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Rising atmospheric CO2 alters the ocean biochemistry in the process known as ocean acidification (OA). It influences biodiversity at different levels, including zooplankton, which is a key component of aquatic communities and plays a pivotal role in the structure and functioning of marine planktonic food webs as a major link between pelagic primary producers and planktivorous. The effect of OA on the fitness of individual zooplanktonic species has been reported by many studies mostly developed under laboratory conditions. In this context, this chapter reviews the OA effects on zooplankton and describes the potential of natural shallow-water CO2 vents as in situ laboratories. The impact on zooplankton assemblages is shown from a study in the North Atlantic (Azores islands) and the suitability of this area for future studies on marine organisms and ecosystems. Sites with naturally elevated CO2 conditions are described, including which variables and limitations must be considered. Results shown are highly relevant to improve our predictions of the responses of zooplankton to climate change stressors including OA. Future studies including long-term multigenerational exposure to multiple stressors (e.g. increased pCO2 and food shortage) are a priority to understand the adaptation capacity of common species and how the zooplankton communities will shift.

Noticeable changes in global temperatures, climate and ocean carbon chemistry are the result of carbon dioxide increase in the atmosphere. This increase has been mitigated by the oceans capacity to absorb one-fourth of the carbon dioxide in the atmosphere, although this CO2 intake affects oceans carbonate chemistry [i.e., ocean acidification—(OA)]. The detrimental effect of OA in the development and shell formation has been studied in several species of bivalves, although no information is available on the wedge shell Donax trunculus, a gastronomically appreciated species and an important economical resource in several southern European countries. We evaluated the effect of pCO2 increase on hatching and early life stages of D. trunculus, considering two ocean acidification scenarios (ΔpH −0.3 and ΔpH −0.6). Our results showed that elevated pCO2 caused a delay in hatching into D-larvae and reduced larvae survival. In the extreme scenario (ΔpH −0.6), some trochophore larvae persisted to day 9 of the experiment and more abnormal larvae were produced than in the ΔpH −0.3 and control treatments. At day 5, normal veligers under extreme acidification were smaller than in other treatments, but by day 9, these differences were attenuated and the average size of normal D-larvae varied inversely to the pH gradient. Possible underlying mechanisms for these complex response patterns are discussed, including the existence of phenotypic plasticity or genetic pre-adaptive capacity in this D. trunculus population to cope with future environmental changes.