Seasonal Upwelling Effects on Seawater Carbonate Chemistry and Ocean Acidification in Northwest Africa

Abstract

The northwest coast of Africa, influenced by the Canary Current, is one of the most productive areas in the world, primarily due to the abundance of small pelagic fish. Upwelling brings high-nutrient sub-surface water to the surface, thus promoting the formation of abundant biomass and considerable fishery resources. Moreover, these upwellings bring sub-surface water high in dissolved inorganic carbon (DIC) with low pH to the surface, reducing the saturation of calcium carbonate and aragonite, making these areas vulnerable to ocean acidification. This can have adverse effects on calcifying marine organisms. The objective of this study is to explore the links between hydrological parameters, carbonate system characteristics, and upwelling activity along the Moroccan Atlantic marine ecosystem between Cape Blanc (21°N) and Cape Cantin (33°N), a region where studies on ocean acidification remain very limited. Upwelling activity was widely observed throughout the study area during the spring season, while in autumn, it was restricted to specific areas, notably between Cape Cantin and Cape Ghir, Cape Draa, between Cape Boujdour and Dakhla, as well as at Cape Blanc, with latitudinal fluctuations. In terms of carbonate chemistry, they are marked by low pHT-insitu, relatively low aragonite saturation, and high DIC concentrations. The intensification of the upwelling phenomenon in spring amplifies the negative ocean acidification state in coastal areas, notably by lowering the pH to 7.8 and reducing the aragonite saturation state to 1.2, thereby impacting the Moroccan upwelling ecosystem, with effects particularly pronounced in the Cap Blanc region. This work provides valuable insights into the carbonate system along the Moroccan Atlantic coast, laying a solid foundation for future research. A long-term biogeochemical analysis is essential to quantify acidification and understand its ecological consequences.