Articles | Volume 13, issue 23
Biogeosciences, 13, 6419–6440, 2016
Biogeosciences, 13, 6419–6440, 2016

Research article 30 Nov 2016

Research article | 30 Nov 2016

What drives the spatial variability of primary productivity and matter fluxes in the north-west African upwelling system? A modelling approach

Pierre-Amaël Auger1,2,5, Thomas Gorgues1, Eric Machu1,3, Olivier Aumont1,a, and Patrice Brehmer2,4 Pierre-Amaël Auger et al.
  • 1Univ. Brest, CNRS, IRD, Ifremer, Laboratoire d'Océanographie Physique et Spatiale (LOPS), IUEM, 29280, Plouzané, France
  • 2Institut de Recherche pour le Développement (IRD), Laboratoire de l'Environnement MARin (LEMAR), UMR 195 CNRS/IRD/UBO/Ifremer, Campus UCAD-IRD, BP 1386, Dakar, Senegal
  • 3ESP/CAD, Laboratoire de Physique de l'Atmosphère et de l'Océan Siméon Fongang (LPAO-SF), BP 5085, Dakar-Fann, Senegal
  • 4Institut Sénégalais de Recherche Agricole (ISRA)/Centre de Recherche Océanographique de Dakar-Thiaroye (CRODT), Pole de recherche de Hann (PRH), BP 2241, Dakar, Senegal
  • 5Instituto Milenio de Oceanografia and Escuela de Ciencias del Mar, Pontificia Universidad Catolica de Valparaiso, Valparaiso, Chile
  • anow at: Laboratoire d'Océanographie et de Climatologie: Expérimentation et Analyse Numérique (LOCEAN), Institut Pierre-Simon Laplace (IPSL), UPMC/CNRS/IRD/MNHN, 4 Place Jussieu, Case 100, 75252 Paris Cedex 05, France

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.

Short summary
A box modeling approach reveals that horizontal currents drive the spatial distribution of phytoplankton biomass and primary production in the north-west African upwelling system. Alongshore (cross-shore) currents limit (enhance) cross-shore exchanges north (south) of Cape Blanc. Off Cape Blanc, a meridional convergence makes ambiguous the response of coastal nutrient upwelling to wind forcings, and high production is based upon nutrients and remineralized matter injected by horizontal currents.
Final-revised paper