Thanks to Humans, Caribbean Reefs Are Basically Dead

Thanks to the dynamic duo of overfishing and climate change, Caribbean reefs are on the brink of death. That’s according to the results of a study, published by the International Union for Conservation of Nature, that performed the most comprehensive survey of the Caribbean reef system ever.

According to Christine Dell’Amore at National Geographic, the results released at an IUCN conference show that coral coverage in the reefs has dropped from 50-60 percent in the 70s to around 10 percent today.

Reef structures are built out of calcium carbonate released by living coral, which are excellent nutrient fixers in nutrient-sparse tropical waters. (Tropical waters are so clear because they lack the suspended nutrients of waters at higher latitudes.) It’s because of that prodigious ability of coral to aggregate nutrients that coral reefs are some of the most biodiverse ecosystems on Earth. In a healthy reef, living coral don’t cover 100 percent of the outer layer of the reef because they also compete with fast growing algae and grasses that take advantage of the buffet coral lay out for them.

While algae and grasses grow faster than corals, they’re normally kept in check by sea urchins, which slowly graze over a reef, and grazing fish like parrotfish. But Caribbean sea urchins experienced a massive die-off in the 70s, possibly due to disease, that has been shown to allow grasses and algae to displace coral. Since then, grazing fish have done the bulk of the work of keeping grasses in check, but overfishing has reduced that ability. The problem is that while coral provides the base of the nutrient cycle in a reef, it can’t compete against algae and grasses in the absence of predators, which will eventually drive out all of the coral and leave the reef barren.

Compounding the problem is the fact that zooxanthellae, which live inside living coral and are the main drivers of that whole nutrient-gathering thing, are very temperature sensitive. When water temperatures increase rapidly, the zooxanthellae ditch the coral en masse (an event known as bleaching), which leaves the coral pretty much dead. Because climate change has caused Caribbean waters to increase in temperature rather rapidly, the region has experienced more bleaching events.

With that double whammy, the Caribbean has become the “poster child” (as an NSF program director said to the New York Times) for reef system collapse. At the same time, the reasons behind that collapse are incredibly clear: While bleaching is part of the idiotically slow-moving climate debate, preventing algae and grasses from displacing coral is a matter of making sure that remaining healthy reef systems are carefully managed and protected from overfishing and pollution. Without those changes, those still-vibrant Caribbean waters will end up looking like one big dead swimming pool.

Follow Derek Mead on Twitter: @derektmead.