Seasonal upwelling drives surface water biogeochemistry with implication for ocean acidification along the Northwest African coast

Chaimaa Jamal^1,2*Ahmed Makaoui²Melissa Chierici³David Cervantes³Adil Chair^2,4Hasnaa Nait Hammou^2,4Mohammed Idrissi²Fatima Zohra Bouthir²Omar Ettahiri²Samia Yousfi¹Mouna Latifa Bouamrani¹

• ¹Ben M'sik Faculty of Sciences, University of Hassan II Casablanca, Casablanca, Morocco
• ²Institut National de Recherche Halieutique (INRH), Casablanca, Morocco
• ³Norwegian Institute of Marine Research (IMR), Bergen, Hordaland, Norway
• ⁴Université Chouaib Doukkali, El Jadida, Casablanca-Settat, Morocco

This study assesses the impact of intensified upwelling on the marine carbonate system by analyzing the surface distribution of key environmental parameters along the Northwest African coast, between Cape Blanc (21°N) and Cape Cantin (33°N), a region where ocean acidification observations remain limited. Here we focus on surface water variability based on data from two oceanographic surveys conducted aboard the R/V Dr. Fridtjof Nansen in spring and autumn 2022. The analysis is based on observational data of temperature, salinity, dissolved oxygen, chlorophyll a, pH, and total alkalinity, and derived carbonate system variables. In spring, upwelling activity was widespread across the study area, whereas in autumn, it was localized near Cape Draa (28°30N), between Cape Boujdour and Dakhla (25°30N–23°30N), and at Cape Blanc (21°N). Both spring and autumn are influenced by low oxygenated South Atlantic Central Water (SACW), which reaches the surface along the coast. The upwelling of SACW is rich in dissolved inorganic carbon (DIC 2160 to 2250 µmol/kg), leading to decrease in pH (~7.85 to 7.95) and aragonite saturation state (ΩAr ~1.5 to 2.5). The strongest acidification signals were observed at Cape Draa (28°30N) and Cape Blanc (21°N) during autumn, where the lowest pH (7.8) and ΩAr (1.5), along with the highest DIC (2250 µmol/kg), were recorded. The study clearly show that lowest acidification state and highest DIC values were related to the influence of the upwelling of SACW at Cape Blanc. It was also evident that areas with high chlorophyll a coincided with higher ΩAr and pH in spring. This suggest that primary production (PP) in spring counteracts the effect of upwelled low pH water along the coast. Areas of high PP, such as at Cape Draa (28°30N), experience increased DIC levels and enhanced acidification after the bloom season, potentially influenced by organic matter remineralization. This study underscores the impact of upwelling and biological drivers on the biogeochemistry and carbonate system dynamics along the Northwest African coast. It highlights the necessity of long-term monitoring to assess ocean acidification trends and their ecological implications.