Winners and losers under hydroxide-based ocean alkalinity enhancement in a Tasmanian plankton community

Ocean alkalinity enhancement (OAE) is an emerging carbon dioxide CO₂ removal approach for climate change mitigation and can be implemented with various alkaline materials that convert dissolved CO₂ into (bi)carbonates, enabling additional atmospheric CO₂ removal. A key knowledge gap is how alkaline materials affect marine life. This study investigated effects of OAE via sodium hydroxide (NaOH) on a coastal Tasmanian plankton community. Natural communities were enclosed within microcosms assigned to three groups: a control, an unequilibrated treatment (NaOH addition), and an equilibrated treatment (NaOH and sodium bicarbonate (NaHCO₃) addition). The unequilibrated treatment simulates carbonate chemistry changes before atmospheric CO₂ uptake and the equilibrated treatment the changes thereafter. Treatments increased alkalinity by ~25% (+500 μmol · kg^-1), theoretically enabling a 21% increase in the marine inorganic carbon sink. Hydroxide-based OAE had minimal effects on the plankton community in the equilibrated treatment, in which CO₂ and pH excursions were small. In the unequilibrated treatment, we observed a slight delay in the phytoplankton bloom, arguably because NaOH addition caused reorganization in the diatom community before the bloom reached its maximum chlorophyll a level. Although the community remained diatom-dominant, community composition was moderately different from the control and equilibrated treatments. The zooplankton community displayed no detectable change except for the invasive Noctiluca scintillans, which became less abundant in the unequilibrated treatment, arguably due to phytoplankton community shifts. We concluded changes in plankton community composition observed were relatively small compared to the rather extreme hydroxide-based alkalinity perturbation and the profound climatic benefit of such a CO₂ sink enhancement.