Temperature affects the early life history stages of corals more than near future ocean acidification

Climate change is projected to increase ocean temperatures by at least 2 C, and levels of pH by similar to 0.2 units (ocean acidification, OA) by the end of this century. While the effects of these stressors on marine organisms have been relatively well explored in isolation, possible interactions between temperature and OA have yet to be thoroughly investigated. OA at levels projected to occur within this century has few direct ecological effects on the early life history stages of corals. In contrast, temperature has pronounced effects on many stages in the early life history of corals. Here, we test whether temperature might act in combination with OA to produce a measurable ecological effect on fertilization, development, larval survivorship or metamorphosis of 2 broadcast spawning species, Acropora millepora and A. tenuis, from the Great Barrier Reef. We used 4 treatments: control, high temperature (+2 degrees C), high partial pressure of CO2 (pCO(2)) (700 mu atm) and a combination of high temperature and high pCO(2), corresponding to the current levels of these variables and the projected values for the end of this century under the IPCC A2 scenario. We found no consistent effect of elevated pCO(2) on fertilization, development, survivorship or metamorphosis, neither alone nor in combination with temperature. In contrast, a 2 degrees C rise in temperature increased rates of development, but otherwise had no consistent effect on fertilization, survivorship or metamorphosis. We conclude that OA is unlikely to be a direct threat to the early life history stages of corals, at least in the near future. In contrast, rising sea temperatures are likely to affect coral population dynamics by increasing the rate of larval development with resulting changes in patterns of connectivity.