Zooplankton community responses to Ocean Alkalinity Enhancement

Reducing atmospheric CO2 concentrations while limiting further ocean acidification has increased interest in marine carbon dioxide removal (mCDR) approaches. Ocean Alkalinity Enhancement (OAE) aims to increase seawater total alkalinity (TA), lower surface-water pCO2, and thereby enhance air-to-sea CO2 uptake and storage predominantly as bicarbonate. Despite its theoretical potential, the ecological consequences of OAE remain insufficiently constrained, particularly for zooplankton that shape marine food webs and mediate carbon transfer to depth. This thesis quantifies how OAE-driven shifts in carbonate chemistry affect zooplankton performance and community dynamics, with explicit attention to non-CO2-equilibrated conditions and to the relative importance of direct physiological stress versus indirect, food-web mediated effects.
Three experimental studies combined mesocosm deployments with controlled laboratory incubations. First, two mineral-based OAE approaches were evaluated in coastal post-bloom waters using slaked lime and olivine across a ΔTA range up to 600 µmol kg-1. The appendicularian Oikopleura dioica showed no detectable changes in abundance, feeding performance, or house production across treatments, suggesting resilience of this gelatinous zooplankton and limited risk to larvacean-mediated particle export within this alkalinity range. Second, a spring bloom mesocosm experiment applied a wider ΔTA gradient reaching 1250 µmol kg-1 and compared immediate versus delayed mixing. Zooplankton recruitment, abundance, and diversity increased under moderate alkalinity additions but declined when ΔTA exceeded approximately 750 µmol kg-1. The decline coincided with delayed phytoplankton bloom development and reduced availability of suitable prey during critical larval stages, consistent with a trophic mismatch mechanism rather than solely direct chemical stress. Third, laboratory incubations with the copepod Temora longicornis showed dose-dependent metabolic impairment with increasing ΔTA, consistent with elevated energetic costs associated with acid-base regulation. At the same time, OAE altered prey elemental properties, improving aspects of food quality and partially buffering copepod performance when nutritional conditions were favorable.
Together, these findings support a precautionary ecological threshold near ΔTA of about 750 µmol kg-1, beyond which risks of disrupted recruitment, altered community composition, and cascading food-web effects may increase. The thesis provides mechanistic evidence and experimentally derived constraints that can inform environmentally responsible OAE design, including guidance for deployment intensity, mixing considerations, and monitoring strategies aligned with future monitoring, reporting, and verification frameworks.