Historical analysis of environmental conditions during Florida Red Tide

•Environmental conditions during periods of large bloom and no bloom were examined.•Statistical analysis of seasonal and non-seasonal variations were conducted.•Periods of large bloom are found to only occur when the Loop Current is in the north.

The most frequent and intense harmful algal blooms, of the toxic dinoflagellate Karenia brevis, occur on the West Florida Shelf. Many aspects relating to such blooms remain poorly known, including environmental conditions during their occurrence. Here the position of the Loop Current, river runoff, and along- and cross-shore winds are analyzed, isolating persistent periods of no bloom (less than 100 cells L−1) and large blooms (more than 105 cells L−1). From 1993 to 2007, 9 instances are detected with a large bloom and 37 instances are found with no bloom. A two-sample unequal variance T-Test analysis reveals that the difference between periods of large blooms and periods with no bloom are statistically significant to the 90% confidence level for the Loop Current's position. Periods of large blooms are found to occur only when the Loop Current is in its northern position, due to the enhanced retention on the shelf.