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

F Diaz,P Raimbault

doi:10.3354/meps197051

Abstract

Nitrogen regeneration fluxes of ammonium (NH 4 + ) and nitrate (NO 3 - ) 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 15 N-tracer techniques. Profiles of dissolved inorganic nitrogen (DIN) concentrations were almost uniform with values of 600, 150 and 35 nM for NO 3 - , NO 2 - and NH 4 + , 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 NH 4 + and NO 3 - , respectively. However, a deepening (to 60 m) of maximum NO 3 - 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. NH 4 + 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 NH 4 + demand. Underestimation of NH 4 + uptake rates due to 15 N isotope dilution had only a small effect on the ∫-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 NO 3 - demand at the surface and in the nitracline, respectively. Consequently, a great part of NO 3 - uptake did not correspond to new production and should be considered as regenerated production, particularly in the NO 3 - 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 NH 4 + and NO 3 - , 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 ∫-ratios.

Highlights

The role of coastal zones in the global biogeochemical carbon cycle, and their potential behavior to be a carbon source or sink, has been much debated by researchers for several years (Walsh et al 1981, RoweO Inter-Research 2000 Resale of full article not permitted et al 1986, Smith & McKenzie 1991).Concepts of new and regenerated production, derived from 15N-tracer experiments (Dugdale & Goering 1967) and providing a model of the relation between primary productivity by phytoplankton communities and the 2 principal modes of nitrogen supply to the euphotic layer, enable us to know the potential role of coastal zones in the global carbon cycle
Inputs of ammonium by terrestrial and Rhone River discharges must be considered as new nitrogen so that NH,+ uptake measured may no longer be considered as regenerated production
In spite of these nutrient inputs, the concept of new and regenerated production may still apply to this marine coastal area for at least 2 reasons: (1) hydrodynamics in the area are dominated by the near-shore presence of oligotrophic Modified Atlantic Water (MAW) (i.e. open ocean conditions] and (2) the influence of the Rhone River plume is only weak [Fig. 1) especially for NH4+

Summary

Introduction

The role of coastal zones in the global biogeochemical carbon cycle, and their potential behavior to be a carbon source or sink, has been much debated by researchers for several years (Walsh et al 1981, RoweO Inter-Research 2000 Resale of full article not permitted et al 1986, Smith & McKenzie 1991).Concepts of new and regenerated production, derived from 15N-tracer experiments (Dugdale & Goering 1967) and providing a model of the relation between primary productivity by phytoplankton communities and the 2 principal modes of nitrogen supply to the euphotic layer, enable us to know the potential role of coastal zones in the global carbon cycle. A specific feature of coastal environments is that nutrient recycling, and regenerated production, contributes to a significant fraction of the primary productivity in the euphotic layer (Eppley & Peterson 1979, Eppley et al 1979). In this regard, to accurately assess new and regenerated production, as well as the f-ratio, is of great interest in characterizing the importance of coastal environments in global biogeochemical cycles

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