Abstract
Atmospheric deposition is recognized as a potentially large source of inorganic nutrients to many ecosystems. In marine systems, where nitrogen (N) is the nutrient typically limiting phytoplankton growth, rainwater is often a significant source of N. Although a considerable portion of atmospheric N deposition is in the form of organic N, only the inorganic N in rainwater has been considered by most previous studies. Laboratory experiments presented here indicate that dissolved organic nitrogen (DON) from rainwater can stimulate productivity of coastal marine bacteria and phytoplankton. A large percent of rainwater DON was potentially biologically available; 45–75% was rapidly utilized by the microorganisms. The magnitude of the response of bacterial and phytoplankton biomass to equivalent amounts of DON‐N or ammonium‐N was similar. However, the community composition of phytoplankton at the end of the experiment differed in treatments receiving DON and inorganic N. Diatoms and dinoflagellates accounted for >90% of the phytoplankton biomass in treatments receiving rainwater DON. In contrast, small (≪2 μm) monads accounted for >85% of the biomass in the treatments receiving ammonium. The results indicate that DON in rainwater can be an important source of N to ecosystems.