Why is ugly, thick brownish seaweed cursing South Florida beaches?

South Florida’s beaches faced a sargassum assault this summer that some scientists believe is part of the largest spread of the nomadic marine weed on record, and one that could continue through September.

From the Keys through the Treasure Coast, islands of the brown algae floating on berry-like bladders have stained beaches and sailed through inlets thick enough that one Palm Beach County lifeguard saw a black racer snake drift by on one large mat.

While a reprieve may be underway locally, county officials report two waves of sargassum have swept ashore since May, with no guarantee another surge isn’t lurking.

In June, sargassum spread through 1,158 square miles of the Caribbean Sea. That’s three times the sargassum coverage during the same time in the record-high year of 2015.

“Right now there is still a lot of sargassum in the Caribbean, so I think these events will last for a while,” said Mengqiu Wang, a researcher at the University of South Florida’s Optical Oceanography Laboratory, which has tracked sargassum since 2000. “There still could be a high chance the sargassum could show up again in Florida.”

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At Riviera Beach’s Phil Foster Park a glut of seaweed greeted tourists and locals alike the week of July 4.

That’s where Tom Lutz, a Palm Beach County Ocean Rescue lifeguard, saw the snake riding the seaweed, and when Georgia visitor Nathalie Latimer struggled through the living barricade to scuba dive.

“We couldn’t believe how much seaweed there was, it was just an incredible amount,” Latimer said. “It was definitely hard to push through. If I was just a swimmer, I wouldn’t want to swim with it, but the diving was great.”

The sargassum takes a circuitous route from the Sargasso Sea in the middle of the North Atlantic to reach South Florida. It follows a path south out of the Sargasso Sea past the Caribbean and through the Yucatan Strait into the Gulf of Mexico. From there it can hit Texas — a particular problem for Galveston — get caught in a spinning eddy in the Gulf, or make a right turn where it hooks up with the Gulf Stream.

It also passes near areas where major rivers discharge, such as the Mississippi in the Gulf of Mexico and the Amazon in the Caribbean.

Florida Atlantic University research professor Brian LaPointe, who studies algae and seaweed at Harbor Branch Oceanographic Institute in Fort Pierce, believes the high-nutrient river discharges are feeding the seaweed.

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LaPointe is in the second year of a three-year NASA grant to study how nutrients are changing in the sargassum. What he’s found so far is nitrogen levels have increased, likely from heavy doses of fertilizer and sewage runoff.

“We have altered the nitrogen cycle on our planet and it started with the invention of fertilizer,” LaPointe said. “We think this is what is behind the increased abundance of sargassum.”

Another possible source of the proliferation, which Wang and LaPointe both said is a hypothesis, is that Saharan dust is contributing to sargassum growth by raining down particles of phosphorous.

On Caribbean islands and in Mexico the seaweed has become a formidable menace. The government of Barbados declared it a national emergency in June, while Mexico News Daily reported the government is spending the equivalent of $3.08 million this summer to clean beaches along the Yucatan Peninsula where seaweed was piling up several feet deep.

But the Florida Keys also suffers when sargassum fills up canals, boat harbors, and mangrove stands, where it rots and forms dead zones.

“We’re talking a catastrophic problem in the Caribbean,” LaPointe said. “2015 was the worst year to date, and that’s when we began to realize this was not a one-off event, it’s happened repeatedly.”

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Some cities in Broward County are clearing beaches of excess seaweed, or mixing it into the sand.

That’s less of an option in Palm Beach County, which typically has a greater density of sea turtle nests, said Kelly Martin, a senior environmental analyst for the county.

“We’re so unique here and the nesting is so dense, we can’t mark them all so we can’t do mechanical raking,” said Martin, noting that the seaweed hasn’t hurt turtle nesting this year. “The seaweed is definitely coming and going.”

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Sargassum can benefit the environment in that it provides shelter for hatchling sea turtles, who feed on the tiny crabs and other organisms that live in the seaweed. Birds also pick through it to find food.

In large quantities, however, it can tangle in boat propellers, and, if left piled up on the beach, can prevent turtles from laying eggs.

“A good day is when there is no seaweed,” said Houston Park, a Palm Beach ocean lifeguard. “It tends to have flies or sea lice in it. People ask where there’s a beach with no seaweed. This summer, it’s been pretty heavy.”

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