Scientists Continue Tracking 14-mile Algae Bloom

Scientists are continuing to keep tabs on one of the largest algae blooms in Tampa Bay’s history, which stretched more than 14 miles this summer from Safety Harbor to Weedon Island.

First observed in May following heavy rainfall, the extensive algae bloom was most likely fueled by stormwater runoff flowing into the bay. Stormwater carries lawn fertilizers, pet waste, sewer overflows and even exhaust from automobiles into the bay, leading to excess nitrogen that causes microscopic algae to multiply rapidly.
At the height of the bloom, the streaked, rusty-brown water was easily visible from the Howard Frankland Bridge.

Algal blooms are not uncommon in summer months, when warm temperatures fuel the growth of algae. But the size and duration of this bloom has worried experts.

“The fear was that there would be a dissolved oxygen crash (leading to fish kills) after the algae died off, but that hasn’t happened this year except in isolated areas,” said Ed Sherwood, a scientist with the Tampa Bay Estuary Program. The shading effect of so much algae also could damage seagrasses, adds Sherwood, but the impact won’t be known until next year.

Dorian Aerial Photographics

Rusty-brown streaks fouling the water near the Howard Frankland Bridge signal the presence of an algal bloom that at its height this summer stretched 14 miles from Safety Harbor to Weedon Island.

This is the second year in a row when a large and persistent bloom has fouled the bay’s waters. Last year’s bloom, which occurred in July, killed catfish, menhaden, pinfish, triggerfish, puffer fish, stingrays, blue crab and brittle stars, after algae consumed oxygen in the water.

Both blooms have been attributed to Pyrodinium bahamense, the same dinoflagellate species that gives Puerto Rico’s Phosphorescent Bay its name. Documented in Tampa Bay since the 1960s, it casts a soft greenish glow on dark summer nights when the water is stirred.

While Pyrodinium can produce a toxin responsible for poisoning shellfish, the bloom in Tampa Bay isn’t producing toxins in levels that would produce mass fish kills, says Sherwood. And last year’s fish kills resulted from depleted oxygen in the water, not toxins.

Environmental officials monitoring the Tampa Bay bloom say conditions improved in September. “The large-scale bloom has largely dissipated, but elevated concentrations were observed in Feather Sound and the Largo inlet,” said Richard Boler of the Hillsborough County Environmental Protection Commission. Pinellas County also reported a decline in chlorophyll a, an indicator of algae presence, from 32.9 ug/l to 19.4 ug/l in Old Tampa Bay.
Because Pyrodinium leaves a cyst in the sediments that can live for years, officials say it is likely to reoccur in Tampa Bay next year.

A recurring Pyrodinium bloom in the Indian River Lagoon over several years has caused mass fish kills and prompted bans on harvesting puffer fish.