The increases in ocean temperature and pCO2 due to climate change are projected to affect the growth and future prevalence of Harmful Algal Blooms (HABs) in nearshore waters, but systematic studies on the effects these climate drivers have on harmful algal species are lacking.In particular, little is known about how future climate scenarios will affect the growth of the toxic dinoflagellate Alexandrium catenella, which produces the toxins responsible for paralytic shellfish poisoning (PSP) that threaten the health and economy of coastal communities in the Gulf of Maine. I examined growth responses of A. catenella and two other naturally co-occurring dinoflagellates from Gulf of Maine-Scrippsiella sp., and Amphidinium carteraein mono and mixed species cultures. Experimental treatments included elevated temperature, lower pH, and the combination of elevated temperature and lower pH as projected for the year 2100 (20°C; pH 7.8), relative to current conditions (15.5°C; pH 8.1). Results show decreased growth rates of A. catenella under elevated temperature and lower pH, and that the decreased in growth rate was largely attributable to the effect of temperature. In contrast, the growth rates of Scrippsiella sp. and A. carterae increased under elevated temperature and lower pH conditions, with temperature being the primary driver of the response. These trends did not change substantially when these species were grown in mixed cultures (A. catenella + Scrippsiella sp., and A. catenella + A. carterae), indicating that allelopathic or competitive interactions did not affect the experimental outcome under the conditions tested. These findings suggest that A. catenella blooms will become less prevalent in the Gulf of Maine with continued climate change, shifting instead to a dominance by other dinoflagellate species. ii ACKNOWLEDGMENTS First of all, I would like to thank everyone who helped me in completing this thesis and who helped me in other academic stuff. To Mark Wells and Lee Karp-Boss, I am very grateful to have both of you as my advisors. I can't say anything but thanks for your dedication, support, and patience in mentoring me during my master's study here. To my committee members, David Townsend and Lawrence Mayer, thank you for your valuable suggestions and comments on my thesis research and writing. Both of you are my inspiration for becoming a scientist. I would like to thank Nils Haentjens for helping me run the Imaging FlowCytoot and using his code in MATLAB software, to Lee's lab (past) students, especially to Grace Weise and Stephanie Ayres who taught me how to culture phytoplankton and make media, to Maura Thomas who helped me analyzing the concentration of nutrients in some of my cultures, to Susan Brawley who gave me constructive comments on my research, and to Susanne Thibodeau, Jodie Feero and Carrie Love for their assistance in administration and ordering my experimental material.
