Red Tide and Offshore Benthic Mortality (2005)

The 2005 bloom of Karenia brevis, the Florida red tide organism, was extensive and persistent, spanning more than 500 square miles and lasting nearly 13 months. This bloom killed fish, manatees, dolphins and sea turtles. Shellfish harvesting areas were closed for more than a year because brevetoxins, the neurotoxins produced by K. brevis, had accumulated in clams and oysters. Additionally, separation of the water column into distinct layers trapped K. brevis in the bottom water layers between New Port Richey south to Sarasota, leading to deaths of bottom-dwelling animals. 

Bloom Cycle

The bloom was first detected approximately 20 miles west of St. Petersburg in January 2005. During the next two weeks, the bloom moved rapidly inshore. K. brevis concentrations remained high in the nearshore region between St. Petersburg and Lee County’s Sanibel Island through late March. By June 2005, the main patch of K. brevis spread from the Tampa Bay area and adjacent coastal waters northwest to the Florida-Alabama border. The bloom eventually ended in February 2006.

Effects of the Bloom

As with most K. brevis blooms, fish kills were common; however, the extent and duration of this bloom resulted in severe effects on fish populations. Commercial fishing boats reported severe declines in catches. When FWRI researchers compared fish abundance during this period to that of the previous 10 years, they found population declines of juvenile spotted seatrout, sand seatrout and red drum. Researchers also noted changes in the sand seatrout spawning aggregations in the Tampa Bay area.

Unique to the 2005-2006 bloom was the widespread deaths of bottom-dwelling animals caused by oxygen depletion in bottom waters from New Port Richey south to Sarasota, approximately three to 23 miles offshore. Researchers estimated the bloom affected bottom communities within a 2,162-square-mile area. During the first week of August 2005, diving and fishing charter businesses reported mass die-offs of fish and other reef animals. Reports also mentioned a smell similar to rotten eggs and divers' silver jewelry and coins turning black, conditions that indicate low oxygen concentrations. Researchers confirmed low dissolved oxygen in the affected areas. Organisms affected included fish (ranging from baitfish to goliath grouper), sponges, corals, worms, molluscs, crabs, sea urchins, starfish and sea turtles. Bottom visibility was also considerably reduced.

Causes of the extensive die-offs

Several factors contributed to the widespread animal die-offs. First, the 2005-2006 bloom extended across a large area and persisted for a long period. Such blooms have greater effects than localized and short blooms. It was suggested that the 2004 hurricane season triggered pulses of groundwater release in offshore springs that provided substantial nutrients to the 2005 red tide bloom. Then, in summer 2005, the sun heated up coastal surface waters leading to a layering of warmer, less dense water over a cooler, denser layer near the bottom. This layering trapped red tide cells in the cooler bottom layer, where bottom-dwelling animals were present. Through time, bacterial decomposition of dead animals and K. brevis cells decreased oxygen concentrations to the point that low oxygen (hypoxia) and no oxygen (anoxia) near the bottom caused mass die-offs. The considerable temperature difference between the two water layers further prevented dissolved oxygen in surface layers from reaching the bottom, worsening the oxygen depletion. Fish killed on the surface by the red tide toxin may have also sunk to the bottom during decay, further reducing oxygen.