Some Hawaiian corals may be resilient to warmer and more acidic seawater, study finds

The longest experimental study on corals, a 22-month project that replicated current and future ocean conditions, suggests Hawaiian corals can remain resilient as climate change makes seawater warmer and more acidic.

The study, led by Oregon State University postdoctoral researcher Rowan McLachlan, was published today in Scientific Reports. The authors, who include Christopher Jury and Robert Toonen of the University of Hawaiʻi at Mānoa, say the research is important because understanding the factors behind coral health is crucial to efforts to save the Earth’s embattled reefs.

“Our study found that in the absence of local stressors such as nutrient pollution, overfishing, bottom trawling, physical damage from tourism, etc., at least half of Hawaiian reef corals will be able to survive and persist in up to 22 months exposure to ocean warming and acidification,” McLachlan said. “The results provide hope that if we can mitigate climate change and keep within the targets of the Paris Climate Agreement, coral reefs will persist in some form, albeit with reduced abundance and genotypic diversity.”

But it remains unknown how corals will fare if changes in temperature and acidity are more severe than those used in the research, or how they will fare under current conditions for longer periods of time.

“Our results do offer some hope but the approximately 50% mortality we saw in some species in this study is not a small thing,” McLachlan said.

The majority of previous research about the capacity for coral reefs to survive and persist under ocean warming and acidification was gained from short-term experiments, one-month duration on average, and based in indoor laboratories in empty glass tanks.

“While informative, these studies are limited in their ability to predict how coral will respond in the wild where the environment is much more complex,” McLachlan said.

In the short period between 2014 and 2017, more than 75% of the planet’s coral reefs experienced bleaching-level heat stress, and 30% suffered mortality-level stress. (Bleaching is the breakdown of the symbiotic relationship between corals and the algae they rely on for energy.)

Coral reefs are in less than 1% of the ocean but are home to nearly one-quarter of all known marine species. Reefs also help regulate the sea’s carbon dioxide levels and are a vital hunting ground scientists use in the search for new medicines.

McLachlan, a doctoral student in the Andréa Grottoli lab at Ohio State University during the study, and collaborators collected samples of the three most common species of Hawaiian corals: Montipora capitata, a branching and plating stony coral; Porites compressa, a branching species also known as finger coral; and Porites lobata, which forms boulder-shaped lobes.

The scientists placed the samples in four different outdoor tanks:

• current ocean conditions
• more-acidic water (minus 0.2 pH units)
• warmer water (plus 2 degrees Celsius)
• warmer and more acidic water

“We strived to design the experimental tanks to mimic the natural reef environment as closely as possible,” McLachlan said.

The tanks were placed outside under natural light and included pumps simulating natural water flow. They were filled with seawater pumped in directly from a neighboring reef and included sand, rocks, fish, algae, crustaceans and other organisms in addition to corals.

McLachlan said these conditions made the study “arguably more realistic and accurate in predicting the outcome of corals in the future under global climate change.”

Results suggest that warming oceans will harm coral species: 61% of the coral samples exposed to the warming conditions survived, 31% less than those exposed to current ocean temperatures.

The Porites species showed more resilience than M. capitata in the combined warming/acidification treatment. There the survival rates were 71% for P. compressa, 56% for P. lobata and 46% for M. capitata.

The ability of the Porites species to withstand rising temperatures and acidification, and their role in reef building, offer hope that some reef ecosystems can persist despite changing ocean conditions, McLachlan said. 

Other co-authors were Grottoli, James Price, Agustí Muñoz‑Garcia and Noah Weisleder of Ohio State; and Stephen Levas of the University of Wisconsin-Whitewater.

The National Science Foundation and the HW Hoover Foundation supported the research.