Impacts of ocean acidification on an intertidal predator-prey system : a multiple life-stages perspective

Abstract

The anthropogenic-induced increase in atmospheric CO2 concentrations, being absorbed by the oceans has led to ocean acidification (OA), causing a reduction in pH and changes in seawater carbonate chemistry. Particularly, coastal and intertidal habitats are predicted, in the near future, to experience more severe acidified conditions. Differential responses to OA of species related by trophic links are expected to alter interspecific interactions, and have the potentials to drive further impacts to marine communities and ecosystems. Despite this threat, knowledge on the impacts of OA on predator-prey systems remains limited. Given that many marine organisms experience complex life cycles, the evaluation of the impacts of OA on individual species should be extended to consider multiple life stages responses. Consequently, the direct and indirect effects that two species involved in a trophic interaction could induce on each other, under OA conditions at different life stages, is still underexplored. This thesis, through an experimental approach, addresses this knowledge gap and aspects of an intertidal predator-prey system under projected OA conditions. I investigated multiple life-stages’ responses to different pH levels, covering present and future conditions, in of two species that, at the adult stage, are part of a predator-prey interaction. Predator-prey interactions were investigated by testing the inducible effect of predator pressure in conjunction with OA on the prey, and the preferential consumption by the predator on prey acclimated to different pH levels. The study prey, the rock oyster Saccostrea cucullata, is a coastal foundation species and its main predator, the whelk Reishia clavigera, likewise plays an important ecological role on rocky shores. Laboratory cultures of the two species’ embryos and larvae in closed-systems and intertidal acidified mesocosms to acclimatise juveniles and adults, under projected OA scenarios, were implemented to expose the species to different conditions at varying life stages. As reflected by the overall mortality, the planktonic larval stages were more sensitive than the benthic stages for both species, with the whelk being more susceptible to OA compared to the oyster, which showed higher tolerance from early life stages. The reproductive strategy of embryonic encapsulation exhibited by whelk, however, led to milder OA impacts on its planktonic larval phase. The indirect effect of predator pressure negatively impacted juvenile but not adult oysters’ survival. Under the combined effect with OA, predator presence reduced the growth of juvenile oysters, which showed increased energetic requirements. In contrast to expectations, the whelk feeding rates did not differ either when the predator or prey were exposed to OA. Although feeding at the same rate of control whelks, Reishia held at low pH (7.4) lost more tissue weight and maintained higher metabolic rates, suggesting an alteration in the assimilation and/or excretion rate under OA. The findings presented, therefore, although considering locally-relevant pH conditions, provide important insights for studying future OA effects on intertidal communities, through investigating responses of multiple life-history stages of functionally important and widely distributed species such as Reishia and Saccostrea on Indo-Pacific rocky shores.