by Dawn Barlow
Both ocean temperatures and pH are projected to increase due to climate change in the near future—it is predicted that temperatures will be raised by 2°C and that acidity will increase by ~0.2 pH units by the end of the century. While much investigation has been done on the effects of temperature and acidity on the ability of adult corals to form the structure necessary to maintain the integrity of the reef, Chua et al. (2013) investigated the direct effects of increased temperature and acidity on the early life history stages of corals. They looked at fertilization, development, survivorship, and metamorphosis of coral larvae under control conditions as well as under elevated temperature and acidity, both separately and in combination. When the two factors were combined, the results were inconsistent. Overall, the conclusion drawn from this study was that acidification alone is unlikely to be a direct threat to early life history stages of corals, at least in the near future. Increasing temperature, on the other hand, was found to increase the rate of larval development and thereby affect coral population dynamics by changing patterns of connectivity.
Chua et al. carried out this investigation in the lab using larvae samples of Acropora millepora and A. tenius collected from the Great Barrier Reef and kept in aquaria until spawning. The larvae were exposed to four treatments: ambient temperature and ambient pCO2, ambient temperature and elevated pCO2, elevated temperature and ambient pCO2, and elevated temperature and elevated pCO2. Experimental temperatures were maintained using aquarium heaters, and experimental pCO2 levels were maintained using a CO2 mixing system developed by Munday et al. (2009) that involves bubbling mixed gasses through sump tanks. The two levels of CO2 concentration and temperature were selected based on the European Project on Ocean Acidification protocol.
The results of this study were highly variable, and given the number of treatments tested, not many conclusions could be drawn from the results. No interactions were shown between temperature and pCO2, but during development, motility was achieved more rapidly by the coral larvae under elevated temperature conditions. The effects of elevated temperature and pCO2 were also variable between the species of coral and treatments, but overall mean survivorship trended toward being lower under combined elevated conditions. The strongest result was that temperature on its own accelerated development. Elevated pCO2 did not have any effect on metamorphosis, either on its own or in combination with elevated temperature. Neither temperature, elevated pCO2, nor the combination of the two had any significant effect on fertilization.
From the results of this experiment, Chua et al. concluded that elevated pCO2 rarely affects development in early life stages of corals, and only in combination with elevated temperature, contrary to previous predictions. When temperature was increased by 2°C the early life stages of development of corals were affected, which is consistent with metabolic theory. Though some coral species may be more susceptible to temperature and elevated pCO2 stress than others, the overall conclusion of this study is that acidification will likely have a minimal effect on the ecology of early life history stages of corals. The ecological implications of more rapid rates of development under elevated temperatures are likely to be highly dependent on the region, species, and local conditions such as reef density and hydrodynamics. In the near future, the temperature increase rather than the increase in acidity associated with global warming is more likely to have ecological consequences for the early life stages of corals.
Chua, C. M., Leggat, W., Moya, A., Baird, A. H. Temperature affects the early life history stages of corals more than near future ocean acidification. Marine Ecology Progress Series 475, 85–92. http://www.int-res.com/abstracts/meps/v475/p85-92/
Munday, P.L, Dixson, D.L., Donelson, J.M., Jones, G.P. Pratchett, M.S., Devitsina, G.V., Doving, K.B, 2009. Ocean acidification impairs olfactory discrimination and honing ability of a marine fish. Proceedings of the National Academy of Science 106, 1848–1852.