The influence of ocean alkalinity enhancement on biological carbon sequestration

Limiting global warming to below 2°C will likely require negative emission technologies alongside decarbonization to address hard-to-abate CO₂ emissions. Ocean alkalinity enhancement (OAE) has been proposed as one such approach. By increasing seawater alkalinity, OAE shifts the carbonate system, allowing for atmospheric CO₂ uptake. However, OAE may also alter marine communities and the biological carbon pump, potentially affecting long-term carbon sequestration. Understanding these effects is therefore essential for assessing its net carbon removal benefit.

This doctoral thesis investigated how OAE-induced carbonate chemistry perturbations affect marine communities and processes regulating the magnitude and efficiency of the biological carbon pump. Particular emphasis was placed on particle formation, transformations during sinking, and export stoichiometry. Three experiments were conducted across contrasting ecosystems and OAE application scenarios.

The results demonstrate that OAE effects depended strongly on perturbation intensity and ecosystem context. Under moderate, CO₂-equilibrated OAE in an oligotrophic subtropical community, export magnitude and efficiency remained largely unchanged. At higher alkalinity, however, abiotic carbonate precipitation occurred, consuming added alkalinity and releasing CO₂. In a eutrophic, diatom-dominated coastal bloom exposed to stronger, CO₂-unequilibrated perturbations, OAE reduced silica ballasting during export, consistent with enhanced biogenic silica dissolution under elevated pH. A further incubation showed community restructuring despite stable biomass: a dominant diatom species declined with alkalinity, while particulate C:N ratios co-varied with community composition, indicating effects on export-relevant traits.

Taken together, the findings show that OAE may leave export processes unchanged under moderate application scenarios, but stronger perturbations can weaken carbon sequestration through community restructuring, altered stoichiometry, export-phase transformations, and abiotic feedbacks. The central conclusion is that apparent OAE success may diverge from actual success: carbonate chemistry and biomass may respond as intended while biological and abiotic feedbacks reduce net CO₂ removal.