MEPS 197:51-65 (2000)  -  doi:10.3354/meps197051

ABSTRACT: Nitrogen regeneration fluxes of ammonium (NH₄⁺) and nitrate (NO₃^-) as well as losses of dissolved organic nitrogen (DON) by phytoplankton were investigated over a 2 mo period (spring 1997) in a NW coastal Mediterranean area (Gulf of Lions) using ¹⁵N-tracer techniques. Profiles of dissolved inorganic nitrogen (DIN) concentrations were almost uniform with values of 600, 150 and 35 nM for NO₃^-, NO₂^- and NH₄⁺, respectively, except at the end of the study period when the upper layer became nitrogen-depleted (<50 nM down to 40 m). Chlorophyll (chl) distributions showed a surface maximum (to 0.85 mg m^-3) and a deep maximum (to 1.25 mg m^-3) at 40 m. Plankton DIN utilization (net uptake) was most of the time highest at the surface, with rates reaching 62 and 40 nM d^-1 for NH₄⁺ and NO₃^-, respectively. However, a deepening (to 60 m) of maximum NO₃^- uptake rates with a corresponding deepening of the nitracline sometimes occurred during the experiment. Therefore, f-ratio profiles depicted maximum surface values (~0.40) at the beginning of the experiment and a deep maximum at the end. NH₄⁺ regeneration rates were 1 order of magnitude higher (up to 220 nM d^-1) than nitrification and DIN loss (as DON) rates, and could largely sustain more than 100% of the plankton NH₄⁺ demand. Underestimation of NH₄⁺ uptake rates due to ¹⁵N isotope dilution had only a small effect on the f-ratio calculation (overestimation <5%). Nitrification occurred from the surface (10 to 20 nM d^-1) down to the base of the euphotic layer (30 nM d^-1), and corresponded to 90% and >>100% of the plankton NO₃^- demand at the surface and in the nitracline, respectively. Consequently, a great part of NO₃^- uptake did not correspond to new production and should be considered as regenerated production, particularly in the NO₃^- depleted surface layer. Profiles of DIN loss (as DON) well paralleled those of DIN net uptake with values highest at surface reaching 35 and 14 nM d^-1 for NH₄⁺ and NO₃^-, respectively. DIN loss rates represented on average ~23% of gross DIN uptake (gross DIN uptake = DIN losses + DIN net uptake) whatever the substrate was, indicating that (1) DIN loss (as DON) did not depend on the nitrogen source, and (2) DIN uptake was mostly due to phytoplankton and not to bacterioplankton, although the study area tended to be globally nitrogen-depleted and based on regeneration. Failure to account for DIN losses had no significant effect on the computation of f-ratios.

KEY WORDS: New production · Nitrogen regeneration · f-ratios · DON release · ¹⁵N tracer