Gross and net nitrogen uptake and DON release in the euphotic zone of Monterey Bay, California

¹⁵N tracer techniques were used to measure rates of NH₄/⁺ and NO₃/^- uptake, dissolved organic nitrogen (DON) release resulting from both ammonium (NH₄/⁺) and nitrate (NO₃/^-) uptake, and NH₄/⁺ regeneration at discrete depths 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 NH₄/⁺, and the primary fate of nitrogen uptake was particle production. In September 1993, NH₄/⁺ and NO₃/^- uptake were more in balance, and the primary 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.