there are 2 major forms of N available at the base of the planktonic food web: dissolved organic N (DON) and dissolved inorganic N (DIN). In DON, N is associated with organic C, which may promote both heterotrophs and autotrophs. In environments where DIN nitrate is the prevailing N form and dissociated dissolved organic C (DOC) is available, heterotrophs may also be promoted, but they may compete with the autotrophs for DIN. The influence of associated or dissociated CN nutrient sources on the interaction between organ- isms and the food web function is poorly known and has not been studied before. To approach this question, we performed a microcosm experiment with a coastal microbial food web, where N and C nutrient sources were provided either associated in 1 molecular compound (DON), or dissociated in 2 separate molecular compounds (DIN and DOC). The results showed that association or dissociation of C and N input had marked effects on all trophic levels, most probably through its effect on bacteria−phytoplankton interaction, which switched between increased coupling and increased competition. The biomass of all components of the food web benefitted from the association of C and N in a single DON molecule. Our study indicated that the degree of association between C and N is an important factor affecting the productivity and efficiency of the microbial food web. Therefore, the C and N association should be considered when studying aquatic systems.