ARI SHAPIRO, HOST:
Now we have some bad news for tourists headed to parts of Mexico's Yucatan Peninsula looking for sparkling Caribbean waters.
VERENA RIVETTA: They will be seeing a sort of chocolate sea. And they cannot swim in the water for hundreds of meters.
SHAPIRO: That's Verena Rivetta. She is the sales director for the Full Moon hotels in Tulum, Mexico. Overwhelming amounts of sargassum some seaweed from the Atlantic have been washing up and carpeting Tulum's beaches.
AUDIE CORNISH, HOST:
It's a problem because as the seaweed rots, it releases hydrogen sulfide gas, and...
RIVETTA: It smells really badly. It's a sort of a rotten egg smell, something like that.
SHAPIRO: Mexico has spent millions of dollars trying to clear its beaches. Rivetta says the seaweed problem is seriously affecting her business.
RIVETTA: We made a report for the six first months of this year, and our sales is falling down for 8.5%.
CORNISH: So where is all this seaweed coming from? A giant mat sargassum seaweed stretching from the coast of West Africa to Mexico. Scientists call it the Great Atlantic Sargassum Belt.
MENGQIU WANG: Before 2011, we didn't even see sargassum the central Atlantic. But right now, we're just seeing tons of them.
SHAPIRO: That's University of South Florida researcher Mengqiu Wang. Wang and her colleagues recently published a study documenting record levels of sargassum in the western Atlantic Ocean.
WANG: In open ocean, sargassum is not necessarily a bad thing. It is actually a critical habitat and a refuge to a lot of marine life like the fish, the crabs, the turtles, the birds. It only becomes a problem when too much of the sargassum started to pile up on a beach and, you know, affecting the tourism, affecting the traditional fishery and be harmful to the environment.
SHAPIRO: Wang suggests deforestation and fertilizer use along the Amazon River are contributing to the rapid growth.
WANG: The nutrient can be leeching out and - into the ocean to stimulate sargassum growth.
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CORNISH: Back in Tulum, Verena Rivetta at Full Moon hotels has taken matters into her own hands.
RIVETTA: We have contracted several workers.
RIVETTA: Here, they are called sargasseros (ph). And we have these kind of people for 10 hours per day, collect and take away the sargassum every day of the week.
SHAPIRO: And for tourists still worried about polluted beaches, hotels in the area provide free buses that shuttle tourists to sargassum-free beaches.
(SOUNDBITE OF WAVES) Transcript provided by NPR, Copyright NPR.