How Long Do Corals Need to Recover?

Discovering clues about an ecosystem’s recovery from a catastrophic event – check.

Developing new international partnerships for Scripps Institution of Oceanography, UC San Diego – check.

Learning to love eating fresh tuna for nearly every meal – check!

These were three of the things that we accomplished this summer during our one-month stint on a tiny island in the Maldives.

As marine conservation biologists, we are concerned with the worldwide degradation of coral reefs due to human activities such as fishing, pollution, and the effects of global climate change. Coral reefs are complex ecosystems, so we and the other members of marine ecologist Jennifer Smith’s research group studies the ecology of remote reefs to get an idea of how they are supposed to work without human interference. Once we understand this, we can set realistic and effective conservation goals. It is exciting work, and it means that we frequently travel to very tiny islands that are very far away from our comfortable home base at Scripps.

Our most recent tiny island trip was to the brand new Korallion Lab, a marine science research station on in Lhaviyani Atoll in the Maldives. Scripps is one of the lab’s founding member institutions, along with Woods Hole Oceanographic Institution, University of Newcastle in the United Kingdom, and ReefCheck Italia.

The Maldives are an interesting place to work because they were hit hard by a massive coral bleaching event in 1998. Corals are sensitive to temperature, so a temperature increase of a few degrees causes them to become stressed, expel the tiny algae that live in their tissues (losing their characteristic color, hence the term ‘bleaching’), and eventually die. Bleaching events happen worldwide with unfortunate frequency, but the 1998 event in the Indian Ocean was the most severe ever recorded. Levi Lewis, a fellow Scripps graduate student, and I were on a mission to discover if there are any lingering effects of that bleaching event on the Maldives’ coral, fish, and other marine life.

Unfortunately, there were few marine scientists working in the Maldives prior to 1998, so we lack comprehensive before-after comparisons to quantify what the bleaching did to Maldivian reefs. But studying Maldivian reefs today can provide clues about what these waters used to look like and even offer hints of what the future might hold. For example, consider this piece of evidence: we saw formerly thriving reefscapes composed of huge, centuries-old corals that were still structurally intact but completely dead and overgrown with algae. If these huge corals had been dead for a long time, they would have been eroded into smaller pieces. If there were only one or two dead corals, it might be a matter of disease or coral-eating predators. However, swimming around an area the size of several football fields, we would see hundreds of these huge ghost coral colonies, suggesting that a fairly recent and dramatic event killed all the coral at the same time.

We routinely observed two types of evidence that give us hope for the future of Maldivian reefs. Many of these ghost reefscapes were speckled with small colonies of live corals that appeared to be only a few years old. Such a scene hints that some coral species are beginning to re-colonize the area. And in sharp contrast to the dead reefs, we encountered several thriving reefs filled with diverse, living coral and schools of colorful fish. Because these thriving reefs were composed of many large, old coral colonies, we think that the 1998 bleaching had little impact on select coral communities in the region.

Why have some areas become ghost reefs that were apparently decimated by the 1998 bleaching event, while other areas appear to have been resilient to the bleaching and persist as thriving, diverse underwater landscapes?

The answer is complicated.

One of tools we use to assess reef health is the Coral Health Index (CHI). Developed by a multidisciplinary group of reef scientists, including Scripps marine ecologist Stuart Sandin, CHI streamlines an array of coral reef variables that we could measure into just three simple metrics: fish biomass, abundance of calcifying organisms on the seafloor, and population of pathogenic bacteria in the water. High fish biomass tells us that the food web is intact: generally, if there are a lot of big fish, then the rest of the food web is probably there, too. An abundance of calcifying organisms tells us that the reef is actively growing over time. Finally, low levels of pathogenic bacteria tell us that the water is clean and minimally impacted by human activities.

So, there we were, measuring CHI all around Lhaviyani Atoll, a sparsely populated group of islands about 35 kilometers (22 miles) in diameter. We hoped to find a pattern that could tell us why some reefs survived the 1998 bleaching unscathed, some were beginning to recover, and still others were decimated with no signs of recovery. What we learned was puzzling: there was no obvious pattern distinguishing the healthy from the unhealthy reefs.

Now back at Scripps, Levi and I continue to process samples and analyze data, trying to make sense of what we learned. The Maldivian government is acutely interested in protecting the country’s reefs, so our results will help legislators make decisions about how to take action in the future. More broadly, the scientific community has much to learn about why reefs respond differently to bleaching events and other disturbances.

Oh, and about all that fresh tuna that we ate during our research trip: tuna fishing is the second largest economic sector in the Maldives (after tourism). Since the country is comprised entirely of tiny limestone islands with no soil, agriculture is virtually nonexistent, leaving fish as the only source of protein. We ate tuna for breakfast, lunch, and dinner. Sometimes wrapped up in Bangladeshi-style roti bread (at breakfast), sometimes as a tuna salad sandwich (lunch), and sometimes fried into delicious nuggets (dinner). Though we may have filled our lifetime cap on mercury, all that tuna kept us happy and well-fueled for the tiring work of trying to figure out just what will happen to the Maldives’ beautiful coral reefs.

– Jill Harris is a marine biology graduate student with Levi Lewis in the laboratory of Scripps marine ecologist Jennifer Smith.