What's the smelly brown gunk hugging Captiva's shoreline?

Hasn't Lee County had enough stinky gunk in its waters lately? Apparently not, as evidenced by the spongy rafts of floating foulness currently bedeviling Captiva. It's blanketing Roosevelt Channel and the Captiva Drive shoreline, lined with homes that start at about $1 million.

Here’s how island resident Carroll Wetzel described the mess in an email he sent earlier this week hoping to get it identified.

“The stuff (is) clogging up the mangroves and around our dock. It is not like the blue green algae, which I gather … is a thin layer. This is brown and puffy like cotton. It smells horribly. Smells terrible,” he wrote. “One of my neighbors who is very close to it has closed up at his house and turned on the AC because of the smell.”

Photos of the spongy mats have been turning up on social media as well, with speculation that – what with the color and revolting smell and all – it might be raw sewage.

After a series of emails flew back and forth between the city of Sanibel, the Lee County Health Department and members of the local media, Rick Bartleson settled the fecal matter question: It’s not poop. It’s algae – cyanobacteria, to be exact.

Yes, cyanobacteria is the awful toxic stuff that fouled Lee County freshwater, devastated wildlife and triggered a state of emergency declaration last year, but this cyanobacteria (there are thousands of separate species) is a different variety.

Which variety precisely is still in question, but Bartleson, a research biologist with the Sanibel-Captiva Conservation Foundation’s marine lab, is confident it’s not one of the cyanobacteria in the microcystis group, the kind that caused all the recent trouble. Bartleson also is confident that, stench aside, this stuff isn’t toxic.

More:Septic leaks, fertilizer run-off targets of Lee water clean-up efforts

Instead, he said, its sudden bloom is a sign of an ecosystem out of balance from excess nutrient loading. In plain speech, that means it’s been over-fertilized. The fertilizer, in the form of nitrogen-rich water, gets washed into the Gulf from excessive runoff, which can come from development, wetland destruction and ditching, he said.

“The literature suggests that its abundance is influenced by nutrient pollution,” said biologist and Sanibel’s director of natural resources James Evans. “It is commonly an indicator of eutrophication,” scientist-speak for what happens when too many nutrients enter the water, usually through human disruption, such as runoff from lawns, septic tank leaks or air pollution.

Although cyanobacteria is usually blue and/or green – hence its common moniker, blue-green algae – this stuff is decidedly closer to the puce section of the color wheel (the . brownish red area).

That’s because, “in addition to the chlorophyll, it has a lot of phycoerythrin,” Bartleson said. “which is pink.”

Another reason for the bloom may be that red tide killed off the underwater critters like sea hares that would normally graze it down and keep it in check, Bartleson said.

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“We had hypoxic (oxygen-starved) water coming into Blind Pass after an upwelling event when we had the dead zone last year,” he said. “That did in a lot of grazers.”

How would Bartleson rate this event on the crisis scale?

“Low. Seagrasses are temporarily shaded and starved when they get covered up, but the mats float away eventually.”

One word of caution, though: Keep your dog away from it.

“It can look sort of like poop and a dog might eat it,” said Bartleson. “A couple of dogs got sick after visiting the causeway when it was thick there a few years ago.

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