The weird way that human waste is killing corals

This story was originally published by Wired and appears here as part of the Climate Desk collaboration

At first glance, the parrotfish seems like the goofiest animal to grace Hawaii’s coral reefs, with its mouth full of beak-like teeth. But the reef wouldn’t be the same without this fish, which nibbles fast-growing algae off the corals. This grazing allows sunlight to reach the symbiotic algae that live within the coral polyps, letting them produce energy. In the process, the parrotfish gnaws off some of the coral’s calcium carbonate skeleton and poops it out as sand — some 800 pounds of the stuff each year per fish. This builds Hawaii’s famous beaches.

But this intricate relationship between reef species is now strained, thanks to human waste. Hawaiian cities have centralized wastewater treatment infrastructure, but the state also has some 88,000 cesspools — pits that readily leak sewage into the sea. Also, septic tanks retain solid human waste but release nitrogen-rich liquid. All of this stimulates algae. (It’s similar to the way runoff from farms using nitrogen fertilizers can cause blooms in nearby waterways.) If there aren’t enough herbivorous fish off the Hawaiian coast due to overfishing, there’s nothing to keep the green stuff in check. It chokes out the corals, starving them of energy, and prevents baby corals from taking hold.

This ecosystem is already under threat as ocean temperatures rise. When corals get stressed, they release the symbiotic algae that give them energy and colour. That phenomenon is called bleaching.

Writing last week in the journal Nature, scientists showed how vital herbivorous fish and unsullied waters keep this system in balance. Using nearly two decades of data, they showed that healthy Hawaiian reefs — with cleaner water plus plenty of parrotfish and other “scraper” fishes — fared far better during a vicious marine heat wave than reefs with fewer scrapers and with more human wastewater and other coastal pollution. The researchers' modelling suggests that by reducing human threats from the land and in the sea, instead of limiting harm to only one or the other, corals and their relatives would be three to six times more likely to grow back four years after a disturbance like a heat wave.

“Even with a severe heat wave in 2015, 20 per cent 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, co-author 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.”

Such heat waves are growing ever more terrible. Previous research has shown that by 2014, half the ocean’s surface was logging temperatures once considered extreme — meaning extreme heat is now the new normal. Since March, global sea surface temperatures have been soaring, especially in the North Atlantic. Late last month, temperatures off Florida reached 101 degrees, leading to the mass bleaching of corals.

“It is a really scary time for oceans around the world, here in the U.S. and in so many other places experiencing record temperatures,” says Tom Dempsey, oceans program director at the Nature Conservancy. (Dempsey wasn’t personally involved in the paper, but the Nature Conservancy contributed to the research.) “It’s grim news. We are facing a really daunting set of challenges. And I think what's exciting about this paper is that it lays out a plan of action.”

That plan is for coastal regions around the world to improve their wastewater management—and fast. Less nitrogen flowing into reef ecosystems will mean fewer nutrients to feed relentless algae blooms.

In other words, conserving reefs isn’t just about fixing marine problems, like reducing overfishing to protect critical ecosystem operators like parrotfish. It also requires fixing problems on land. “Far too often — certainly in Western conservation and Western resource management — we have created silos that are unhelpful and actually prevent wholesale, comprehensive, and effective action,” says Dempsey. “We tend to think about fish as a food problem, and we think about wastewater as a human health problem. But all of these things are connected to the reef — and the reef is centrally connected to the health of human communities and to the health of our planet.”