The Weird Way That Human Waste Is Killing Corals

“Even with a severe heat wave in 2015, 20 percent of those reefs that were in cleaner water with the herbivore fish not only went got through the heat, but some improved,” says Arizona State University ecologist Greg Asner, coauthor of the study, who leads the Allen Coral Atlas reef mapping project. “There are vast areas of the same problem all over the planet. And so what it means is that while we’re hyper-focused on climate change’s effects on coral reefs—which we should be, and don’t get me wrong, it’s critical—the other one that’s killing reefs is coastal wastewater and coastal pollution. It’s a global problem for sure.” 

This study looked at 120 miles of Hawaiian coastline between the years 2003 and 2019. Asner and his colleagues gathered data on sea temperatures and surveyed reefs—calculating, for instance, how much biomass fish represent. On land, they calculated the amount of urban runoff in a given area—all the gunk washing off of streets, including motor oil and other nasty chemicals. They also calculated wastewater effluent, and therefore how much nitrogen might be heading into the sea. “The number one problem we have, of all the land issues, is human waste going into the ocean,” says Asner. “We have a ridiculous amount of wastewater pollution being generated by individual homes.” 

(They didn’t tally the bevy of pharmaceuticals that are passing through human bodies and entering wastewater via the sewer system. Scientists have just begun to study which of those drugs might be having an adverse effect on corals, Asner says, so that aspect requires further research.)

Rising heat is delivering a major blow to a system already strained by pollution and overfishing. “In 2015 we had a massive wake-up call: The first and so far largest marine heat wave arrived and cooked our corals for more than 12 weeks. We lost up to 50 percent in some areas, and more than 25 percent across the board,” says Asner. “The three combined—the pollution, the low herbivore fish, and the heat—it’s not additive, it’s multiplicative. It causes a very large decline in the reef’s health in those heat waves.”