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

Abstract

International audienceA 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

On the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic

“…In line with the results of Auger et al (2016) and Pastor et al (2013), we find that the area around Cape Blanc, corresponding to the region of convergence of the coastal flows and formation of the Cabo Verde frontal zone, is a key region of the CanUS. The relevance of the central Cabo Verde frontal zone in the CanUS was discussed in Auger et al (2016) and Pastor et al (2013) in terms of chlorophyll and nutrient convergence on the shelf and of their subsequent offshore advection, visible as a persistent bloom in the region offshore of Cape Blanc. Here we confirm and strengthen these results, affirming that this sector of the CanUS has a central role in the collection and subsequent transport offshore of the organic carbon produced over wide areas of the shelf.…”

“…In both model and observations, the southern MauritanianSenegalese subregion is the most productive area of the CanUS and is characterized by a reduced offshore gradient of NPP, while the northern Moroccan subregion is characterized by a sharp offshore gradient of production. The convergence of the coastal currents in the region of Cape Blanc fuels a persistent offshore bloom (Auger et al, 2016) that clearly appears both in the SeaWiFS product and in the model.…”

“…A complex circulation pattern determines strong subregional differences in the CanUS in terms of circulation, mesoscale activity, seasonality of upwelling, and biology ). The high productivity in the CanUS is sustained both by local coastal upwelling and by meridional alongshore advection of nutrients (Carr and Kearns, 2003;Pelegrí et al, 2006;Pastor et al, 2013;Auger et al, 2016). Sufficiently long water residence times in the nearshore region, favorable light, and temperature conditions also contribute to sustaining high levels of production .…”

“…These models need to have relatively high resolution in order to resolve these mesoscale dynamics, forcing most studies to employ regional models instead of global ones. However, so far relatively few high-resolution modeling studies have focused on the CanUS compared to other upwelling regions and even fewer have employed a fully coupled biogeochemical model (Auger et al, 2016;Fischer and Karakaş, 2009;Lachkar and Gruber, 2011;Pastor et al, 2013).…”

“…In the most recent study, Auger et al (2016) used a regional ROMS configuration coupled to the biogeochemical and ecological module PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies) to highlight the important role of the lateral redistribution of nutrients and phytoplankton on the CanUS shelf in determining the complex seasonal pattern of chlorophyll and in the fueling of the persistent Cape Blanc offshore bloom. Despite their specific focus on phytoplankton, they extended the work of previous studies that concentrated on the lateral transport of organic carbon in limited portions of the CanUS.…”