Articles | Volume 13, issue 10
Research article
18 May 2016
Research article |  | 18 May 2016

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

Darren R. Clark, Claire E. Widdicombe, Andrew P. Rees, and E. Malcolm S. Woodward

Abstract. 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.

Short summary
Based in the Mauritanian upwelling system, the article describes a Lagrangian study of biogeochemical processes within a freshly upwelled body of water as it advects offshore. We report rates of primary production, nitrogen assimilation, and regeneration and describe how these processes relate to the dynamics of the upwelling regime. This system is perhaps the least studied of the four major eastern boundary upwelling systems and so these measurements provide important new insights.
Final-revised paper