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Data: European Space Agency Sentinel 3 satellite via NOAA; Map: Axios Visuals

An algae bloom has taken over Florida's largest freshwater lake and spread to surrounding areas, turning waters green and threatening drinking water, as well as tourism and wildlife.

Why it matters: Big algae blooms hit waterfront economies that are especially depending on boating and fishing, like those in Florida. This marks the ninth large algae bloom in the state since 2004.

In response to the bloom, Gov. Rick Scott declared a five-county state of emergency on July 9. In response, officials are forgoing the normal water release schedule to keep water and algae in the lake, hoping that this will allow conditions in areas downstream to improve. Water sent into the Caloosahatchee River, for example, resulted in a major bloom in that waterway as well.

How this happened: Algae blooms occur when environmental conditions favor the rapid growth of algae, which feed off of nutrients found in fertilizers and waste products. Nearly calm, warm waters like those found on Lake Okeechobee can be especially ideal for such blooms.

The big picture: Algae blooms' harmful effects can be profound. In 2014, for example, 500,000 people in Ohio were left without drinking water for three days in 2014 due to an algae bloom in Lake Erie.

And they can be impossible to rein in once they develop. On June 12, just 1% of Lake Okeechobee was covered with a thick green algal bloom. Twenty days later, it covered 436 miles and 90% of the lake. Water releases from Lake Okeechobee have spread the bloom to other areas, including in Fort Myers and Naples.

Warming air and water temperatures may make algae blooms more common, since toxic blue-green algae prefer warm water. The algae is made up in part of toxic cyanobacteria, which, when present in large enough numbers, can deplete the oxygen in the water to create so-called "dead zones" in the water column below.

NOAA algal bloom specialist Richard Stumpf said that in Florida and other warm climates, blooms can last all year.

Who's to blame: South Florida is naturally a relatively nutrient-poor environment. As agriculture and Florida's population have grown, more nutrients have been added to the water, encouraging algal growth.

  • Phosphorus and agriculture have the most to do with these problems. Phosphorus-based fertilizers used in agricultural areas near the lake have increased the nutrients substantially.
  • Human solid waste has been trucked up from South Florida and dumped on farms near Lake Okeechobee, also upping nutrients to even higher levels.

Who's hurting: Human health in the region can deteriorate as the blooms proliferate. The algae is also having economic ramifications, too, as recreational activities have slowed or stopped on some waterways too loaded up with a carpet of algae.

What can be done: Paul Gray, a staff scientist for Audubon Florida, told Axios that there are a number of measures that can be taken to reduce the chances of algal blooms, like helping landowners keep nutrients on their property and building filter marshes so that plants can remove the nutrients from the water. But Gray said this is expensive, and due to little private or public funding "we've been very slow putting them on the ground."

What's next: The Army Corps of Engineers is charged with managing Okeechobee's water levels, and regulates the flow of water between the lake and surrounding waterways.

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