The algae bloom was most likely fueled by pollutants — fertilizers, yard waste and animal feces — that were washed into the bay from the rains that hit the region over the past two months, according to Pinellas County officials. The recent heat wave has helped spur the explosive growth of the microscopic algae.
Hillsborough County's Environmental Protection Commission officials first noticed the bloom on June 4. It later showed up as a thick brown streak across the bay in an aerial photo snapped by a visiting California scientist who was soaring above the Howard Frankland Bridge on June 11. The size surprised experts.
"It's been pretty expansive," said Ed Sherwood, a scientist with the Tampa Bay Estuary Program.
Lately the bloom has been thickest in the waters along the Courtney Campbell Parkway. One scientist described the bloom there as being "like soup."
More than one type of algae appears to be involved in the bloom, but the predominant type is called Pyrodinium bahamense.
That's the same species that gives one of Puerto Rico's tourist attractions, Phosphorescent Bay, its name, said Cindy Heil, senior research scientist with the state's marine science laboratory in St. Petersburg. On a dark night, when stirred up, Pyrodinium produces a soft, greenish glow.
That same species has been documented in Tampa Bay since the 1960s, she said. This is the second summer in a row with a Pyrodinium bloom in Tampa Bay. Last year's bloom, which occurred in July, killed catfish, menhaden, pinfish, triggerfish, puffer fish, spadefish, stingrays, blue crab, brittle stars and small Florida crown conch.
Pyrodinium has been known to produce a toxin that's poisonous to shellfish, although so far this year's bloom has not affected the oyster and clam beds in Boca Ciega Bay, Heil said.
What killed the fish last year wasn't poison but rather a lack of oxygen, she said. At night the blooming algae absorb all the oxygen in the water, leaving none for any fish nearby. They suffocate.
An algae bloom of this size could produce a large fish kill, but so far that hasn't happened. Pinellas officials are "just keeping our fingers crossed that we can escape that problem this year," said Kelli Hammer-Levy of the county Department of Environmental Management.
No one knows what caused last year's bloom to subside, or what made this year's bloom so huge. But if the nutrient-rich pollution stops flowing, or the heat eases up, the bloom may break up.
Although this bloom is relatively large, it's not the largest in Tampa Bay's history, said Heil. The biggest was a 1971 Red Tide bloom from the Sunshine Skyway bridge up into Old Tampa Bay. It left thousands of dead fish in the canals and bayous in St. Petersburg, and fishermen reported shark feeding frenzies in the upper bay, she said.
County and state officials are monitoring the bloom and sampling the water regularly to try to predict any further changes. The next sampling will occur Wednesday.
Craig Pittman can be reached at firstname.lastname@example.org or (727) 893-8530.