Limnol. Oceanogr., 44(3), 1999, 573–585

15N tracer techniques were used to measure rates of NH and NO uptake, dissolved organic nitrogen (DON) 1 2 4 3 release resulting from both ammonium (NH ) and nitrate (NO ) uptake, and NH regeneration at discrete depths 1 2 1 4 3 4 throughout the euphotic zone in Monterey Bay, California, at a site, H3, outside the plume of direct upwelling influence. In March 1993, 94% of the inorganic nitrogen taken up was NH , and the primary fate of nitrogen 4 uptake was particle production. In September 1993, NH and NO uptake were more in balance, and the primary 1 2 4 3 fate of nitrogen uptake was DON. We suggest that grazing was an important mechanism resulting in DON release in March and that a combination of grazing and a more physiologically stressed phytoplankton population produced the higher observed rates of DON release in September. During both cruises, the percentage of nitrogen released as DON increased with depth, suggesting that deeper in the water column, a smaller percentage of the nitrogen taken up is incorporated into sinking particles. Based on these data, we suggest that the DON pool acts as an intermediate between DIN assimilation and the net formation of particles for export and will thus affect carbon flow in Monterey Bay. Primary production in marine systems can be characterized as new or regenerated depending on the nitrogen substrate used; NO is generally considered to fuel new pro3 duction (in the absence of significant dinitrogen fixation), while NH fuels regenerated production (Dugdale and Goe4 ring 1967; Eppley and Peterson 1979). Oceanographers have used 15N-labeled NO and NH to measure nitrogen uptake 2 1 3 4 rates and thus estimate rates of new and regenerated production assuming that uptake of 15N label into the cell results in the production of biomass. This assumption, however, is not completely valid. The disappearance of 15N label from the NH or NO substrate pool is often not balanced by an 1 2 4 3 equal appearance of 15N in particulate nitrogen (PN; Glibert et al. 1982; Price et al. 1985; Ward et al. 1989; Slawyk et al. 1990). This observation led some researchers to hypothesize that DON was an alternate fate for the missing 15N (Laws 1984; Ward et al. 1989) Researchers have now directly measured the passage of 15N label from the NH or NO substrate pool into the DON 1 2 4 3 pool (Bronk and Glibert 1991, 1993a,b, 1994; Bronk et al. 1994; Slawyk and Raimbault 1995). If 15N is present in the DON pool at the end of an incubation, then accounting for the accumulation of 15N exclusively within the PN fraction results in an underestimate of the amount of nitrogen taken up by the cell during the incubation. To obtain a better es-