Seaweed invasion in Florida is more than annoying. It carries health risks too.

The beaches of South Florida are currently clogged with bristly, stinky and downright annoying piles of seaweed known as sargassum.

To get in the water, you need to stride through and sometimes swim within the seaweed. Even if you stay on the sand, you may have to endure the stench.

Beyond being annoying, the masses of organic matter can be home to things that sting and give you a rash. They can also release harmful gases.

Sea creatures, stingers and rashes

Though sargassum, which forms far out to sea and offers shelter for baby fish, shrimp, crabs, jellyfish and other creatures, some of the sea life can mean problems for people when it washes ashore.

Jellyfish and jellyfish larvae sometimes get tangled in the seaweed, and their tentacles can really hurt and cause severe skin irritation, rashes and blisters.

The tiny larvae of thimble jellyfish can be in the seaweed as well. They’re known to get trapped between your swimsuit and skin, resulting in a rash known as “seabather’s eruption.”

The small crabs and shrimp in the sargassum, which attract sea turtles and mahi mahi offshore, also can attract stingrays inshore. Stepping on one could result in a painful, venom-laden puncture wound.

Ew, that smell

After about 48 hours on the beach, sargassum starts to decay and can produce hydrogen sulfide and ammonia gases. Hydrogen sulfide smells like rotten eggs and ammonia gives off notes of urine, sweat or dead fish.

Neither are particularly good for you. Both can cause respiratory, cardiovascular and neurological impacts, depending on how long you’re exposed and the concentration of the gas.

Hydrogen sulfide, if you’re exposed long enough, can cause upper airway irritation, nausea, headaches, vertigo, confusion and even memory loss. Asthma or other respiratory problems may worsen as well.

Low concentrations of ammonia may cause coughing and irritation in the nose and throat, but the overall effects are less documented.

Recent papers have hypothesized but not proven that hydrogen sulfide from sargassum strandings could put pregnant women at a higher risk of early onset preeclampsia, a potentially life-threatening pregnancy complication.

Another side effect: nutrients in the seaweed can fuel growth of fecal bacteria already present in South Florida waterways, said Florida Atlantic University’s Brian Lapointe, a marine ecologist and one of the nation’s leading experts on sargassum blooms.

Normally, relatively clean ocean water dilutes the fecal runoff as it comes out of inlets, but if sargassum is present, the nutrients can boost its growth.

How it happened

Though sargassum is a natural occurrence, impacts in Florida have spiked since 2011, when the blooms shifted from the Sargasso Sea south into equatorial waters. Tradewinds and currents there carry the seaweed west, first through the Caribbean, then into the Gulf and eventually around the Florida peninsula.

Scientists don’t fully know why the shift occurred, but Lapointe has done research linking the blooms to flood years in the Amazon Basin.

Nutrient runoff from the big river drifts out into the Atlantic and fuels seaweed growth.

The algae also gains nitrogen from rain loaded with nitrates from fossil fuel pollution, Lapointe said.

Any relief in sight?

Based on satellite imagery, large amounts of sargassum are currently drifting “upstream” of South Florida, in the Gulf and Caribbean. Ocean currency should keep a steady supply of the algae headed toward the region for several weeks — if not months. June, July and August are typically the most intense months.

In their most recent monthly sargassum report, the University of South Florida Optical Oceanography Lab said sargassum amounts are likely to increase in June. “Beaching events around the Caribbean and southeast coast of Florida will continue and likely increase.”

Wind direction is a key factor. An east wind is needed to push the masses ashore. “That strong east wind we’ve had is due to high pressure sitting over the Carolina’s. All that has to line up,” said Lapointe.

“In summer there’s a permanent high across the Atlantic that’s the main driver of the sargassum beachings,” said meteorologist George Rizzuto of the National Weather Service Miami.

South Florida could get a break from the east wind early next week, however, as a front briefly comes in from the north. After that, the prevailing east-southeast winds will resume, meaning as long as there’s seaweed floating by, it’ll wash ashore.