CLEARWATER — Look toward the water when you drive across Tampa Bay this weekend and you might see a strange, scummy cloud, the color of rust.
That's an algae bloom, the bay's first in two years. It's big, it's dangerous for fish and it's growing by the day.
The bloom of Pyrodinium bahamense has exploded in the last month, stretching from Safety Harbor to the Howard Frankland Bridge.
"And it's thick," said Tampa Bay Estuary Program executive director Holly Greening. "You can actually see a boat's wake in the bloom."
Small algae balls, called cysts, that rest on the bay floor were awakened by the right mix of summer heat, ample rain and lawn runoff. Nitrogen and phosphorous fertilizers pollute the water and feed the bloom.
It is so far smaller than a bloom in 2009, which, at 14 miles wide, was one of the largest in bay history. With time and the right conditions, researchers say, it could grow bigger.
That could prove deadly for marine wildlife. At night, the algae absorbs lots of oxygen, stifling the fish. No fish kills were reported in 2009, but a bloom in 2008 killed 10 different species, including catfish, puffer fish, stingrays, blue crabs and brittle stars.
Though the bloom looks like a hazy spill, it is not toxic or viscous like oil.
The algae are great masses of microscopic life forms that float at the surface or, if overheated, swim downward with tiny flagella. When disturbed, they glow greenish-white.
This is the first bay bloom since Pinellas County's ban on the sale or use of nitrogen and phosphorous fertilizers began in June. Tampa will ban the fertilizers next summer. Hillsborough County allows their sale but bans their use before heavy rainfall.
"It does point to the need to be very careful with what we do in our back yard," Greening said.
The bay's biggest bloom, in 1971, stretched from the Sunshine Skyway into Old Tampa Bay. Thousands of dead fish were found floating in the waters of St. Petersburg.
Contact Drew Harwell at (727) 445-4170 or firstname.lastname@example.org.