Isotope-dilution experiments with 15NH4/+ were conducted on near-surface water samples in the Mississippi River plume during May 1992 and July 1993 to quantify community cycling rates for ammonium and determine whether regeneration rates are enhanced by light. Experiments done under natural light in May showed ranges of potential uptake and regeneration rates of 0-0.4 μM h-1 and 0-0.18 μM h-1. Samples collected offshore from the Atchafalaya River and the Southwest Pass of the Mississippi River in July yielded potential uptake rates of 0.4-1.8 μM h-1 under natural light vs. 0-0.45 μM h-1 in the dark. Ammonium regeneration rates ranged from 0.08 to 0.75 μM h-1 in the light and from 0.02 to 0.3 μM h-1 in the dark. The observed light/dark regeneration-rate differences imply a close coupling between phytoplankton production and ammonium regeneration. The ratio of bacterial cell-specific regeneration to uptake rates increased in the outer regions of the plume, indicating a changing contribution of bacterial-sized organisms to nitrogen cycling processes in different regions of the plume.