From the Field

Sewage Pollution: A Significant Threat to Coral Reefs

Coral Reef at Palmyra Atoll National Wildlife Refuge. Photo © USFWS / Jim Maragos

When you think of the top threats to coral reefs, sewage isn’t usually at the top of the list.

Climate change and overfishing are the more familiar hazards, but pollution from untreated sewage is serious threat to reefs and the services they provide for marine life and people.

Assessing the Scale of Sewage Pollution

Sewage pollution is shockingly widespread: A full 96 percent of places that have both people and coral reefs have a sewage pollution problem, according to recent research by Stephanie Wear, The Nature Conservancy’s lead scientist for coral reef conservation.

Most sewage finds its way into the ocean as either poorly treated or untreated discharge, or as stormwater runoff. In places with little to no infrastructure, like the developing world, the majority of wastewater goes untreated. For example, about 85 percent of the wastewater entering the sea in the Caribbean is untreated, according to the United Nations Environmental Program.

“Some of the greatest sewage pollution in the world occurs in developing countries, where you also have the most coral reefs,” says Wear.

But sewage pollution isn’t limited to the developing world. Wear says that existing infrastructure in many places, including the United States, is old or poorly maintained, causing untreated wastewater to leak or overflow into waterways during heavy rain.

When stacked up against the value of coral reefs, the threat of sewage pollution is immense. Coral reefs provide several critical services for people, including shoreline protection, livelihoods from ecotourism, fisheries production, and medicines for common diseases. Environmental consultants estimate these services are worth $31 billion U.S. dollars each year.

A Toxic Cocktail for Corals

It’s not difficult to understand how this influx of sewage is bad for coral reefs — untreated sewage isn’t safe for people, so marine life isn’t likely to fare any better.

The most widely recognized pollutant within sewage is the excess nutrients. “On a reef, there is a battle between coral and seaweed for space and light,” explains Wear. Corals typically maintain the upper hand, but excess nutrients can tip the battle in favor of seaweed.

“Even with the help of herbivorous fish, which act like lawn mowers, corals can lose the battle and be overgrown by seaweed if exposed to excesses nutrients,” she says.

But Wear’s research makes the case that sewage pollution is a far more complex problem than just nutrient overload. “There are lots of other nasty things in sewage that have a potentially fatal impact on corals,” she says. “It’s a toxic cocktail, where the different pollutants within sewage can interact with each other to make things worse.”

So what exactly is in sewage?

Fresh water: The primary component of sewage, fresh water can stress and even kill corals. While corals’ exact tolerance to fresh water is unknown, it’s well documented that influxes of fresh water from storms increase reef mortality.

Endocrine disruptors: These chemicals disrupt the hormone system in both humans and other living things. They include natural and synthetic estrogens, parabens, petrochemicals, and phthalates, among others. Found in many household products, endocrine disruptors are also linked to a myriad of human health problems.

Heavy metals: These chemicals — including mercury, lead, and copper — can lead to bleaching, death, and decreased reproductive success in corals. Wear explains that heavy metals accumulate in the skeletons of corals just like they do in people. They can also increase the strength of pathogens on the coral’s surface, making the coral susceptible to infection.

Pathogens: Sewage is teaming with viruses and bacteria, and fecal contamination is a major cause of illness around the world. Wear says that the same holds true for corals. In the Caribbean, researchers discovered that a bacterium associated with hospital-acquired infections in humans, Serratia marcescens, was causing white pox disease in threatened elkhorn corals. Outbreaks of this disease killed more than 70 percent of corals in the Florida Keys. “A human pathogen caused a disease in a marine animal, and the source was sewage,” says Wear. “That’s insane.”

Toxins: Different places will have a different suite of toxins, but the possibilities are myriad. One important class of toxins is pharmaceuticals. Wear explains that any and all drugs that people take end up in sewage, including antibiotics. Antibiotics are especially problematic for corals, which have a protective mucus layer that is home to a diverse community of bacteria that function much in the same way as the microbes in our guts. Wear says that scientists suspect that antibiotics may impact this bacterial community, which makes the corals more susceptible to disease.

Elkhorn coral at Jardines de la Reina, Cuba. Photo © Ian Shive
Elkhorn coral at Jardines de la Reina, Cuba. Photo © Ian Shive

Conservation for Human Health

Aside from benefiting coral reefs, Wear says that solving the sewage pollution problem will also benefit human health. “We need to work beyond our comfort zones as conservationists and managers and reach out to the broad community to address this issue,” she says.

Coral reefs and the people that depend upon them will benefit from healthier water, Wear adds, while people in coastal areas will benefit from improved sanitation and sewage treatment. What’s more, improved sewage treatment conserves water.

“Its not just the environmental havoc that sewage causes on reefs,” says Wear, “but we are also wasting water that we could otherwise use.”

Justine E. Hausheer

Justine E. Hausheer is an award-winning science writer for The Nature Conservancy, covering the innovative research conducted by the Conservancy’s scientists in the Asia Pacific region. She has a degree from Princeton University and a master's in Science, Health, and Environmental Reporting from New York University. Justine's favorite stories take her into pristine forests, desolate deserts, or far-flung islands to report on field research as it's happening. When not writing, you can find her traipsing after birds, attempting to fish, and exploring the wild places around her home in Brisbane, Australia. More from Justine

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  1. I’m doing a project on how to conserve and save the coral reefs and this is pretty useful! 😀

  2. I’m doing a project but can you help me on ‘who does coral pollution effect’ ? Really need the help

    1. Hi Kitty, What kind of project and what kind of help are you looking for? Thank you!

  3. i agree but could you tell me more about the effects on the animals

  4. Hi,
    I am a grade 9 student and I was searching on the internet how can sewage affect coral reefs and this is very helpful information.?
    Thank you.?