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Can Coral Reefs Strangled by Algae Recover?

“Experimental support for alternative attractors on coral reefs”, Russell J. Schmitt, Sally J. Holbrook, Samantha L. Davis, Andrew J.Brooks, Thomas C. Adam, Proceedings of the National Academy of Sciences Mar 2019, 116 (10) 4372-4381; DOI: 10.1073/pnas.1812412116 

When a coral reef suffers from bad environmental conditions like warmer water, sometimes the coral can recover—but sometimes algae will grow over the ecosystem instead. Scientists in French Polynesia tested what makes a reef either rebound or convert to undersea algae fields after a disturbance.


Why Too Much Algae Hurts Coral Reefs

Coral reefs have declined worldwide because of a number of threats, including being overrun by macroalgae (algae that can be seen with the naked eye, i.e. seaweed) if there are too many nutrients or not enough algae-eating fish in the ecosystem. This switch from a coral dominated ecosystem to an algae dominated one is called a phase shift. (Image by joakant on Pixabay)Coral wages a constant battle against algae for space and dominance. On a healthy coral reef, it’s a battle that the coral mostly wins.

But in recent decades, scientists and reef managers have noticed with alarm that vitally important coral reef ecosystems have been overgrown with algae. Colorful coral reefs quickly became unrecognizable, with only the algae-coated outlines of dead coral hinting at the reef’s former bounty.

Warming waters, agricultural pollution, or overfishing can all cause this sudden shift to algae dominance, but it’s often a combination of factors that tip the scales in favor of the algae. This abrupt change in ecosystem type is called a phase shift, and a big part of conserving coral reefs is predicting when it might happen to a particular reef—and how to prevent it. A recent study in Moorea, French Polynesia helps us better understand how and why coral-algal phase shifts happen.


A Fragile Balance

A phase shift can be imagined like a marble (or an ecosystem) balanced on top of a hill. A stressed coral reef might only need a small push to fall onto the side of the dotted line where algae dominates. Nudging this marble slightly to the right (by making the environment slightly worse for the coral or slightly better for the algae) would make the marble roll into the basin where algae dominates. Once there, it would take a lot more effort to get the marble back onto the coral-dominant side of the hill, where a coral reef can exist.

Imagine a marble balanced on top of a hill. If you nudge it gently, it will roll down the hill, but once at the bottom it becomes much harder to return the marble to the top. Coral reefs that are overtaken by algae face a similar problem—once the ecosystem has slipped into algae-dominance, it’s much harder to get a healthy coral reef back.

To learn more, scientists used two distinct parts of a coral reef in Moorea as a natural laboratory. The coral growing on Moorea’s forereef, or the side of the reef closer to the ocean, had been very resilient and remained strong across years of natural disturbances. In the lagoon, however, patches of the reef had historically succumbed to algae after disturbances. The researchers used both the lagoon and forereef to test how they handled disturbances.


Testing the Reef 

Moorea, French Polynesia, where the forereef and lagoon served as natural laboratories (Image by Mariamichelle on Pixabay)

To imitate an overfished coral reef, the researchers set up cages with different sized holes over portions of the reef. Small holes excluded larger algae-munching fish from swimming inside. Half of the cages contained coral, while half went over algae. At the end of the experiment, the scientists realized that it was not just the hole size of the cages affecting whether or not algae grew inside, but whether the cage had started off with coral or algae inside.

The researchers also tested how patches of the forereef and lagoon would react to algae removal. They removed either all, some, or none of a common type of algae (Turbinaria) from the experimental sites, then tracked whether it grew back over several years. The healthy, algae-eating fish population on the forereef ensured that the algae didn’t grow back on those test sites. In the lagoon, Turbinaria made a full recovery within a year at the sites where some of it had been removed. It was only at sites where algae had been completely removed from the lagoon that it didn’t grow back right away.


A Tool for Better Management?

In certain conditions, either coral or algae can dominate an ecosystem (the space on the hill between the two dashed lines). However, the scientists in this study found that the ecosystem doesn’t switch back until it crosses both dashed lines. That means that if algae have taken over what used to be a coral reef, algae will dominate the ecosystem until conditions go all the way back across the dashed line on the left.

The study confirmed what scientists had previously suspected but been unable to test: that returning an area to the exact same environmental conditions before algae took over often isn’t enough to bring back a coral reef. In the hill analogy, it isn’t enough to push the ecosystem (marble) to the top. You have to push it over the top and then farther down the other side for the ecosystem to have a chance to flip back.

Facing challenges like ocean acidification, a warming ocean from climate change, and overfishing means that coral reefs need all the extra help they can get. The challenge of recovering coral reefs overrun by algae may sound like yet more bad news on an endless list of problems facing coral reefs.  But the researchers hope that the opposite may be true. If managers and scientists can predict the range of conditions where either coral or algae can dominate (i.e. where the dotted lines are on the hill for a particular reef), they’ll be better armed to protect existing coral reefs and even help recovery when possible. That way, we can preserve coral reefs and prioritize the ones that are most likely to resist an algae-takeover.


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