River of Grass

Why Red Tide isn’t Red – Video

Over the last year, Florida has experienced a double whammy of algae blooms — one offshore and one closer to home.

A red tide — a choking cluster of toxic algae — floating off Florida’s southwest coast has killed 115 manatees, more than 350 sea turtles and hundreds of tons of fish since it began last October. Then, starting in early June, harmful blue-green algae began to flood Lake Okeechobee — the largest body of fresh water in Florida. At one point this summer, Lake Okeechobee’s waters were 10 times too toxic to touch.

Thanks to these combined harmful algae blooms, Florida by some estimates could lose up to $2 trillion in tourism and business revenues.

Based on the timing and location of the two blooms, it is tempting to tie them together. Lower Florida has plenty of warm water and agricultural nutrients like nitrates and phosphates, both of which can contribute to the growth of the organisms involved in these blooms.

Yet their origins contrast as much as their colors. While blue-green algae appear to spawn in direct connection with the nutrients poured into Lake Okeechobee, it is unclear why Florida’s red tide forms, and why they make toxins in the first place is one of the great mysteries of marine science.

Why red tide isn’t red
Let’s start with one major misconception about red tide. It isn’t red.

Red tide refers to a random assortment of phytoplankton — single-celled marine organisms. Similar to photosynthesis in plants, these phytoplankton make pigments capable of capturing sunlight to make food.

Under a microscope, one of these phytoplankton looks golden-brown, but when millions collect into a bloom, our eyes can mistake this mass as being red.

Under a microscope, one of these phytoplankton looks golden-brown, but when millions collect into a bloom, our eyes can mistake this mass as being red. In truth, the color of these blooms can range from burgundy to yellow, and this optical illusion applies to phytoplankton species that do not produce toxins.

“Just about all phytoplankton look red to our eye at high enough biomass,” Clarissa Anderson, a biological oceanographer at the Scripps Institution of Oceanography, told the PBS NewsHour. “It is not connected to any special pigments but more a function of the cone [cells] in our eyeballs and the way things shift near red wavelengths. I never use the term ‘red tide’ for this very reason, since we can easily have a benign organism blooming nearshore that discolors the water red.”

There are two main types of phytoplankton: dinoflagellates, which swim with whip-like tails, and diatoms, which are immobile and travel via ocean currents. In Florida and along the Gulf Coast, the red tide of everyone’s nightmares is caused by a dinoflagellate called Karenia brevis. Finish Reading on PBS.