(narrator) Every time we hear about corals, it's about them dying.

And it's true, corals are in trouble.

But one of the biggest problems for these corals is something we can fix.

It's our poop.

And where it goes.

[hollow whooshing] ♪ Off the Florida Key ♪ Ahem.

Sorry.

I'll stop.

But yeah, off the Florida Keys, there's a place called the Looe Key Reef.

Brian LaPointe has been studying the Looe Key Reef for more than 30 years.

He and graduate student Kristiana Heath want to know how water quality has affected these corals.

Quick side note, I know this show is called "Overview," but today we're also taking you under.

[water splashing] We're still using drones.

They're just waterproof drones.

So it counts.

It takes millions of years for these underwater forests to grow.

But in the last few decades, this reef has gone from lush, dense colonies to bare skeletons.

(Kristiana) Looe Key reef is a really special place to me.

I grew up going there.

It's almost the epitome of my childhood.

Just in that short time, I've seen the decline.

It's sad.

It's heartbreaking.

(narrator) Warming oceans cause corals to bleach or expel the symbiotic algae living in their tissue, leaving behind a white skeleton instead of a thriving coral colony.

But corals could protect themselves from rising temperatures if they weren't also dealing with pollution coming from land.

South Florida is home to more than nine million people and tens of millions of tourists visit every year.

(Brian) With all those people come a lot of wastewater, sewage, and of course, as the residential population grows, more fertilizers, more deforestation.

All of these result in increased nitrogen in the water.

(narrator) Healthy oceans have a chemical balance of carbon, nitrogen, and phosphorus.

But humans have been disrupting that balance by adding nitrogen in various forms like sewage, crop fertilizer, and manure.

For corals, that extra nitrogen stresses them out and makes them more likely to bleach when water temperatures rise.

(Brian) Corals have evolved over millions of years, likely in a condition of nitrogen limitation.

So all of a sudden, humans come along and totally alter the global nitrogen cycle.

(narrator) But while excess nitrogen is killing corals, it's creating a bonanza of growth in algae.

And not in a good way.

Take sargassum, a kind of floating algae.

Under normal circumstances, it serves an important role in the ocean.

Sargassum provides habitat for fish, turtles, and at least ten endemic species.

But extra nitrogen is causing a massive sargassum bloom in a process called eutrophication.

Eutrophication is when nutrient pollution, in this case nitrogen, sends algae into a feeding frenzy, causing a bumper crop that sucks up all the oxygen as it dies, suffocating fish and other marine species in the process.

Over the last decade, sargassum blooms have intensified, stretching from the shores of West Africa to the Gulf of Mexico, and weighing more than 20 million tons.

As it dies, sargassum not only smothers coral reefs but piles on beaches in South Florida and the Caribbean, plaguing communities that rely on tourism.

(Brian) That the sargassum, just like the coral reefs, are the canaries in the coal mine.

They are the first responders to the increasing nitrogen.

These are the largest harmful algal blooms on our planet.

(narrator) You might have heard different versions of this story before, the annual algal bloom on Lake Erie, the Massachusetts-sized dead zone in the Gulf of Mexico, and the toxic algae that causes red tides in the Chesapeake Bay.

All examples of eutrophication.

It's an issue all over the world and it's happening for several reasons.

Close to a third of the planet's land area has been converted into farmland.

And runoff from fertilizer and manure leeches into bodies of water and eventually the ocean.

Deforestation exacerbates the problem because without vegetation to filter and trap runoff, those nutrients run freely into the water.

And of course, there's the sewage problem.

Lots of people means a lot of toilets.

And in some Florida counties, half of those toilets are on septic systems, meaning they drain into backyards.

Septic systems work when there's at least two feet of dry soil between the septic field and the groundwater to filter the sewage.

But sea level rise has pushed groundwater closer to the surface, which means unfiltered sewage is fouling groundwater and making its way to the ocean.

Centralized wastewater treatment plants are better but most of them only partially remove nitrogen.

And three miles off the coast of Miami, we found where that wastewater goes.

Every day, this pipe discharges 143 million gallons into the ocean.

And we met a new friend along the way.

I'm sorry about all the sewage, pal.

(Brian) When you begin to look at the amount of nitrogen coming from septic tanks and wastewater, it's staggering.

(narrator) You can see the water's discoloration from the pipe hundreds of feet in the air.

Local fishermen call it the boil.

These nitrogen-enriched waters turn green, a stark contrast to the turquoise blue that the water should be.

(Brian) We are allowing these sewage outfalls to operate on living coral reefs that are habitat for endangered species.

(Kristiana) It can start to feel a little helpless when you really think about the complexity of the issue.

But that's really the opposite thing that needs to happen.

We need to take it as a call to action.

(narrator) Down in the Keys, communities have started taking action.

This is the Cudjoe water treatment plant.

Every home on the Keys has transitioned away from septic and has been plumbed into plants like this.

Like other treatment plants, the Cudjoe site uses sewage-eating bacteria to break down waste.

The difference here is that wastewater flows through an extra round of bacteria to remove most of the nitrogen.

And instead of pumping the water into the ocean, the plant injects it below the water table, using a 3,000-foot-deep well.

Oh.

And those outfall pipes?

The state will phase them out by 2025, a result of legislation relying on Brian's research, which might mean less nutrients clogging up Florida's coastal waters and hope for the only living reef in the continental U.S. (Brian) We are just now starting to turn that corner, at least in Florida.

Accessibility provided by the U.S. Department of Education.

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