Rising levels of carbon dioxide have led to warmer oceans, with a quarter of the atmospheric CO2 dissolving into the oceans and causing them to become more acidic. Such changes can have a negative impact on various marine ecosystems, including corals. However, a new study led by Ohio State University (OSU) has found that some Hawaiian corals are surprisingly resilient to warmer, more acidic conditions.
The scientists collected samples of the three most common coral species in Hawaii – Montipora capitata, Porites compressa, and Porites lobata – and placed them in outside tanks with four different conditions: a control tank with current ocean conditions, an ocean warming condition (plus 2 degrees Celsius), an ocean acidification condition (minus 0.2 pH units), and a combination of both warming and acidification.
The tanks were designed to mimic ocean reefs by including sand, rocks, fish, crabs, starfish, and urchins, and were allowed natural variability in temperature and pH levels over the course of each day and during different seasons, as they would have in their natural habitat.
Although over the course of the 22-month study the three coral species did experience significant mortality under conditions simulating the ocean temperatures and acidity expected in the future, none of them completely died off, and some were even thriving by the end of the study.
“We found surprisingly positive outcomes in our study. We don’t get a lot of that in the coral research field when it comes to the effects of warming oceans,” said study lead author Rowan McLachlan, who conducted the study as a doctoral student at OSU.
Overall, 61 percent of the Hawaiian corals exposed to warming conditions survived (compared to 92 percent exposed to current ocean temperatures), with the two Porites species being more resilient than M. capitata in the combined warming and acidification condition. At the end of the study, the survival rates were 71 percent for P. compressa, 56 percent for P. lobata, and only 46 percent for M. capitata.
“Of the coral that survived, especially the Porites species, they were coping well, even thriving,” said Dr. McLachlan. “They were able to adapt to the above-average temperature and acidity.”
“We don’t know how corals will fare if changes in temperature and acidity are more drastic than what we used in this study,” he added. “Our results do offer some hope but the approximately 50 percent mortality we saw in some species in this study is not a small thing.”
Further studies that include local stressors such as pollution or overfishing – which may have additional negative impacts on corals in some regions – are needed in order to better understand the future effects of climate change on coral reefs.
The study is published in the journal Nature Scientific Reports.