| Summary: | Biological impacts of ocean acidification have mostly been studied using future levels of CO₂ without consideration of natural variability or how this modulates both duration and magnitude of CO₂ exposure. Here we combine results from laboratory studies on coral reef fish with diurnal in situ CO₂ data from a shallow coral reef, to demonstrate how natural variability alters exposure times for marine organisms under increasingly high-CO₂ conditions. Large in situ CO₂ variability already results in exposure of coral reef fish to short-term CO₂ levels higher than laboratory-derived critical CO₂ levels (~600 μatm). However, we suggest that the in situ exposure time is presently insufficient to induce negative effects observed in laboratory studies. Our results suggest that both exposure time and the magnitude of CO₂ levels will be important in determining the response of organisms to future ocean acidification, where both will increase markedly with future increases in CO₂.
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