The significance of nitrogen regeneration for new production within a filament of the Mauritanian upwelling system

• Published in: Biogeosciences Vol. 13; no. 10; pp. 2873 - 2888
• Main Authors: Clark, Darren R, Widdicombe, Claire E, Rees, Andrew P, Woodward, E. Malcolm S
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 18.05.2016; Copernicus Publications
• Subjects: Advection; Assimilation; Biological activity; Communities; Comparative analysis; Continental shelves; Continental slope; Diatoms; Flagellates; Helium; Helium isotopes; Light penetration; Marine microorganisms; Mineral nutrients; Mixed layer; Mixed layer depth; Nitrogen; Nutrient concentrations; Nutrients; Ocean circulation; Offshore; Penetration depth; Phytoplankton; Plankton; Primary production; Productivity; Public transportation; Regeneration; Regeneration (biological); Stoichiometry; Studies; Sulfur; Sulfur hexafluoride; Sulphur; Tracers; Tracers (Chemistry); Upwelling; Water depth; Water masses
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-13-2873-2016

The Lagrangian progression of a biological community was followed in a filament of the Mauritanian upwelling system, north-west Africa, during offshore advection. The inert dual tracers sulfur hexafluoride and helium-3 labelled a freshly upwelled patch of water that was mapped for 8 days. Changes in biological, physical, and chemical characteristics were measured, including phytoplankton productivity, nitrogen assimilation, and regeneration. Freshly upwelled water contained high nutrient concentrations but was depleted in N compared to Redfield stoichiometry. The highest rate of primary productivity was measured on the continental shelf, associated with high rates of nitrogen assimilation and a phytoplankton community dominated by diatoms and flagellates. Indicators of phytoplankton abundance and activity decreased as the labelled water mass transited the continental shelf slope into deeper water, possibly linked to the mixed layer depth exceeding the light penetration depth. By the end of the study, the primary productivity rate decreased and was associated with lower rates of nitrogen assimilation and lower nutrient concentrations. Nitrogen regeneration and assimilation took place simultaneously. Results highlighted the importance of regenerated NH4+ in sustaining phytoplankton productivity and indicate that the upwelled NO3− pool contained an increasing fraction of regenerated NO3− as it advected offshore. By calculating this fraction and incorporating it into an f ratio formulation, we estimated that of the 12.38 Tg C of annual regional production, 4.73 Tg C was exportable.