New 18-year study shows that despite increasing resistance, coral reefs in the Lakshadweep are not recovering well from climate change disturbances
The coral reefs of the Lakshadweep are not doing well. They haven’t been well for a while now. Just how badly they are faring is something we are only just waking up to. In a study just published in the journal Coral Reefs, NCF’s Oceans and Coasts Programme reports on two decades of monitoring the coral reefs of the Lakshadweep Archipelago and its findings are bitter-sweet.
Lakshadweep’s reefs have been battered by a dramatically changing climate in the past two decades—the islands have witnessed three large El Niños—sudden increases in ocean temperatures that kill large tracts of coral. The good news is that with every subsequent El Niño event, less coral is dying—the reefs are becoming more resistant. The bad news is that their ability to recover from each event has declined dramatically—by almost four-fold. The even worse news is that the frequency of these disturbance events is increasing all the time—killing the reef before it is able to limp back to health again.
Corals are incredibly complex animals—they form unique partnerships with microscopic algae that live within their tissues and give corals the energy to grow and flourish. Corals are colonial animals, and in a healthy reef, hundreds of colonies scramble for space in shallow, light-filled tropical waters, forming dense reefs. The coral reef is one of the most diverse ecosystems on the planet, with millions of undersea critters finding homes and food within these spectacular structures.
As complex and life-giving as they are, they are also extremely sensitive to changes in temperature. Raise the temperature just a little over what is normal for the time of year, and they expel the algae they depend on—turning a deathly white as they lose their colour—and their main source of food. Without the algae, bleached coral are way more vulnerable to disease and death.
The Lakshadweep archipelago has experienced three such mass bleaching events in the last twenty years—in 1998, 2010 and 2016—each was linked to an El Niño current, and each was of a higher intensity than the previous one.
What some of the once-healthy reefs in Lakshadweep look like today
Since 1998, NCF’s Oceans and Coasts team has been working to better-understand how corals respond to these rapidly changing temperatures. A large team of people has contributed to this effort—sampling year after year for the last two decades, in one of the few programmes of its kind in the subcontinent. Long-term studies of this nature are rare, and it is often difficult to find financial support for them. Which is why this work has been largely a labour of love by many dedicated researchers over the years. In all, the team monitored six reefs across three islands (two each in Kavaratti, Agatti and Kadmat) with all their myriad coral genera, to study two key trajectories: how resistant the reefs were to climatic anomalies, and how well they recovered from them.
They found that even though the severity of each El Niño event increased over time, less coral died with every subsequent event. On the face of it, this increasing resistance means that the reef is doing much better, right? Learning to cope with higher temperatures perhaps?
Unfortunately, this hasn’t been the case. While coral mortality has showed signs of decline, corals are taking much longer to recover with each new El Niño event.
Says Shreya Yadav, “Increases in sea surface temperature were once a decadal event – when they happened, the ecosystem had enough time to heal from it. In the last twenty years, something seems to be broken in the circulatory system of the ocean. El Niño events are becoming increasingly frequent and intense. Each successive year competes with the one just past to beat temperature records. For the reef it means less and less time to recover even if it gets more resistant to each event.”
To understand why this is happening, the Oceans and Coast team looked at the coral themselves. Reefs have plenty of species of coral—and each of them has a unique way of dealing with the trials and tribulations of life. These ‘life-history traits’ are what determine how each coral will respond to different stresses.
“Not every coral responds to temperature equally”, says Teresa Alcoverro, “Not all bleach, and even if they bleach, not all die. With every successive El Niño, we have seen a gradual filtering out of the most vulnerable species. In the reefs of today, only the hardiest survive.”
Of course, this means that, over the last two decades, the composition of reefs have gone through a drastic transformation. The genus Acropora, once found all over the Lakshadweep reefs is virtually extinct—reduced to patches in the lagoon and a few other locations. The Acropora are iconic species—growing in intricate antler-like branches, thick thickets, or large table-tops. They are typically fast growing, and contribute disproportionately to the growth of the reef. Among their branches a myriad species of invertebrate and fish seek refuge or food. After the first large die-off in 1998, these fast-growing species were critical to the recovery of the reef. In the first decade of our monitoring, things were looking pretty optimistic, since many reefs were bouncing back from dramatic losses. Until 2010.
“We thought we understood how the Lakshadweep reefs behave”, reflects Rohan Arthur. “The El Niño of 2010 changed all that. In the second decade of our monitoring we’ve had to completely rethink how our reefs were behaving. There were fewer young coral recruits in the reef every year, and we were seeing a completely different set of species taking over the reef than we were used to.”
A range of slower-growing, more resistant coral took over in areas where Acropora was once dominant. As a result, the rate of recovery had dropped dramatically. A back-of-the-envelope calculation indicates that at current rates, the Lakshadweep would require at least three decades without additional disturbances to recover to pre-2010 levels. Of course, this was not a luxury the reefs had. It was only a few years later—in 2016—when the El Niño hit yet again, resetting the slow trajectory of recovery.
Twenty years and three El Niños on, we are rapidly losing critical habitat for fish, invertebrates and other critters as reefs shed their structural complexity, turn flatter, and take much longer to recover from stressful events. There is a growing fear that the reef is no longer able to keep up with the natural processes of erosion—making the atolls increasingly vulnerable to storms, land loss and salinisation of freshwater supplies. For the 70,000 people that live on the Lakshadweep, the declining condition of the reef affects them directly, and can determine whether the islands will continue to be suitable for living in the decades to come.
Says Rohan: “There is a lot of talk about ocean optimism – about restoring faith in the ability of ecosystems to recover from even the worst disturbances. While this may certainly be true for some parts of the world, the Lakshadweep reefs are a sobering reminder of the reality that many, if not most, tropical reefs face. This may be difficult to hear, but it is something we need to confront head on. At global levels, we have to rededicate ourselves to bettering the 1.5 degree targets proposed by the IPCC. By all accounts, 1.5 degrees may not be sufficient for low-lying coral atolls. And for the Lakshadweep, we need a radical rethink of our developmental priorities for the islands. The future of the reefs and the habitability of these crumbling atolls depends on it.”
Photographs: Shreya Yadav
View the full paper here.
- Shreya Yadav, Teresa Alcoverro, Rohan Arthur (2018). Coral reefs respond to repeated ENSO events with increasing resistance but reduced recovery capacities in the Lakshadweep archipelago. Coral Reefs (2018).