The purpose of this study was to investigate the bioavailability of dissolved organic nitrogen (DON) produced during a phytoplankton bloom. The experiments were conducted with natural plankton communities as batch growth experiments over approximately 30 days with nitrogen limitation. Five to six times during the exponential and stationary phases of each experimental bloom the bioavailability of DON was measured over 60 days together with DOC and oxygen consumption. The overall aim was to quantify remineralization of the added nitrate. The results showed that maximum 33 % of the added nitrate was remineralized, and with large variation during different growth phases. In contrast natural samples had a steady release of DIN. The C:N ratio in DOM was crucial to whether or not a release of DIN was detected. The results showed that a C:N ratio below 17 in DOM was essential for a subsequent release of DIN. Otherwise nitrogen was retained in the bacterial biomass. We hypothesize that photochemistry and grazing may increase mineralization of DON in shallow ecosystems.