| Summary: | Understanding how copepods may respond to ocean acidification (OA) is critical for risk assessments of ocean ecology and biogeochemistry. The perception that copepods are insensitive to OA is largely based on experiments with adult females. Their apparent resilience to increased carbon dioxide (pCO 2 ) concentrations has supported the view that copepods are ‘winners’ under OA. Here, we show that this conclusion is not robust, that sensitivity across different life stages is significantly misrepresented by studies solely using adult females. Stage-specific responses to pCO 2 (385–6000 µatm) were studied across different life stages of a calanoid copepod, monitoring for lethal and sublethal responses. Mortality rates varied significantly across the different life stages, with nauplii showing the highest lethal effects; nauplii mortality rates increased threefold when pCO 2 concentrations reached 1000 µatm (year 2100 scenario) with LC50 at 1084 µatm pCO 2 . In comparison, eggs, early copepodite stages, and adult males and females were not affected lethally until pCO 2 concentrations ≥3000 µatm. Adverse effects on reproduction were found, with >35% decline in nauplii recruitment at 1000 µatm pCO 2 . This suppression of reproductive scope, coupled with the decreased survival of early stage progeny at this pCO 2 concentration, has clear potential to damage population growth dynamics in this species. The disparity in responses seen across the different developmental stages emphasizes the need for a holistic life-cycle approach to make species-level projections to climate change. Significant misrepresentation and error propagation can develop from studies which attempt to project outcomes to future OA conditions solely based on single life history stage exposures.
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