How Ocean Warming and Acidification are Impacting Coral Reef Growth

Interview conducted by Joan NugentMay 28 2021

Thought LeadersDr. Christopher CornwallRutherford Discovery FellowVictoria University of Wellington

AZoCleantech speaks with Dr. Christopher Cornwall from Victoria University of Wellington. Christopher is the lead author of a paper published in the journal Proceedings of the National Academy of Sciences¹ that documents the findings of an international collaboration of scientists. The research looks at the effect of rising levels of ocean temperatures and acidification on coral reef growth.

Can you tell our readers about the findings of your recent research?

In our recent research, we assessed how coral reef growth would be impacted by future ocean warming and acidification. We find that the response of reef growth to climate change will largely be dictated by the amount of CO₂ we emit. For example, under both representative pathway concentrations (RCP) 4.5 and 8.5 (roughly equating to intermediate and worst-case scenarios), coral reefs will, on average, be undergoing net erosion by the end of the century.

However, under RCP 2.6, 63% of all reefs will be able to continue to grow, and a few of these will even be able to grow at the same rate at which sea levels are rising. In the 4.5 and 8.5 scenarios, this means that coral reefs will no longer be capable of providing ecosystem services such as a habitat for an incredible array of biodiversity and protecting shorelines.

Did you encounter any problems when carrying out your most recent research? If so, how did you overcome them?

Understanding how a whole reef will respond is not just a simple product of how fast the corals are growing today, and how fast they will grow in the future. Multiple species contribute to reef growth, either through calcium carbonate production (adding to the reef, e.g., corals and calcified algae), or through erosion (taking away from the reef, e.g., bioeroding sponges, sea urchins and parrotfish). This meant we needed to understand how a myriad of processes will be impacted by climate change: coral and coralline algal calcification, bioerosion by sponges, bioerosion by cyanobacteria/endolithic algae and sediment dissolution.

These processes all vary by reef, with no one reef is the same as another. Also, the effects of ocean warming will manifest differently depending on geography, with some regions having more frequent or intense coral bleaching events than others. Therefore, we utilized as many different study sites of our own, or others whose data was freely available in the literature, to assess the responses of each reef to changes.

Can you give us more information on what is meant by the term ‘bleaching events’?

Coral mass bleaching events are when the dinoflagellate symbiont is expelled from the coral animal host. This occurs when thermal anomalies are great, usually during marine heatwaves. These marine heatwaves are increasing in duration, magnitude, and frequency due to ocean warming, and these components of marine heatwaves are all predicted to increase in the future.

During these coral bleaching events, the white “bleached” skeleton and usually unpigmented tissue are all that remain, leading to the term “bleaching.” Depending on the intensity of the heating event, some of this coral will die. Our study used a common metric known as “degree heating weeks,” which records the number of anomalous days where temperatures go 1 degree above the summer monthly maximum mean. For example, 7 days at 1 degree above this temperature is the same as 1 day at 7 degrees above, and are both recorded as 1 degree heating week.

Image Credit: Shutterstock.com/ Rich Carey

What can the loss of coral reefs mean for the diversity of marine animals?

Coral reefs are home to more than 830,000 species and provide coastal communities with food and income through fisheries and tourism. We did not directly quantify the fate of future species diversity on these reefs. However, many of these species will not be able to persist on the remaining reef assemblages - the reefs that remain in our RCP8.5 scenario will largely be dominated by coralline algae. On average, these reefs will erode faster than they can grow, removing the habitat potential over time from these reefs.

How important is the production of calcium carbonate for coral reefs?

The production of calcium carbonate largely dictates the potential of reef vertical accretion. Therefore, without calcium carbonate production, the very coral reef itself will erode once the rates of carbonate production halt, then are reduced below zero. Depending on these rates of erosion and the amount of reef framework present, many locations will still exist as dead calcium carbonate reef that will not provide the same sort of food and habitat for many fish and invertebrates species.

Places like Pacific and Indian Ocean atolls with human communities will be badly impacted. If the reef cannot grow anymore, it means it is unlikely that many of these locations will keep pace with the effects of sea level rise. This means they will likely be inundated by encroaching sea level rise or less efficient at protecting against storm surges.

How important is it that immediate action is taken to protect coral reefs?

Reducing our greenhouse gas emissions, particularly CO₂, remains the most likely mechanism to keep future coral reefs as close as they are to today’s reefs as they possibly can be. Even under lower emissions scenarios, coral reef assemblages will be transformed, with heat tolerant species and genotypes dominating. However, under higher CO₂ emission scenarios it is unlikely that even these corals will remain. If we miss the RCP2.6 trajectory, and fall more inline with the 4.5 scenario, coral reefs will be greatly impacted, but may remain growing in some places for the next few decades.

However, by the year 2100 coral reefs will still be largely gone in this scenario, with little difference between the 4.5 and 8.5 scenarios by 2100. In essence, we need to reduce our CO₂ emissions drastically and quickly for coral reefs to provide the same ecosystem services in the future. It is also unknown whether heat tolerant remaining corals will possess similar rates of carbonate production as the present-day corals, as tradeoffs between heat tolerance and growth could occur. For example, in the Kimberley region of Western Australia on a place called Tallon Island, extremely heat tolerant corals that can withstand 40 °C waters exist, but they are prostrate and encrusting, and few branching corals exist in the more thermally variable environments.

What makes coral reefs such delicate ecosystems?

The key vulnerability of coral reef ecosystems to climate change is the susceptibility of corals to marine heatwaves. We project that the effects of ocean acidification under our highest emissions scenario are dramatic, with mean declines in global net carbonate production being greater than the present-day rates of net carbonate production of ~30% of our reefs. The effects of ocean warming on the calcification physiology of corals and coralline algae are similar. However, when we factor for marine heatwaves, these impacts are roughly 5 times the effect of ocean acidification. This is because the coral-dinoflagellate symbiosis is in a fine balance, and when temperatures increase by much more than what the corals are acclimatized to, then they undergo coral bleaching. This makes them extremely susceptible to the impacts of ocean warming when marine heatwaves are accounted for.

In contrast, for example, coralline algae are red calcifying seaweed, that possesses their own photosynthetic machinery. It takes roughly 5 times as much heating as corals for coralline algae to suffer negative impacts. And even then, coralline algae and other seaweeds do not bleach in a similar manner to corals. Instead, they pale and die, but parts of them can be quickly overgrown by juvenile coralline algae that settle on them after such events. Corals on the other hand, require a decade or more to recover from such events. In contrast, these calcified algae are much more vulnerable to the effects of ocean acidification, and once that intensifies, their ability to act as a bdand aid for coral loss will be severely reduced.

In terms of the research you have carried out on coral reefs, what are some things you know that you wish more people knew?

Coral reef ecosystems can be transformed from a state that has existed for tens of thousands of years into another depauperate state within weeks after coral bleaching events. Climate change is not occurring just in the future, it has already begun and has started to transform ecosystems across the globe. Not only in coral reef ecosystems, but also mangrove systems, kelp forests and “rocky reefs” that we find in more temperate locations.

What’s next for you and the team at Victoria University of Wellington?

Here we have gone some way to answer the question of, How will climate change impact coral reef net carbonate production on a global scale? However, there are several avenues of research that remain unanswered.

Firstly, we need to better understand under what circumstances corals and coral reef taxa can acclimatize to ocean acidification, warming and potentially marine heatwaves. How do these stressors drive changes in population and species assemblages on reefs, and what consequences do they have for the provision of three-dimensional habitat, carbonate production and resident biodiversity?

Myself and my colleagues have done some work in this space. Of most note, we have shown that coralline algae can gain tolerance to ocean acidification over multiple generations in artificial laboratory experiments. However, much more work needs to be completed with corals and temperature stress before we can predict future reef trajectories.

Coral reefs are not the only regions that produce calcium carbonate. There are many different types of temperate reef that also provide calcium carbonate frameworks, either as ecologically important veneers on rock, or as proper reefs meters deep themselves. How these “reefs” will respond is also of utmost importance, and something we are wanting to determine here.

Within the biological taxa we examined, there was a range of responses. Some species are naturally more tolerant to different climate change stressors as a taxa, while extensive between species variability also exists. Here at Victoria University of Wellington, we examine what physiological traits drive tolerance to ocean warming, acidification, and marine heatwaves in coralline algae. Past work with the ARC Centre of Excellence in Coral Reef studies has also focused on similar traits in corals, based on work done by Steeve Comeau and Malcolm McCulloch.

Lastly, our model here used a few study sites in the Pacific. The Pacific is very data deficient, and most data we used came from sites that possess research stations owned by the USA. The New Zealand government has funded a Centre of Research Excellence (CoRE, named “Coastal People; Southern Skies”) where members of my theme will obtain further data on carbonate production rates and also of species responses to climate change in the Pacific Islands.

Where can readers find more information?

Your readers can access the full paper here. Resources on important research into multigenerational responses of coral reef taxa to climate change can be found in a paper titled Rapid multi-generational acclimation of coralline algal reproductive structures to ocean acidification as well as in the paper titled A coralline alga gains tolerance to ocean acidification over multiple generations of exposure. Research into traits that control species’ responses to ocean acidification can be found here. 

About Dr. Christopher Cornwall 

Christopher completed an MSc at Victoria University of Wellington and a Ph.D. at the Department of Botany at the University of Otago in 2013, examining the capacity of seaweed to act as a refuge for ocean acidification for calcifying species. In 2013 he took up a Research Fellow position at the Institute for Marine and Antarctic Studies in Tasmania. He then worked as a Lecturer at the ARC Centre of Excellence for Coral Reef Studies University of Western Australia node at the School of Earth Science and UWA’s Oceans Institute and Oceans Graduate School examining the calcification mechanisms in coralline algae.

Christopher is currently a Rutherford Discovery Fellow, the 5-year fellowship has been funded by the Royal Society of New Zealand to start his own group at the School of Biological Sciences focusing on New Zealand’s iconic kelp forest ecosystems. Dr. Christopher Cornwall was awarded The 2020 Prime Minister’s MacDiarmid Emerging Scientist Prize a few months ago. In July 2021 Christopher will start a role as leader of the Understanding theme in the Centre of Research Excellence Coastal People: Southern Skies.

References

1. Cornwall. C. E., Comeau. C., Korndere. N., et al, [2021], ‘Global declines in coral reef calcium carbonate production under ocean acidification and warming,’ PNAS, [DOI: 10.1073/pnas.2015265118]