Blooms of the toxigenic, harmful dinoflagellate Karenia brevis are an almost annual occurrence in the eastern Gulf of Mexico, typically initiating after the summer months and terminating in early spring or earlier. The question of whether blooms have been expanding in frequency or duration has been debated for years. Irregularity of data prior to the mid 1990s has hampered statistical analyses of long term trends. Recently, a bloom severity index was developed that captures changes in bloom occurrence in a rigorous way. Based on this index, bloom severity has increased over the past 5 decades. More severe blooms are related to increases in Gulf of Mexico temperature anomalies since the mid 1990s, as well as increases in river flow of the major river, the Caloosahatchee. Applying a measure of the nitrogen input from anthropogenic sources, there is also a direct and significant relationship with increasing bloom duration per year, and this relationship strengthens when decadal, rather than annual, averages are applied. Clearly, the combination of regional warming, changes in river flow, and increased nitrogen pollution continue to stress the waters of the eastern Gulf of Mexico and more blooms of this toxic dinoflagellate may be anticipated in the future.

harmful algal blooms, Karenia brevis, nutrient pollution, climate change