Abstract
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.
Highlights
The combination of north-east trade winds and the Coriolis effect due to earth’s rotation drives the upwelling of deep nutrient-rich waters into the photic zone of coastal regions in eastern ocean boundaries
The system can be separated into two regimes; the region between 15 and 20◦ N undergoes periodic upwelling which dominates during winter and spring, whereas the region north of 20◦ N is characterized by year-round coastal upwelling with maximum intensity during summer and autumn (Mittelstaedt, 1991)
Optimum parameter analysis (Rees et al, 2011) demonstrated that the filament studied was dominated by NACW (50–80 %)
Summary
The combination of north-east trade winds and the Coriolis effect due to earth’s rotation drives the upwelling of deep nutrient-rich waters into the photic zone of coastal regions in eastern ocean boundaries. The maturation and development of biological communities within upwelled water masses as they advect offshore reflects a characteristic feature of eastern boundary upwelling ecosystems (EBUE), the spatial separation of nutrient sources and sinks. The Mauritanian region within this system is characterized by a relatively wide shelf area, over which upwelled water influences biological productivity (Mittelstaedt, 1991), while extended filaments and island-induced eddies are additional features (Arístegui et al, 2009). The study region, located between 20 and 22◦ N, lies at the confluence of the two source waters upwelled in this system
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