ABSTRACT: The dynamics of N and its interactions with labile dissolved organic C (DOC), bacteria, and phytoplankton were studied to determine potential effects of dissolved organic matter (DOM) and light on N dynamics in surface waters of the Mississippi River(USA) plume in the Gulf of Mexico. Bacterial uptake of added labeled N compounds (¹⁵NH₄⁺ or ¹⁵N-labeled dissolved free amino acids, DFAA) was stimulated more by high-molecular-weight (HMW, &GT1 kDa) DOM than bylow-molecular-weight (LMW, &LT 1 kDa) DOM. An index that inversely indicated the presence of labile DOC was defined as the fraction of assimilated Amino acid-¹⁵N that was Recovered as ¹⁵N-Ammonium (ANRA), following the additions ofhigh-levels (4 uM) of ¹⁵N-DFAA. ANRA ratios were high in the absence of other available carbon sources because heterotrophic bacteria were forced to use the added amino acids as a carbon source for respiration rather than as a nutrient sourcefor biomass formation. In dynamic light/dark experiments, conducted with in situ populations of organisms, uptake rates of added ¹⁵NH₄⁺ were significantly enhanced both by the presence of light and by the additionof HMW DOM. Uptake rates of added ¹⁵N-labeled DFAA were increased by the addition of HMW DOM but not by light. ANRA ratios were consistently lower in the presence of added HMW DOM than in controls. Added HMW DOM thus appeared to stimulate theincorporation of assimilated DFAA into bacterial biomass. Bacterial growth rates were relatively high in both light and dark bottles with DFAA additions and in light bottles with HMW DOM plus NH₄⁺ additions, but they remainedcomparatively low in dark bottles with added NH₄⁺. These results are consistent with the idea that bacterial N dynamics in these euphotic waters may be tightly coupled to photosynthetic activities over short time scales.