Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal strategy that encompasses a range of approaches aimed at increasing the carbon dioxide (CO₂) uptake capacity of seawater through the addition of alkaline materials. Perturbations to carbonate chemistry underpin all OAE applications, with the most pronounced changes occurring under CO₂-unequilibrated alkalinization, where CO₂ uptake is intended to occur after alkalinity has been released into the marine environment. Although our understanding of the ecological impacts of OAE on microbial communities that form the base of marine food webs is rapidly growing, no experimental studies have been conducted in Arctic waters. To help address this knowledge gap, we conducted a 7-day bottle incubation experiment (5-12 June 2026) using surface seawater and its microbial plankton community (<200 µm) collected from the Fram Strait (74°55.800′N, 11°17.572′W) aboard Le Commandant Charcot during cruise LCC280526. The experiment tested six sustained CO₂-unequilibrated alkalinity treatments, each in triplicate, spanning ambient conditions to an increase of approximately +450 µmol kg⁻¹ in increments of ~90 µmol kg⁻¹. This generated pH and pCO₂ gradients from approximately 8.06 and 406 µatm under ambient conditions to 8.80 and 58 µatm in the highest OAE treatment, respectively. To assess the effects of unequilibrated OAE on the enclosed phytoplankton community, we monitored daily photosynthetic health (Fv/Fm) and minimum fluorescence (F₀, as a proxy for chlorophyll concentration) using fast repetition rate fluorometry, and every other day cell abundance and size structure using flow cytometry. Pigment composition, and thus community composition, was determined at the beginning and end of the experiment using high-performance liquid chromatography. The results will be discussed within the broader context of previous studies investigating OAE impacts on phytoplankton communities across ocean regions to begin assessing the relative sensitivity of Arctic phytoplankton communities to OAE.
 

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