Climate change is impacting marine ecosystems worldwide through a suite of associated stressors, including ocean acidification, warming, and hypoxia. However, our current understanding of the effects of climate change are primarily confined to single species, single stressor studies. To gain a broader understanding of its impacts at the community level, it is critical to examine interactions between species under multiple stressors. To address this, we examined the effects of ocean acidification and hypoxia within giant kelp forest grazing communities of the California Current, systems that are characterized by rich diversity and the natural co-occurrence of low pH, low oxygen waters associated with upwelling. We measured interaction strengths between cultured juvenile giant kelp and four species of invertebrate grazers under the two stressors in laboratory factorial experiments. Two sets of four-day experiments were run, one using the microscopic stage and the other using the macroscopic stage of juvenile kelp to additionally test for the effect of kelp size on interaction strengths.
Results/Conclusions
In experiments with the microscopic stage of juvenile kelp, interaction strengths varied across species, with the brown turban snail (Chlorostoma brunnea) showing the strongest interaction, followed by the kelp isopod (Idotea resecata), kelp curler amphipod (Peramphithoe humeralis), and the purple urchin (Strongylocentrotus purpuratus). Overall, hypoxia negatively impacted interaction strengths more than acidification, suggesting that hypoxia can drive changes in feeding behavior even over short timeframes.
General patterns in species’ responses to the two stressors did not change depending on kelp size; however, the effects of grazers varied with kelp life stage. With the macroscopic stage, brown turban snails maintained high grazing rates. Purple urchins had a stronger interaction with the macroscopic stage, suggesting their mouthparts are more efficient at grazing larger stages of kelp. The crustaceans experienced a weakening of interaction strengths when offered the macroscopic stage, possibly due to their smaller size and finer feeding structures. The brown turban snail, an often overlooked grazer in kelp forests, emerges as a dominant grazer on these smaller stages of juvenile kelp. By examining outcomes of species interactions within a multiple stressors context, this study provides key insights into how kelp forest communities might function in the future.