More sustained, more severe blooms and shifting monthly patterns of the toxigenic dinoflagellate Karenia brevis on the West Florida Shelf

Various recent reports, based on different approaches, data sets and time periods, have yielded different conclusions with regard to whether blooms of the Florida red tide dinoflagellate, Karenia brevis, have increased over time. Without question, however, there have been a number of recent blooms that have been long lasting, continuing through the summer months normally taken to be outside the ideal temperature niche for K. brevis. Here, using a recently developed bloom severity index, the time series of blooms from 1970 to 2019 is examined, focusing on how monthly patterns have changed over time. More severe blooms have been found since the mid 1990s, now lasting 4- to 5-months longer than in previous decades, a trend related to the Oceanic Niño Index (El Niño -Southern Oscillation). Since the mid-1990s, water temperature anomalies have been related to bloom severity with lags of 3 to 6 months. The most significant temperature increases have occurred in the latter months of the year when K. brevis growth typically is highest. Increased flow from the Caloosahatchee River, and its total nitrogen load, are also predictors of recent bloom severity with lags of several months. Cells that survive the now-warmer winter dry season have adequate nutrients and may experience more favorable nitrogen forms as the summer wet season develops, and as nutrients are recycled, may “over summer”. The stresses of increased warming and increased nutrient pollution on K. brevis blooms will continue to make managing these blooms a challenge for management as climate change trajectories continue.