Editor’s note: Two large-scale algae outbreaks in Florida are killing fish and threatening public health. Along the southwest coast, one of the longest-lasting red tide outbreaks in the state’s history is affecting more than 100 miles of beaches. Meanwhile, discharges of polluted fresh water from Lake Okeechobee and polluted local runoff water from the St. Lucie and Caloosahatchee watersheds have caused blooms of blue-green algae in downstream estuaries on both coasts. Karl Havens, a professor at the University of Florida and director of the Florida Sea Grant Program, explains what’s driving this two-pronged disaster.
What’s the difference between red tide and blue-green algae?
Both are photosynthetic microscopic organisms that live in water. Blue-green algae are properly called cyanobacteria. Some species of cyanobacteria occur in the ocean, but blooms—extremely high levels that create green surface scums of algae—happen mainly in lakes and rivers, where salinity is low.
Red tides are caused by a type of algae called a dinoflagellate, which also is ubiquitous in lakes, rivers, estuaries and the oceans. But the particular species that causes red tide blooms, which can literally make water look blood red, occur only in saltwater.
What causes these blooms?
Blooms occur where lakes, rivers or near-shore waters have high concentrations of nutrients—in particular, nitrogen and phosphorus. Some lakes and rivers have naturally high nutrient concentrations. However, in Lake Okeechobee and the St. Lucie and Caloosahatchee estuaries, man-made nutrient pollution from their watersheds is causing the blooms. Very high levels of nitrogen and phosphorus are washing into the water from agricultural lands, leaky septic systems and fertilizer runoff.
Red tides form offshore, and it is not clear whether or to what extent they have become more frequent. When ocean currents carry a red tide to the shore it can intensify, especially where there are abundant nutrients to fuel algae growth. This year, after heavy spring rains and because of discharges of water from Lake Okeechobee, river runoff in southwest Florida brought a large amount of nutrients into near-shore waters of the Gulf of Mexico, which fueled the large red tide.
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