The effects of rising ocean temperature and pCO2 on the physiology and growth of giant kelp, Macrocystis pyrifera, and grazing by purple urchins, Strongylocentrotus purpuratus

OF THE THESIS The Effects of Rising Ocean Temperature and pCO2 on the Physiology and Growth of Giant Kelp, Macrocystis pyrifera, and Grazing by Purple Urchins, Strongylocentrotus purpuratus by Matthew B. Brown Master of Science in Biology with a Concentration in Ecology San Diego State University, 2013 As climate change rapidly alters the world’s oceans, marine life will have to acclimate and/or adapt to warmer and more acidic conditions. While there is a growing body of literature on the individual effects of elevated temperature and CO2 on marine biota, few studies have examined the synergistic effects of these factors, especially regarding how they impact species interactions. In coastal environments of temperate latitudes, forests of kelp (large brown seaweeds in the Order Laminariales) provide habitat and food for numerous species, support enhanced biodiversity, and provide important ecosystem services. Consequently, impacts to these important ecosystem engineers can have disproportionately large effects on coastal ecosystem functioning. To determine how climate change might impact kelp forest ecosystems, I examined two of the more conspicuous and ecologically important kelp forest species, namely the giant kelp, Macrocystis pyrifera, and the purple sea urchin, Strongylocentrotus purpuratus. First, I performed three separate experiments in order to determine the effects of elevated temperature and pCO2 on M. pyrifera growth and photosynthetic performance. In my first experiment I cultured M. pyrifera meristematic tissues under three pCO2 levels (500, 1000, 1500 atm CO2) and examined how this impacted their growth, steady-state photosynthetic oxygen evolution, and changes in their tissue carbon:nitrogen ratios. In my second experiment, I used a fully factorial design with two temperatures (12 o C and 15 o C) and two pCO2 levels (500 atm and 1500 atm CO2), and examined how these impacted kelp growth, steady-state photosynthetic carbon uptake, and tissue carbon:nitrogen ratios. In my third experiment, I used the same fully factorial design (12 o C and 15 o C; 500 atm and 1500 atm CO2), but examined changes in kelp photosynthetic pigment composition and carbonic anhydrase activity (an estimate of their ability to use HCO3 in photosynthesis). Counter to my expectations, elevating only pCO2 in the water had no effect on kelp growth rates, photosynthesis or tissue carbon:nitrogen ratios in either of the first two experiments. In contrast, in the second experiment, elevating only seawater temperature resulted in a significant reduction in both photosynthesis and growth, and an increase in tissue carbon:nitrogen ratios. However, when seawater temperature and pCO2 were increased together, the kelps exhibited significant increases in photosynthesis and growth relative to the other treatments. This suggested that rising ocean temperatures may interact with rising pCO2 to elicit responses that are different than when either of these factors is increased by itself. In my third experiment, elevating pCO2 in the water significantly reduced carbonic anhydrase activity, suggesting a reduction in HCO3 -based