Understanding how marine plankton communities respond to Ocean Alkalinity Enhancement (OAE) across contrasting environments is fundamental to building a robust environmental impact assessment framework. The Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP) addresses this challenge through a globally standardised microcosm experiment conducted at 17 locations worldwide, spanning tropical to temperate systems.
Here we present preliminary results from the OAEPIIP experiment conducted at GEOMAR Helmholtz Centre for Ocean Research Kiel in March 2025, using natural plankton communities from the Boknis Eck time-series station (Eckernförde Bay, south-western Baltic Sea; T = 6 °C, S = 16). Nine 55-L microcosms were assigned to three treatments (n=3 each): an unalkalised seawater control, an equilibrated OAE, and a non-equilibrated OAE treatment, both targeting a total alkalinity increase of +500 µmol kg⁻¹. The equilibrated treatment simulated alkalinised seawater re-equilibrated with atmospheric CO₂ prior to release, whereas the non-equilibrated treatment simulated direct NaOH addition, resulting in elevated pH (~9) and strongly suppressed fCO₂ (<50 µatm), potentially inducing transient carbon limitation for phytoplankton.
Preliminary results show that bulk parameters (Chl a, Particulate Organic Carbon, Particulate Organic Nitrogen) declined over the course of the experiment, consistent with a post-bloom community state and did not differ significantly among treatments. In contrast, flow cytometry revealed that the non-equilibrated treatment was consistently distinct from both control and equilibrated treatments across multiple phytoplankton size classes, with differences observed in cell abundances and the temporal trend. Nutrient dynamics showed a transient delay in silicate drawdown in the non-equilibrated treatment during the early experimental phase, consistent with observations from previous OAE studies.
These results suggest that the carbonate chemistry state at the point of alkalinity addition may be a critical driver of ecological responses to OAE, with direct implications for how different OAE implementation strategies are evaluated in environmental impact assessments.
 

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