(Read this first paragraph in a David Attenborough voice and then throw in a big reflective pause)
Coral reefs are like the Beijing, London or New York of the oceans. They have the greatest biodiversity of any ecosystem type on the planet, but remarkably cover less than two percent of the ocean bottom. During the Anthropocene, the current geological age, where human activity has been the dominant influence on climate and the environment, coral reefs have become among the most threatened ecosystems on earth.
Photo by my good friend Selena McMillen while we were diving in Menjangan Island, Bali.
Humans pose the greatest threat to coral reefs. Unsustainable and destructive fishing, pollution, climate warming, ocean acidification, and invasive species are all contributing to the destruction of these special places. In some spot’s reefs have been utterly devastated, while in many other locations the reefs today are a pale comparison of what they once were. Seeing a quiet bleached reef firsthand is terribly upsetting. So, what can be done? A recent special report from the Intergovernmental Panel on Climate Change (IPCC) states that with an additional half-degree of warming above today’s levels, tropical coral reefs will face “very frequent mass mortalities,” although coral adaptation is possible. If temperatures rise an additional degree to 2 degrees above pre-industrial levels, coral reefs are in danger of vanishing entirely.
Under the stress of warmer and more acidic waters, corals expel their symbiotic algae. These microalgal dinoflagellates photosynthesize nutrients that are released to the coral tissues and provide enough energy for the corals to deposit calcium carbonate to build their skeletons. Recently a research team lead by Christopher Jury, of the University of Hawaii, found a set of what they are calling “supercorals” in Hawaii’s Kāne’ohe Bay, that can already tolerate warmer temperatures and more acidic waters. These robust “supercorals” even managed to recuperate after sewage was diverted into the bay in the 1930s bringing in low pH and high temperatures for many years. In Kāne’ohe Bay, coral cover on shallow reefs (0–2 m) was depleted by an average of greater than 70% by the early 1970s. But when the sewage input was relocated in 1977, coral percentage cover in the bay increased rapidly over the next 20 years, returning to near pre-eutrophication levels (50–90% coral cover). Eutrophication is a big word that basically means the over-enrichment of water by nutrients such as nitrogen and phosphorus, aka sewage.
“I began to realize that the temperature and chemistry conditions in Kāne’ohe Bay are very similar to the conditions that many people predict will kill corals off globally, yet the reefs in the bay seem to be thriving, making the area incredibly valuable as a possible window into the future,” said Jury.
Maybe these “supercorals” could respond well to warm, acidic waters, but how well? So, turns out Kāne’ohe Bay is the perfect place to run some experiments. Jury and his colleagues compared the pH and temperature tolerances of three dominant coral species from within Kāne‘ohe Bay to the tolerances of conspecifics (members of the same species) from a control site 18 km to the southeast, with the goal to demonstrate that corals from Kāne‘ohe are more resistant to acidification and warming. Second, the researchers explored the potential for adaptive responses that may underlie coral resilience to anthropogenic impacts. The research team collected branches of three common coral species from about two dozen colonies in both Kāne’ohe Bay and the control site at Waimānalo Bay. The corals went back to the lab, given time to acclimate and were then exposed to sea water with different combinations and levels of temperature and acidity for five weeks in a fully factorial design.
They found corals from Kāne’ohe Bay tolerated warmer and more acidic waters significantly better than the corals from Waimānalo Bay. The corals from Kāne’ohe Bay also grew more than twice as fast as the Waimānalo Bay corals. All results they were expecting to see. But, the impact of decreased pH on calcification was more variable, depending on temperature. The corals that managed to survive the sewage years, the more tolerant ones, was part of the genetic variations in the population that survived. These corals then proceeded to pass on their genes to those in the extant Kāne’ohe Bay population.
So that seems like good news, right? So perhaps we could help with some of these more physiologically tolerant species in order to restore life to barren coral reef graveyards. And what do you know there are people like Dr Sarah Frias-Torres doing just that, coral gardening. Click here to find out more about coral gardening. It’s just like normal gardening, but underwater.
Corals of the same species can have different pH and temperature tolerances among different habitats. The results of Jury’s study indicate that reducing our human influence offers some hope for coral reef resilience. It would be a real shame to lose these incredibly diverse and beautiful ecosystems because of our own behaviour. Please take the time to learn more, teach others and show that you care.
Please see the publication to read the juicy details, findings and discussion of this important work https://royalsocietypublishing.org/doi/10.1098/rspb.2019.0614
Jury CP and Toonen RJ (2019) Adaptive responses and local stressor mitigation drive coral resilience in warmer, more acidic oceans. Proceeding of the Royal Society B: Biological Sciences. https://doi.org/10.1098/rspb.2019.0614
