Nitrogen regeneration and dissolved organic nitrogen release during spring in a NW Mediterranean coastal zone (Gulf of Lions):implications for the estimation of new production

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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 N-15-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 (similar to 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 N-15 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 much greater than 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 similar to 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.