EFFECTS OF OCEAN ACIDIFICATION ON PREDATOR-PREY INTERACTIONS IN ECHINODERMS by Megan Lillian Hatfield Vaughan Hon. B.Sc., Dalhousie University, 2011 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in The Faculty of Graduate and Postdoctoral Studies (Zoology) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2015 © Megan Lillian Hatfield Vaughan, 2015 Abstract The need to understand future changes in marine ecosystems has become critically important as increasing atmospheric carbon dioxide (CO2) drives rapid ocean acidification (OA). OA may improve or reduce the performance of marine species, and the relative impacts on interacting species will largely determine changes at the community level. The goal of this thesis was to determine the effects of acidification on predator-prey interactions between red sea urchins (Strongylocentrotus franciscanus) and sunflower stars (Pycnopodia helianthoides), a key predator-prey pair in Northeast Pacific kelp forest ecosystems. I tested this question using laboratory mesocosm experiments. Sea urchins were acclimated to ambient (pCO2 ~ 500 μatm, pH ~ 8.0) or acidified (pCO2 ~ 1000 μatm, pH ~ 7.7) conditions, with or without a caged sea star, for 22 weeks in a recirculating seawater system. In Chapter 2, I investigated the effects of OA on the growth, calcification, and feeding rate of P. helianthoides. High CO2 had a significant positive effect on sea star growth, but no effect on calcified tissue mass. In addition, the consumption rate of turban snails (Chlorostoma funebralis) by sea stars was significantly higher in the high CO2 treatment. In Chapter 3, I examined the effects of OA on the responses of S. franciscanus to sea star cues. Predator presence and high CO2 negatively and additively affected sea urchin growth rates, but did not affect alarm responses to predator cues. Significantly higher grazing rates on kelp (Macrocystis pyrifera) were also observed in the presence of predators. Predators, but not CO2, had a significant negative effect on urchin calcified mass. Urchin spine length was also significantly reduced under acidified conditions. Overall, these findings suggest P. helianthoides responds positively to ocean acidification, but S. franciscanus may suffer reduced fitness at seawater pCO2 levels predicted for the end of the century. Differential effects ii of ocean acidification on this predator-prey pair could increase the strength of the trophic interaction and lead to stronger top-down control in the future. iii Preface Chapters 2 and 3 were conceived by myself in collaboration with Chris Harley. I collected and analyzed the data, and wrote the manuscripts. Chris Harley contributed to interpreting the results and edited the manuscripts. Renee Bechmann at the International Research Institute of Stavanger also contributed to experimental design. iv Table of Contents Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents .......................................................................................................................... v List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix List of Symbols and Abbreviations ............................................................................................ xi Acknowledgements ..................................................................................................................... xii Dedication ................................................................................................................................... xiv Chapter 1: Introduction ............................................................................................................... 1 1.1 Ocean Acidification and the Marine Carbonate System .................................................. 1 1.2 Effects on Marine Organisms and Implications for Species Interactions ........................ 4 1.2.1 Effects of OA on calcification ................................................................................ 5 1.2.2 Effects of OA on growth ......................................................................................... 6 1.2.3 Effects of OA on olfaction ...................................................................................... 7 1.3 Thesis Overview and Objectives ..................................................................................... 8 1.3.1 Study species ........................................................................................................... 9 1.3.2 Research questions ................................................................................................ 10 Chapter 2: Effects of Ocean Acidification on the Growth, Feeding Rate, and Calcification of Pycnopodia helianthiodes ................................................................................. 11 2.1 Introduction .................................................................................................................... 11 2.2 Methods.......................................................................................................................... 12 2.2.1 Collection site and experimental set-up ................................................................ 12 v 2.2.2 Manipulation and measurement of seawater chemistry ........................................ 13 2.2.3 Measurement of sea star growth, calcification, and feeding rate .......................... 14 2.2.4 Statistical analysis .................................................................................................. 15 2.3 Results ............................................................................................................................ 15 2.4 Discussion ...................................................................................................................... 19 Chapter 3: Nonconsumptive Effects of a Predatory Sea Star on Red Sea Urchins (Strongylocentrotus franciscanus) under Acidified Conditions ............................................... 22 3.1 Introduction .................................................................................................................... 22 3.2 Methods.......................................................................................................................... 25 3.2.1 Collection site and experimental set-up ................................................................ 25 3.2.2 Measurement of sea urchin growth and grazing rate ............................................ 26 3.2.3 Measurement of sea urchin alarm response to predator cues ............................... 26 3.2.4 Measurement of sea urchin calcified tissue .......................................................... 27 3.2.5 Statistical analysis ................................................................................................. 27 3.3 Results ............................................................................................................................ 28 3.4 Discussion ...................................................................................................................... 35 Chapter 4: Conclusion ................................................................................................................ 40 4.1 Summary of Results ...................................................................................................... 40 4.2 Effects of OA on Predator-Prey Interactions ................................................................ 41 4.3 Study Limitations .......................................................................................................... 43 4.4 Recommendations for Future Research ........................................................................ 43 Bibliography ................................................................................................................................ 46 Appendices ................................................................................................................................... 55 vi A. Seawater Chemistry ........................................................................................................ 55 B. Chapter 2 Supplementary Figures ................................................................................... 60 C. Chapter 3 Supplementary Figures ................................................................................... 63 vii List of Tables 2-1. Cox Proportional Hazards analysis testing the effect of CO2 treatment (control/high) on the survival of turban snails (Chlorostoma funebralis) fed to sunflower stars (Pycnopodia helianthoides). Sea star size (diameter) was included as a covariate. The significance of a covariate was tested by comparing the log likelihood of a submodel (excluding the covariate) to the log likelihood of the full model. (*) indicates a significant effect. 3-1. Linear mixed effects models testing the effect of CO2 treatment (control/high) and predator treatment (absent/present) on A) log final test diameter, B) log calcified mass,