A fragile jewel

What is climate change doing to our coral reefs? (11 minute read)

Welcome to The Turnstone. Here, I help people understand important issues such as climate change, water quality and conservation. I send my articles out every Sunday - if you’d like them emailed to you directly, you can sign up to my mailing list.

Before I begin today’s articles, I have two updates:

Firstly, one of my research colleagues, Richard Yao, is conducting a survey on attitudes to invasive species in New Zealand. If you are a New Zealand resident, I encourage you to participate in the survey (sorry, it’s not for those outside New Zealand at present).

Protecting New Zealand's remaining native forests from introduced species: what do you think?

Be in to win one of two $250 gift vouchers by filling out this survey.

Aotearoa is facing a biosecurity crisis, with more than 1,500 unwanted plants, animals, marine pests, and diseases. Scion researchers are studying the value that New Zealanders place on protecting native plants and animals.

Your confidential answers will ensure the voices of New Zealanders are heard by decision makers and will help shape biosecurity programmes in the future.

This project is funded by the New Zealand Biological Heritage National Science Challenge.

This survey should take approximately 25-30 minutes to complete.

If you are over 18 and have lived in New Zealand for more than three years, take the survey here: https://tinyurl.com/NZ-Native-Forests  

Secondly, I’m taking a work-related trip overseas soon, to give a talk and run a workshop at a conference in Kenya. Since I’m flying all that way (something I remain conflicted about due to the climate impacts of such a trip), I intend to make the most of it. I’m going to use the opportunity to share some African perspectives on climate change and other important issues, such as the African climate change summit which has just finished. I will also share my thoughts and reflections on what I experience. However, I’m uncertain about the availability and cost of internet, so I apologise in advance if I occasionally miss sending out my articles on exactly the same times and days as usual over the next month.

Now, onto coral...

Warm water laps gently at my feet as I stand on the white sand of a tree-fringed beach. The lagoon with its myriad shades of blue invites me, but I hesitate. I look behind me to a spot above the high tide mark, where I’ve left a sarong, a pair of old sandals and enough coins for my bus fare back to the place I’m staying. I’m told it should be safe to leave them, but it goes against my instincts. Still, I know I don’t want to miss Blue Bay. I’m almost at the end of three months doing volunteer work in Mauritius, and I don’t know when, if ever, I’ll get back. I decide it’s worth the risk.

A mask and snorkel on my face and fins on my feet, I dive in, then swim parallel to the beach. I pass a headland and swim into an inlet. The beach has gone and it’s mostly rocky shore here. I see one man fishing with a line and a few snorkellers, but it’s quiet. If I put my head under the water, I can pretend I'm alone.

Blue Bay has been designated as a marine park to protect its corals. Guidebooks call them ‘coral gardens’, but to me it’s like an underwater, upside-down cathedral, its ceiling painted with frescoes of colourful fish. Most spectacular are the table corals, which grow as huge fans, layered from the shallowest water to the deepest, forming the structure of the cathedral’s roof. I hold my breath and dive down so I’m looking at the coral around me instead of from above. It’s exhilarating, almost as if I’ve flown up into the lofty dome of St Peter’s in Rome. I surface, breath and duck down again, then again.

Table coral (image credit: not my photo, this came from David Mark on Pixabay)

That was more than 20 years ago and I wonder how the corals of Blue Bay are doing. I know that my peaceful swim would be impossible today. There are now dozens of boats ferrying people around to the best spots, and rules about where you can and can’t swim. From some of the accounts I’ve read of the large numbers of people visiting, I doubt I could safely leave my money on the beach and come back to find it still there, as I did in 2002.

But there’s another reason I wonder about the survival of the Blue Bay coral – climate change. Around the world, coral is bleaching as a result of rising sea temperatures. It nags at my mind when I think about Blue Bay, but it’s something I don’t know much about. So, when I hear that there’s someone who’s studied the response of coral to climate change right here in Wellington, I know who I want to talk to.

Dr Chris Cornwall is a marine biologist at Victoria University of Wellington. His main research focus has been seaweed (more on that another time) but he spent several years studying coral reefs in Western Australia and still does research on corals.

To understand coral bleaching and the threat of climate change, it helps to understand what corals are, and what makes the ones which form tropical reefs special. So I ask him to take me back to the basics of coral.

“Coral is an animal, related to anemones, that forms skeletons of calcium carbonate”. This sounds simple enough. Calcium carbonate is the same compound which makes up sea shells, although it can occur in different forms. New Zealand has corals – deep-sea species of great beauty and variety (here’s an illustrated guide from the Department of Conservation) . But, Cornwall tells me, “they’re solitary organisms that don’t form reefs. Much slower growing.”

The reef-forming corals do something remarkable. “They have what we call a symbiont, a type of algae that lives within its cells. The symbiotic algae within the corals are what give them most of those pretty colours that you’ll see on the reef. What happens is that the algae can photosynthesise and provide energy to the corals in exchange for the waste nutrients that the corals make. Corals live in nutrient poor waters compared to, say, our kelp forests here in New Zealand. So this relationship allows them to survive.

“They form these amazing three dimensional structures that provide habitat and food to an amazing array of biodiversity. Without these coral reefs we’re not going to have many different species of fish and invertebrates that rely on these coral reefs. We’re also not going to have things like shoreline protection for many coastal tropical countries. When the corals grow they build upwards, and protect the lagoon and inlying land from things like hurricanes, cyclones and the wave swell. And there are a lot of places where the actual island itself is an atoll and so without the coral they essentially would get washed away or inundated.”

But for something which forms such massive structures – whole islands – coral is also vulnerable. “We should realise that the relationship between the coral animal and the symbiont is quite delicate. In the water, temperatures are relatively stable or at least they are reliable cycles. It’s really difficult for marine species to cope with big, swift changes in temperature. If the temperatures in summer, which is when it is warmest, go 1 or 2 degrees above normal for too long, the coral spits out the symbiont. Essentially, the relationship between the two of them breaks down.

“What’s left is the white coral skeleton, hence the term bleaching. If the temperature didn’t get too warm to begin with or it wasn’t too warm for too long, they might take up other symbionts from the seawater, or if they’ve got enough symbionts left within them, they might recover. But if they don’t have enough symbionts, they won’t recover from that. Generally, after the bleaching occurs, we’re talking about one or two weeks maximum before they start suffering mortality.”

In the same way that many countries have been experiencing heatwaves on land, our oceans have been experiencing heatwaves too. As a result, nearly all areas of reef have seen bleaching. Cornwall lists them off, and it’s a grim picture. “All of the Great Barrier Reef has bleached a couple of times, if not three or four times. The Caribbean is going through a really big bleaching event right now where I think some of the warmest temperatures on record have been found. All of the west coast of Australia that has coral reefs has bleached now. Most of the Indo Pacific… The problem with these marine heatwaves is they’re predicted to become much more extreme and much more frequent.”

Cornwall tells me that climate change is changing the makeup of the reefs. “I’m starting to see some papers come out showing that when we have too many of these bleaching events, there’s certain heat tolerant corals that will proliferate and lots of the other diversity of corals will be lost. So we’re getting communities of corals that are just one or two species.”

But the problem isn’t only the heat. By putting more carbon dioxide into the air, we are also putting more carbon dioxide into the ocean, and that’s making the ocean more acidic. “It’s more difficult for corals, or it costs them more energy, to calcify, which is what they do when they make their skeletons. So some species start to calcify slower. Higher carbon dioxide concentrations in the air also change what we call the speciation of carbon dissolved in the ocean. Most of the dissolved carbon is locked away as bicarbonate at present, a little bit of carbonate and a very small amount of CO2. Under a future ocean, what’s going to happen is more CO2 and less carbonate. That could be problematic for species whose skeletons are exposed directly to seawater because they could potentially dissolve as the concentration of carbonate decreases.”

Among the species most vulnerable to the ocean’s increasing acidity are the coralline algae. They’re not coral, but they do form a crucial part of reefs. “They play a really important role in binding together coral reefs. They act as a settlement substrate for coral larvae. So basically they emit chemical cues in the water that tell larvae to come back to them, this is a good place to live.”

It’s a depressing situation for corals around the world. The delicate balance between coral and algae is under threat from increasing temperatures. The ability to form and maintain their skeletons is under threat from increasing acidity in the ocean. As if these weren’t enough, there’s something else, too. It’s not connected to climate change, but it is connected to a topic I’m really concerned about – water quality.

As Cornwall mentioned, corals naturally grow in waters which are low in nutrients. But we are changing the corals’ environment. We do this directly, such as by releasing treated or untreated sewage into the ocean, but we also do it indirectly, such as when nutrients wash into fresh water from fertiliser and livestock waste. Eventually, this fresh water ends up in the ocean. These nutrients are adding further stress to corals by favouring the growth of algae which grow over corals. Overfishing plays a role too, by reducing fish which eat the algae. The situation is particularly dire in the Caribbean, where the majority of reefs have now been transformed into habitats dominated by algae, sponges and other non-reef-forming species. And it’s only going to get worse as climate change puts even more stress on the delicate relationship between corals and their symbiotic algae.

All of this means that the privilege I had, to experience the magic of Blue Bay, may not be something that future generations can enjoy. And I can see that this weighs heavily on Cornwall. “I don’t know how many times a scientist like me has to say that if we don’t do something now then our children and grandchildren aren’t going to be able to experience the same marine environments that we did. This is not just from a human value point of view, but also we’re causing mass extinction of species. We will certainly lose many marine species here in New Zealand if we continue the way we’re doing and in the tropics they’re even worse off than we are. That’s probably one of the hardest parts of my job.”

When I was in Mauritius, I knew that the environment I saw on land was in trouble. The island was mostly covered with sugar cane or forests dominated by invasive species. So many of the native species were endangered. But I was working with dedicated people who were able to protect a few precious areas from the invasive species, and prove that recovery of the ecosystems was possible.

But it’s not clear whether we can do that for coral reefs. Climate change is so profound, and so pervasive, that we can’t just fence off small fragments to protect.

One of the most magical moments of my work in Mauritius was travelling to Round Island. It’s an inaccessible island, a lump of rock battered by waves and the only safe access is by helicopter. The journey to and from the island gave me a spectacular view of Mauritius. The fringing reef made it look like a multifaceted jewel, the turquoise lagoon, the blue-grey coral reefs and then the deep blue water beyond. I will hold that breathtaking sight in my memory, but I fear before long a memory may be all that survives of the coral.

It’s not Mauritius, but an atoll in the Maldives, but it shows what I mean (image credit: Melanie Newfield).

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