Risks and co-benefits of ocean alkalinity enhancement in marine plankton communities

Limiting global warming well below 2 °C will require deep greenhouse gas emission cuts alongside carbon dioxide removal (CDR). Ocean alkalinity enhancement (OAE) is a promising marine CDR approach. By increasing seawater alkalinity, OAE promotes atmospheric CO₂ uptake and long-term ocean storage. OAE approaches differ in alkalinity source and deployment, affecting seawater perturbation and ecological impacts. Yet consequences for plankton communities remain poorly understood. This thesis investigates OAE effects on mesozooplankton and associated plankton through two large-scale mesocosm experiments and a synthesis of published studies.

Publication I reports a subtropical mesocosm experiment using CO₂-equilibrated OAE. Food web structure and function were largely unaffected by the moderate changes in pH and carbonate saturation. Mesozooplankton remained stable, likely reflecting copepod tolerance. Short-term effects on reproduction and productivity dissipated over time, likely constrained by overarching food limitation.

Publications II and III examine a 54-day mesocosm experiment in a Norwegian fjord under contrasting productivity regimes. CO₂-unequilibrated OAE was applied along lime- and silicate-based gradients. In publication II, mesozooplankton again showed little response. While OAE impacts on bottom-up and top-down pathways were amplified during blooms, these changes were not reflected in mesozooplankton, potentially because large fish populations suppressed them. In publication III focus was placed on calcifying zooplankton, hypothesized to benefit from OAE´s potential alleviation of acidification. However, calcifiers in this experiment showed no evidence for such mechanism.

In line with our experimental results, a synthesis of 24 studies showed that ecological responses were more frequent under more intense OAE scenarios and that food web conditions influenced effects, and that mesozooplankton and higher consumers are severely understudied. To successfully define the boundaries of OAE´s safe operating space, future research should expand our knowledge of plankton consumer responses, particularly within multitrophic contexts and under distinct trophic regimes, including blooms.