Resilience of the gelatinous zooplankton species Oikopleura dioica to ocean alkalinity enhancement

Ocean alkalinity enhancement (OAE) through mineral dissolution is a promising marine carbon dioxide removal strategy because it increases the buffering capacity of seawater and thereby enhances passive storage of atmospheric CO2. However, the ecological consequences of OAE for zooplankton, particularly gelatinous species, remain poorly understood. Here, we assessed the response of a key gelatinous zooplankton species to OAE in a 53-day mesocosm experiment in a temperate Norwegian fjord. Oikopleura dioica is a globally distributed zooplankton member, known for its high secondary production capacity and key role in vertical carbon flux. O. dioica continuously produces mucous feeding structures ('houses'), which efficiently retain submicron particles. Once discarded, these houses can sink rapidly and contribute to vertical carbon exports. To test the impacts of OAE on O. dioica abundances and their house production capacity, we exposed natural plankton communities to non-CO2-equilibrated OAE scenarios spanning a ΔTA range from 0-600 μmol kg-1, using silicate-based (olivine) and calcium-based (slaked lime) minerals. Population dynamics of O. dioica were monitored alongside the plankton community, and targeted bottle incubations were used to quantify house production and feeding rates. We show that O. dioica abundances varied by an order of magnitude within and across treatments. No interaction between O. dioica abundance and alkalinity levels or mineral types could be detected. Instead, O. dioica abundance variations were primarily explained by prey availability (picoplankton). Additionally, house production and feeding rate experiments showed that O. dioica were unaffected by OAE across all treatments. These findings indicate that O. dioica, as a key gelatinous zooplankton member, is physiologically resilient to OAE within the tested range. Future studies should incorporate gelatinous zooplankton into OAE assessments and investigate higher alkalinity perturbations to evaluate potential ecosystem impacts and larvacean-mediated changes in carbon export during under OAE deployments above ΔTA 600 μmol kg-1.