Effects of ocean alkalinity enhancement on Southern Ocean phytoplankton growth and community composition

This dataset was generated to assess the biological impacts of ocean alkalinity enhancement (OAE) in the Southern Ocean. Deck-board incubation experiments were conducted at five sites spanning subantarctic to sea-ice regions during the RV Investigator MISO voyage (January–March 2024). Unfiltered surface seawater was incubated for 5–8 days following additions of three alkalinity sources: sodium hydroxide (NaOH), ground olivine, and steel slag, alongside untreated controls. Measurements collected at the beginning and end of each incubation included carbonate chemistry (total alkalinity and pH), macronutrients (nitrate, phosphate, silicate), chlorophyll-a, biogenic silica, photophysiology (Fv/Fm), and plankton community composition. Phytoplankton and bacterial abundances were quantified using flow cytometry, while genus-level phytoplankton composition was determined by light microscopy. A complementary dark leaching experiment quantified dissolved trace metal release from each OAE material using ICP-MS. The purpose of the study was to distinguish the biological effects of alkalinity change alone from those arising from collateral nutrient and trace-metal release, and to evaluate how different OAE materials influence phytoplankton growth and community structure in iron-limited Southern Ocean waters. The dataset supports assessment of ecological risks and co-benefits associated with ocean alkalinity enhancement as a carbon dioxide removal strategy.