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Why Toxic Algae Blooms Are Likely To Get Worse Before They Get Better

Hans Paerl
UNC Institute of Marine Sciences
A large algal bloom on Lake Taihu in China, caused by cyanobacteria like those now spreading across Lake Okeechobee

  The bacteria that causes toxic algal blooms like the one now affecting the Treasure Coast have been around for billions of years. Cyanobacteria even deserves some of the credit for putting the oxygen we breathe into the Earth’s atmosphere. But runoff from agriculture, storm water and sewage have given this blue-green algae an unnatural boost, accelerating the natural processes that break down dead plants and animals into nutrients that enrich our lakes and streams. 

Climate change is also helping speed up the bacteria’s development—not just because of warmer temperatures (which give the algae more time to develop in cooler, ice-covered lakes)—but for a litany of reasons, including increased levels of carbon dioxide, the gradual expansion of shallow waters through sea level rise and cycles of extreme rainfall and drought. 

In that sequence—already seen in lakes in Florida and Texas as well as China—marine ecologist Hans Paerl says “nutrients get washed into the system, then all of a sudden the system gets stagnant and hot, and calm. That’s the combination for these blooms to be optimized.”

“No matter how you slice or dice it, the major symptoms of climate change, warming and more extreme events,”Paerl says, are favorable for cyanobacteria.

Paerl, who studies algal blooms at the University of North Carolina’s Institute of Marine Sciences, says another factor that makes any fix more difficult is that the nutrients algal blooms rely on are often stored in sediments he likens to a water body’s long-term memory. 

“The sediments in a lake are kind of the memory of the system,” Paerl says. “What you put in takes a long time to get out.” Some nitrogen can be eliminated from the water naturally when it’s transformed into gas. 

Phosphorus—the other key ingredient in algal blooms— is much more difficult, and costly, to remove. Here in South Florida, large areas planted with cattails have helped reduce the flow of phosphorus in some areas by 75 percent, but that reduction does not address existing phosphorus deposits built up over decades of sugarcane farming. Adding chemicals to “lock” the phosphorus into lake sediment, or dredging, are other strategies with their own drawbacks—like finding a place to store huge quantities of phosphorus-laden soil.

Once they’re established, Paerl says, algal blooms tend to create a kind of cycle: “These blooms are pretty smart actually: The algae float up near the surface; they use up all the light.” Once that happens, the plants on the bottom die off, creating—you guessed it—more sediment for blooms to feed on. 

With all that in mind, Paerl says, “really the only knob we can tweak in all this is the input knob.”

That means farms, cities and golf courses need to do better keeping nutrients out of the water in the first place.

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