Decreased pH increases predation rates of a gelatinous predator

Chair: Ken Caldeira

Edd Hammill (1)*, Ellery Johnson (2), Januar Harianto (3), Maria Byrne (3)

1 Utah State University, Logan, UT, 84341, USA
2 University of Technology Sydney, NSW 2006, Australia
3 University of Sydney, NSW 2006, Australia

Background
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. Should ocean acidification reduce the ability of the exoskeleton to perform its defensive functions, pH-mediated changes to exoskeleton thickness need not be fatal in isolation, as sub-lethal changes may increase susceptibility to predation.

Methods
We conducted a two-way experiment to understand how changes in oceanic pH affect trophic interactions between calcified zooplankton and a gelatinous predator. pH levels were set at either ambient (pH 8.2) or reduced (pH 7.8), and crossed with the presence/absence of the cubozoan jellyfish Carybdea rastoni.

Findings
In isolation, reduced pH or the presence of Carybdea rastoni led to changes in zooplankton community composition. Crucially, the combined effects of changes in pH and Carybdea rastoni had a greater impact on overall zooplankton community composition than would be predicted from combining the effect of each factor in isolation. It appeared that this antagonistic effect was generated by low pH increasing predation rates of Carybdea rastoni on the most abundant zooplankton subclass (copepoda).

Conclusions
Our results indicate that the ecological consequences of changes in oceanic pH may be greater than predicted from single-species experiments alone. By adjusting the strength of interactions between predators and prey, changes to ocean pH have the potential to destabilise population dynamics and community structure, despite not being fatal in and of themselves. We suggest that increased acidity may benefit gelatinous predators by increasing prey availability, potentially increasing the risk of jelly fish blooms.