Ocean alkalinity enhancement (OAE) is increasingly considered as a climate mitigation strategy, yet its biological impacts across taxa and life stages remain insufficiently constrained. Understanding organismal responses to altered carbonate chemistry is essential for evaluating ecological risk and informing safe deployment. Here, we synthesize experimental evidence across multiple marine invertebrates, including crabs, krill, copepods, oyster larvae, and echinoderms, to examine sensitivity to OAE under controlled laboratory conditions. Using phase-structured exposure regimes, we investigate responses to both transient and sustained carbonate chemistry changes across early life stages and developmental transitions. Endpoints include survival, molting, development, and calcification, alongside sublethal indicators of physiological stress. Our approach focuses on identifying biological thresholds and response domains that differentiate lethal from sublethal effects. Across taxa, early developmental stages emerge as particularly sensitive, with disruptions to molting and development occurring under conditions that do not induce acute mortality. These patterns suggest a hierarchical sensitivity structure, where sublethal processes respond at lower thresholds than survival, with potential consequences for recruitment and population dynamics. Persistent governance fragmentation, limited MRV capacity, and uneven readiness highlight the need to operationalize biological thresholds as decision-relevant indicators, enabling standardized monitoring, risk assessment, and credible, scalable deployment pathways. We further evaluate how experimentally derived thresholds can inform monitoring strategies by identifying early-warning biological indicators that precede population-level impacts. This work advances a biology-centered framework for evaluating OAE impacts and highlights the importance of incorporating mechanistic, life-stage-resolved responses into assessment, monitoring, and governance of ocean-based climate interventions.