The Coral Reefs We Depend On Most May Fall First

Emily Kelly’s first experience with corals came at age 12 on the Great Barrier Reef in Australia. “There’s just a kaleidoscope of colors. There’s something happening everywhere.” To Kelly, reefs are like a bustling city—grunting fish chatter, snapping shrimp, and parrotfish rasping algae off of every surface. “There’s a lot of noise on that healthy coral reef system, just like New York.”

Two decades later, as a full-fledged marine biologist, she has visited too many silent reefs. “You’re missing out on a lot of the sounds” on a degraded reef, Kelly says. She called them “abandoned cities,” describing the typical vista as “a brown, fuzzy carpet covering a lumpy bottom.” It was a far cry from the pink, blue, and green-hued chaos she’s seen at healthy reefs.

A yellow stingray (Urobatis jamaicensis) moves amongst seaweeds on a degraded reef in Bocas del Toro, Panama.

As these marine metropolises have become ghost towns throughout much the world’s tropical seas, we’ve begun to understand what’s causing them to fail.

Healthy coral reefs are self-repairing and punch above their weight in terms of benefits to humans and biodiversity. They buffer coastlines from the ravages of storms while also providing a home to a quarter of all marine species. They’re also an abundant source of food for many coastal communities.

But their capacity to feed people is not limitless.

Two weeks ago, a study showed that coastal communities, like black holes, have a form of gravity, sucking up fish from nearby reefs. The research team analyzed surveys of 1,800 sites and found that just two bits of information could predict fish numbers on unprotected reefs: the proximity of human settlements and the population density of those settlements, which together they call “human gravity.”

Higher human gravity means more fishing, which can be devastating to corals, says Kelly, who wasn’t involved in the research. Seaweed-eating fish give corals a competitive edge against their aquatic antagonists, and their loss puts corals at risk. “The number one thing that organisms are competing for on reefs is real estate. You have corals, you have other invertebrates, and you have seaweeds that live on the bottom, too,” Kelly says. “All of them are constantly in competition for space.”

Most seaweeds are fast growing—some can double in size in a single day. Corals, in contrast, grow no more than a few inches per year. Some boulder corals can only manage a tenth that rate. Were it not for herbivorous fish, what Kelly calls “the lawnmowers of the reef,” seaweed would smother corals. They even chemically attack the colonies in their quest for elbow room. Large herbivores like the noisy parrotfish are a particularly important set of seaweed munchers.

The stoplight parrotfish (Sparisoma viride) is one example of a herbivorous fish.

Fishing typically drives down numbers of all fish, and co-author Nick Graham noted their previous work has shown that the abundance of herbivores typically tracks the total amount of fish on a reef. As fishing depresses their numbers, coral suffers. But, Kelly says, “Where you have a large number of herbivores and also large bodied herbivores, you end up having much lower amounts of seaweed and also much shorter seaweed.” Seaweeds’ losses are the reef’s gain.

In other studies, juvenile corals were most abundant when there were more and bigger herbivorous fish. This relationship becomes even stronger after taking into account subtle differences between individual herbivores, like how long they wait between bites and the size of their dinner plates (i.e. the amount of reef surface area growing seaweeds). These coral tykes are the key to a healthy future for reefs.

Hale and healthy herbivores even go as far as clearing sites for new construction. When allowed to work at full capacity, these maintenance crews help create something special.

Keeping Up with Sea-Level Rise

At Millennium Atoll, part of the Southern Line Islands a few hundred miles north of French Polynesia, Kelly said they found the reef to be in such good health that the uppermost corals were nearly breaking the surface of the water. They had grown so high, in fact, that baby sharks needed to ride waves to gain access to the labyrinthine hunting ground. “It was amazing,” she says of the expedition to the remote reefs. “Seeing that really put a lot together for me in terms of how a reef can function.”

Millennium Atoll and other pristine sites in the Central Pacific should be able to keep pace with sea-level rise under all but the most pessimistic climate change forecasts. As coral colonies grow, they cement their calcium-carbonate skeletons on top of rocks and dead coral, raising the overall height of the reef in a process that physical geographer Chris Perry refers to as accretion. Perry and others calculate potential accretion by adding together the growth of coral and other calcifying organisms while subtracting loss from animals that eat or otherwise erode them.

Reefs in the Southern Line Islands of the Central Pacific, like this one at Palmyra Atoll, are some of the most pristine in the world.

On many reefs, though, the ability of corals to grow is being hampered by overfishing, bleaching, and other threats, jeopardizing the entire reef. In fact, most reefs in the Caribbean and Indian Oceans are too beleaguered to keep pace with projected sea level rise, according to a study lead by Perry that was released last month. What’s more, this finding is optimistic, as the estimates they use for accretion can’t account for major events like storms that can level whole reefs.

That spells trouble for nearby coastlines. Though the study’s authors did not measure the power of waves directly, they point out that when seas rise more than a couple feet above the crest of a reef, more energy from the surf makes its way to the shore. When ocean swells aren’t tempered by long tumbles over shallow reefs, they can reshape coastlines over time and increase the risk of flooding.

Accretion rates on many reefs in the Caribbean Sea and the Indian Ocean are disconcertingly low, but for different reasons. “The Caribbean [has] a long period or history of reef degradation and that’s being driven by all sorts of different things,” Perry says, including overfishing, water pollution and runoff, and diseases. In the Indian Ocean, however, Perry said low accretion rates are largely driven by coral bleaching.

Corals turn bone white when seawater temperatures rise too high and will die if the bleaching event lasts too long. Climate change-induced bleaching and loss of herbivores are interwoven threats for reefs, says Graham, also an author of the sea-level rise paper. “One of the drivers of a faster recovery rate [from coral bleaching events] is the biomass, or amount of herbivorous fish, on a reef.” Moreover, even when bleaching and herbivore losses are not severe enough to kill corals outright, they can stress them enough to suppress overall accretion rates.

When water temperatures rise too high, corals are forced to eject the colorful algae living inside them. Bleached corals die if the water temperatures do not drop quickly.

By keeping seaweed at bay, herbivorous fish give baby corals a leg up while also keeping adult corals clear of seaweed. But reefs near coastal populations are likely to have fewer herbivores, and thus are less likely to be able to keep up with sea-level rise. “It is the reefs near people that people depend on for shoreline protection, but also that are most compromised ecologically,” says Joshua Cinner, lead author of the human gravity paper.

That means coastal communities face a double whammy when nearby reefs are overfished. By removing the marine maintenance team, communities risk losing the habitat that fosters the fish they harvest. They also expose themselves to destructive storm surges and erosive waves that break on the beach rather than the reef.

Protecting Herbivorous Fish

There is a ray of hope, though. The authors of the human gravity paper found that protected reefs may not help widely roaming top predators like sharks and tunas, but they could preserve seaweed-eating fish. Essentially, no-fishing zones do more to protect a reef that’s close to lots of people, even if they can’t fully protect all fish. Such local measures, when combined with global action on climate change, may give the underwater cities a fighting chance.

One reef that gives Kelly hope is Kahekili on the Hawaiian island of Maui. Since June 2009, it has been classed as a Herbivore Fisheries Management Area, meaning it is off limits to the harvest of herbivores, but still open to other types of fishing. In a relatively short time, coral colonies have flourished, painting more of the ocean floor with their colorful hues.

Hawksbill sea turtles (Eretmochelys imbricata) are critically endangered according to the International Union for the Conservation of Nature.

Kahekili is a place where she can reliably find critically-endangered hawksbill sea turtles feeding in the shallows in the afternoons and where coral pinnacles were crowned with frogfish. “Every afternoon at four o’clock in the summer, the surgeonfish spawn, swimming up in these magical dances, intertwining with one another,” she said.

Perry agrees. “When you see what those really healthy, thriving systems can look like and that they do still exist in places, that gives you hope.”