The Canary Current Easter Boundary Upwelling Ecosystem (EBUE) is one of the most productive upwelling systems in the planet, extending along the Northwestern African coast. Although mesoscale activity plays an important role in upwelling dynamics, large scale primary production (PP) is governed by alongshore wind patterns, which generate Ekman transport and bring nutrients to surface waters. Wind patterns in the Canary Current EBUE present seasonal variations, which depend on the latitude. Three main latitudinal divides are usually identified: a seasonal upwelling zone (SUZ) along the Senegalese-Mauritanian coast, which expands from 13 to 20ºN, a permanent upwelling zone (PUZ), from 20 to 26ºN, and a weak permanent upwelling zone (WPUZ), from 26 to 33ºN. While the WPUZ and PUZ present year-round upwelling (peaking during summer in the northern section), the SUZ exhibits winter upwelling followed by a downwelling period, especially during summer months, due to the appearance of the onshore monsoonal winds.
Nevertheless, PP also depends on the characteristics of the upwelled waters, which differ depending on their source water mass: the South Atlantic Central Waters (SACW) have higher nutrient concentrations than the North Atlantic Central Waters (NACW), thus yielding larger PP. The boundary between these two water masses lies close to Cape Blanc (21ºN) near the coast and extends southwestwards, forming an oceanic front that extends hundreds of kilometres. The southern part of the Canary Current EBUE, which is the most productive subregion, has been relatively poorly studied. Few studies on PP production have been performed in this area, the bulk of them being carried out during the 1970s and 1980s. In this context, satellite-based Net Primary Production (NPP) models represent a very useful tool to study production patterns as they provide a synoptic view of the ecosystem, a view that would be out of reach with in situ measurements. The figure accompanying this post depicts the monthly climatological NPP values for the 2003–2015 period yielded by a modified version of the Vertically Generalized Production Model (VGPM)1, the white line representing the 1000 mg C·m-2·day-1 isoline. Largest NPP values (up to ~6000 mg C·m-2·day-1) are registered around and south of Cape Blanc, in areas where SACW is the source water mass of upwelled waters. Besides, a clear seasonal change in NPP following wind patterns can be observed: during late winter and early spring high NPP values extend south of ~10-12ºN, but as of late summer the most productive area is limited to Cape Blanc (see how during the June–September period the 1000 isoline tends to restrict the most productive areas close to the coast, as if it was a zip). Although NPP estimates from models based on remote sensing measurements are useful resource, they still have important flaws and in situ measurements such as the ones that are being carried out at the Fluxes project are a key element to further improve and validate them.
This represents an important challenge for the near future, as global change and its associated disturbances in environmental conditions are expected to greatly impact marine primary production. Indeed, and although a net reduction in global primary production has been observed over recent years, the regional impact of these changes is far from being homogeneous: tropical and subtropical open-ocean areas are experiencing marked decreases, whereas high latitudes show increases in primary production. However, the fate of EBUEs remains broadly unknown. Being hotspots of primary production and fishery resources, they play an important role in human socio-economic activities and, as such, studying the functioning of these systems is a vital aspect to understand both which changes the future ocean is going to experience and how these will affect the relation of humanity with it.
Markel Gómez Letona