Ocean alkalinity enhancement pelagic impact in the Northwest Iberian Upwelling System under nutrient limited conditions

Abstract. Ocean Alkalinity Enhancement (OAE) is a promising strategy for marine carbon dioxide removal, yet its ecological effects remain largely unexplored in highly dynamic upwelling ecosystems. Here, we examine the response of OAE on a natural plankton community from the NW Iberian Upwelling System (Ría de Vigo) during a diatom bloom in May 2025. Natural in situ plankton communities were sampled aboard R/V Mytilus (May 9^th) and enclosed in nine replicated microcosms exposed to two hydroxide-based OAE treatments and a control (n=3 per group). Microcosms were monitored for 20 consecutive days (9^th-28^th May). Total alkalinity, pH, dissolved inorganic nutrients, chlorophyll-a, biogenic silica, particulate organic carbon and nitrogen were measured, and samples for light microscopy and flow cytometry were collected. All microcosms showed rapid nutrient drawdown, synchronous declines in particulate organic matter, and a transition from a Chaetoceros-dominated diatom assemblage to a more diverse community with increasing contributions of smaller taxa. Microplankton dynamics were characterized by a rapid collapse of the diatom bloom due to inorganic nutrient exhaustion independent of OAE treatments. Only within the smallest size classes there was contrasting OAE responses: while nanoeukaryotes and heterotrophic bacteria remain unaffected, autotrophic picoeukaryotes were negatively influenced by alkalinization, and Synechococcus abundance increased in the OAE treatments. Under nutrient‑limited late diatom‑bloom conditions, OAE does not modify microplankton succession nor alter biomass or the potential efficiency of the biological carbon pump. Pelagic ecological impacts to OAE remain low when resource limitation controls phytoplankton community structure. While OAE did not fundamentally alter the upwelling-driven diatom-bloom dynamics in this highly dynamic ecosystem, the ecological outcome under contrasting hydrographic and seasonal scenarios remains to be evaluated.