The Seine River in Paris has been closed to public swimming for more than 100 years, but it reopened to swimmers this summer. We talked to two people who took a dip.

Megan Hall: Welcome to Possibly, where we take on huge problems like the future of our planet and break them down into small questions with unexpected answers. I’m Megan Hall.

This summer, people in Paris got to do something that was illegal for more than 100 years: swim in the Seine.

The river was closed to public swimming in 1923 because of pollution and boat traffic. But now, after a massive clean-up effort, Paris officials say it’s safe to go back in. So, we decided to talk to some people who gave it a try.

Daniel: I’m Daniel

Megan Hall: That’s the nephew of our founder.

Nienke: And,my name is Nienke.

Megan Hall: That’s Daniel’s partner. They were working remotely in Paris this summer.

Daniel: We were just kind of, living our life, but in Paris.

Megan Hall: They just happened to be there when the city reopened the Seine for swimming. So they decided to check it out.

Daniel: We went to the area near the Eiffel Tower that was open for swimming.

Nienke: Eventually we waited maybe a half hour. Do you think that’s fair? Yeah.

Megan Hall: As they waited, they saw a man updating a sign with details about weather, wind speeds, and pollution in the river.

Daniel: He was this very sort of stern old man, not smiling, just writing numbers, and then

Nienke: He wrote, uh, in French, like happy swimming.

Daniel: Happy swimming.

Megan Hall: This happy swimming was made possible thanks to a push to clean up the river in time for the Paris Summer Olympics.

To do that, the city built large storage tanks for raw sewage. Before, when big storms happened, the sewers were overwhelmed and released everything straight into the river. They also worked hard to stop houseboats from dumping their waste into the water.

The end result for swimmers like Nienke and Daniel?

Daniel: Yeah, so the water, um, itself, um, was surprisingly clean.

Nienke: It was. Kind of chilly, but very, refreshing. The water looked almost a little bit blue. You could see some fish.

Megan Hall: Other Paris residents aren’t so sure about swimming in the Seine. They question whether the water is really clean enough.

Nienke: I certainly wondered, I hope this is as clean as they say, because I have no way of knowing if,  if there is something strange in here. But I think each of us also accidentally had a little taste of the water, and it seemed absolutely fine.

Daniel: It tasted delicious.

Megan Hall: Daniel says he hopes the Seine clean-up encourages other cities to make their rivers safe enough for swimming.

Daniel:  I grew up in New York and I always thought it would be cool to go swimming in the Hudson River and was always told, no, it’s too dirty. And I thought that was very strange, because I have access to parks, to the streets, and it’s just sort of another natural environment that’s accessible as something you can enjoy.

Nienke: I agree. I think it changes your relationship to the city. I think both Dan and I have been in, in cities abroad where if there’s a river we’re, we start asking people, oh, where can I go and swim? And then when the answer is like, you absolutely can’t, that would be dangerous, that’s a very depressing and strange realization, that there’s parts of the city that are so prominent, like the Seine, it’s right there, you see it all the time, I think being able to access it makes you more connected to the city and its resources.

Megan Hall: So far, the swimming program at the Seine has been a success, so much so that the mayor extended the season to the middle of September.

But if you can’t make it to Paris, there are cities in the US that have made these transformations too. Similar efforts to store sewage have made Narragansett Bay here in our home state of Rhode Island much cleaner. In fact, everywhere these kinds of improvements are made, it’s transformed water that was once filthy and dangerous.

That’s it for today. You can find more information, or ask a question about the way your choices affect our planet, at askpossibly.org. You can also subscribe to Possibly wherever you get your podcasts or follow us on Instagram, Facebook, LinkedIn, or Bluesky at  “askpossibly”

Possibly is a co-production of Brown University’s Institute for Environment and Society, Brown’s Climate Solutions Initiative, and The Public’s Radio.

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Megan Hall: Welcome to Possibly, where we take on huge problems like the future of our planet and break them down into small questions with unexpected answers. I’m Megan Hall. 

Today we’re answering a question from listener Alexandra Hahn, who saw something a bit unusual: 

Alexandra Hahn: I wrote to you guys, because last spring, I saw a bunch of dead fish in a river downtown. And I was wondering what they were doing there, what kind of fish they were? How did they die? 

Megan Hall: Reporter Will Malloy talked to Possibly’s co-founder Professor Stephen Porder to tell us what’s going on. Hi Will!

Will Malloy: Hi Megan! 

Megan Hall: So, why were there a bunch of dead fish in the river? 

Will Malloy: There’s actually a name for the phenomenon Alexandra noticed – it’s called a fish kill. A fish kill is when a large number of fish die over a short period of time in the same area. 

Megan Hall: That sounds smelly! But why does that happen? 

Will Malloy: Like people, fish need oxygen to live – only instead of getting it from the air, like us, they breathe oxygen gas that’s dissolved in the water.

Stephen Porder: So they are actually breathing oxygen underwater the same way we’re breathing oxygen out here in the air. 

Will Malloy: And basically, a fish kill can happen when the amount of oxygen in the water drops and there isn’t enough for fish to survive. 

Stephen Porder: If you consume all that oxygen in the water, the fish will die. Just as if you consume all the oxygen in the air. We will die.

Megan Hall: Ok, that makes sense. But why would there be less oxygen in the water all of a sudden? 

Will Malloy: It’s a bit of a complicated process, but one thing that can take oxygen out of the water is actually dead seaweed and algae decomposing. 

Stephen Porder: If a dead piece of seaweed falls to the bottom of the bay, it gets decomposed, that consumes oxygen. 

Will Malloy: You can think of it like the opposite of photosynthesis – when plants photosynthesize, there is a reaction that produces oxygen. When dead stuff breaks down, there are reactions that use up oxygen. 

Megan Hall: Ok, sure, but isn’t stuff dying in the bay all the time? 

Stephen Porder: So there’s always algae and seaweed in the water. But in order to get a fish kill, you need sort of a burst of growth of stuff followed by a die off

Will Malloy: So if there’s a surge of more life – more seaweed, more algae – after a little bit, you’ll actually see more death as some of these plants die, which eats up a bunch of oxygen. 

Megan Hall: What causes this “surge of life’?

Will Malloy: It can come from a lot of factors, but often it comes after something like a big rainstorm. 

Megan Hall: Why would rain create more algae and seaweed in the water? 

Will Malloy: I wasn’t kidding when I said it was complicated! When there is a big rainstorm, the rain can wash fertilizer off of people’s lawns and down into storm drains.  

Stephen Porder: If there’s too much rainfall, our sewage system can’t handle it. 

Will Malloy: Meaning that the sewers overflow. And when some sewers overflow, they can spill out into bodies of water, like rivers, bays, or even the ocean. And that means all that fertilizer and the rest of our sewage ends up in the water. 

Stephen Porder: And so you got a flush of sewage, all of that is nutrients. And those nutrients provide that bursts of life, of growth of stuff in the water

Megan Hall: And after that fertilizer encourages more stuff to grow in the water, those plants eventually die, which eats up the oxygen that fish need to live? 

Will Malloy: Exactly! Now, this doesn’t happen every time it rains. I mean, the conditions have to be just right,  but in general, it’s one big chain reaction that starts with a bunch of rain and ends with a bunch of dead fish. 

Megan Hall: So, what can we do to prevent this from happening?

Will Malloy: In your own garden, try planting things that don’t need fertilizer. And in general, cities and towns can create more green spaces to break up paved surfaces. That way, stormwater can go into the soil instead of the bay. 

Megan Hall: Thanks, Will! That’s it for today. For more information, or to ask a question about the way your choices affect our planet, go to thepublicsradio.org/possibly. Or subscribe to us wherever you get your podcasts. 

You can also follow us on Instagram, Facebook, LinkedIn or X at  “askpossibly”

Possibly is a co-production of The Public’s Radio, Brown University’s Institute for Environment and Society and Brown’s Climate Solutions Initiative.

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