Abstract
Abstract. A comparative box analysis based on a multi-decadal physical–biogeochemical hindcast simulation (1980–2009) was conducted to characterize the drivers of the spatial distribution of phytoplankton biomass and production in the north-west (NW) African upwelling system. Alongshore geostrophic flow related to large-scale circulation patterns associated with the influence of coastal topography is suggested to modulate the coastal divergence, and then the response of nutrient upwelling to wind forcing. In our simulation, this translates into a coastal upwelling of nitrate being significant in all regions but the Cape Blanc (CB) area. However, upwelling is found to be the dominant supplier of nitrate only in the northern Saharan Bank (NSB) and the Senegalo-Mauritanian (SM) regions. Elsewhere, nitrate supply is dominated by meridional advection, especially off Cape Blanc. Phytoplankton displays a similar behaviour with a supply by lateral advection which equals the net coastal phytoplankton growth in all coastal regions except the Senegalo-Mauritanian area. Noticeably, in the Cape Blanc area, the net coastal phytoplankton growth is mostly sustained by high levels of regenerated production exceeding new production by more than twofold, which is in agreement with the locally weak input of nitrate by coastal upwelling. Further offshore, the distribution of nutrients and phytoplankton is explained by the coastal circulation. Indeed, in the northern part of our domain (i.e. Saharan Bank), the coastal circulation is mainly alongshore, resulting in low offshore lateral advection of nutrients and phytoplankton. Conversely, lateral advection transports coastal nutrients and phytoplankton towards offshore areas in the latitudinal band off the Senegalo-Mauritanian region. Moreover, this latter offshore region benefits from transient southern intrusions of nutrient-rich waters from the Guinean upwelling.
Highlights
IntroductionAmong the four major eastern boundary upwelling systems (EBUS), the north-west (NW) African upwelling region is the most spatially and seasonally variable in terms of primaryP.-A
Among the four major eastern boundary upwelling systems (EBUS), the north-west (NW) African upwelling region is the most spatially and seasonally variable in terms of primaryPublished by Copernicus Publications on behalf of the European Geosciences Union.P.-A
Nitrate supply is dominated by meridional advection
Summary
Among the four major eastern boundary upwelling systems (EBUS), the north-west (NW) African upwelling region is the most spatially and seasonally variable in terms of primaryP.-A. Auger et al.: Drivers of productivity off north-west Africa productivity (Carr and Kearns, 2003). This variability may impact the distribution and abundance of fish populations, and their associated fisheries, on a large range of timescales (Arístegui et al, 2009). It constrains the dynamics of nutrient and organic carbon exports from the coastal margin (Helmke et al, 2005; Muller-Karger et al, 2005). The mechanisms that control the spatiotemporal variability of primary productivity are still poorly understood
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