Biogeochemical and phytoplankton community responses to ocean alkalinity enhancement in coastal waters of Terceira Island (Azores)

The data presented herein originate from a bottle incubation experiment investigating the effects of CO₂-semi-equilibrated ocean alkalinity enhancement (OAE) on a subtropical North Atlantic plankton community collected off Terceira Island (Azores, Portugal) in February 2025. Natural seawater was incubated for six days under three alkalinity levels (ΔTA = 0, 400, and 800 µmol kg⁻¹) and two nutrient regimes (moderate and high nutrient availability) to assess interactions between OAE-induced carbonate chemistry changes and episodic nutrient enrichment. Alkalinity was manipulated using sodium carbonate (Na₂CO₃) and calcium chloride (CaCl₂·2H₂O), simulating carbonate-based, CO₂-semi-equilibrated OAE conditions. The dataset comprises carbonate chemistry parameters, dissolved inorganic nutrients, particulate organic carbon and nitrogen concentrations, chlorophyll a, and phytoplankton community composition determined by microscopy. The experiment provides insights into how OAE influences phytoplankton community structure, particulate stoichiometry, and export-relevant traits that may affect biologically mediated carbon sequestration.