Interactive effects of elevated temperature and pCO2 on early-life-history stages of the giant kelp Macrocystis pyrifera

Rising atmospheric CO2 is expected to increase global temperatures and partial pressure of CO2 in surface waters, causing ocean warming and acidification. These changes may have important consequences for the physiological performance of early life-history stages of marine organisms. In this study we investigated the potential for interactive effects of ecologically relevant levels of temperature and pCO2 on germination, dormancy and mortality of zoospores of the giant kelp Macrocystis pyrifera, a foundation species of temperate reef ecosystems. Newly settled kelp spores were cultured in the laboratory for seven days in a factorial design with temperature (13 °C and 18 °C) and pCO2 (~ 370 and ~ 1800 μatm) as experimental factors. The two levels of temperature and the low-pCO2 treatment in our design were consistent with present-day environmental conditions in the kelp forest as measured by autonomous temperature and pH sensors, while the high-pCO2 treatment reflects an extreme, future acidification scenario. Our results revealed that the combined effects of increased temperature and pCO2 can significantly decrease germination rates and increase the mortality of kelp spores. Interactive effects of temperature and pCO2 were detected on spore mortality and dormancy. Spore mortality only differed between pCO2 treatments at high temperature. In contrast, spore dormancy was higher in the treatment with low temperature and high pCO2, which is similar to the environmental conditions experienced during upwelling events in southern California. Our results highlight the importance of considering multiple stressors to understand how the early-stages of foundation species such as M. pyrifera will be affected by global change.