Oceanic zooplankton form the key link between primary producers and higher trophic levels in the world’s largest food chains. This role as a key link means any disruption to predator-prey interactions within the zooplankton community could have far-reaching consequences for oceanic food webs. Single-species experiments show how ocean acidification can dramatically affect zooplankton exoskeletons, reducing overall thickness and producing deformations. In addition, increased acidity has been shown to cause physiological stress, meaning resources may have to be diverted away from muscle development. Should ocean acidification reduce the ability of zooplankton to defend against the threat of predation, the physiological effects of acidification need not be fatal in isolation, as sub-lethal changes may increase susceptibility to predation.
We conducted a two-way factorial experiment to understand how changes in ocean chemistry affect trophic interactions between calcified zooplankton and a gelatinous predator. Zooplankton communities were harvested from coastal eastern Australia, and exposed to acidified conditions and the cubozoan predator Carybdea rastoni. Ocean chemistry was modified by bubbling ambient or CO2-enriched air into a system of flow-through plankton kreisels. The system was able to supply a steady flow of seawater under either ambient (pH 8.2) or acidified conditions (pH 7.8).
In isolation, increased acidity or the presence of predatory Carybdea rastoni led to changes in zooplankton community composition. Crucially, when plankton communities were exposed to both increased acidity and Carybdea rastoni, the impact was greater than would be predicted from the results of each stressor in isolation. It appeared that this antagonistic effect was generated by low pH increasing predation rates of Carybdea rastoni on several taxa within the zooplankton community, including the most abundant zooplankton subclass (copepoda).
Our results indicate that the ecological consequences of increased ocean acidity may be greater than predicted from single-species experiments alone. By adjusting the strength of interactions between predators and prey, changes to ocean acidity have the potential to destabilise population dynamics and community structure, despite not being directly fatal. Crucially, gelatinous predators have been shown to be relatively tolerant of changes in ocean acidity. Given the differences in physiology and susceptibility to ocean acidity among crustacean and molluscan zooplankton compared to their cnidarian predators, we suggest that increased acidity could potentially benefit gelatinous predators by increasing their access to prey.