Karenia brevis blooms on the West Florida Shelf: A comparative study of the robust 2012 bloom and the nearly null 2013 event

•Described and contrasted are two successive west Florida shelf HAB years.•The pathway of K. brevis from offshore to the coast is the bottom Ekman layer.•Upwelling is required, but too much upwelling suppresses bloom development.•Ocean physics when combined with the organism biology offers new HAB insights.

Harmful algal blooms of the dinoflagellate Karenia brevis require an upwelling circulation to manifest along the coastline of the West Florida Continental Shelf. Too much upwelling, however, can impede bloom formation by increasing inorganic nutrient levels to the point where faster growing phytoplankton such as diatoms may out-compete the slower growing K. brevis, as occurred in 1998 and 2010. Both 2012 and 2013 experienced persistent upwelling, but only 2012 exhibited a robust harmful algal bloom. Here we examine the subtle differences in the coastal ocean circulation between those two years that led to the disparate bloom evolutions.