Why Was There a Harmful Algal Bloom in 2015: The Relative Growth of Toxic and Non-toxic Diatoms as a Function of Temperature

A coastwide bloom of the toxigenic diatom Pseudo-nitzschia in 2015 resulted in the largest recorded outbreak and unprecedented levels of the neurotoxin, domoic acid (DA), along the North American west coast. The scientific community has suggested that warmer ocean temperatures were the main cause of this harmful algal bloom (HAB), but little scientific evidence to support the relationship between temperature, and the growth and toxicity of Pseudo-nitzschia has been provided for local isolates of these diatoms. To gain insight into bloom dynamics, a laboratory study was conducted to examine the growth of toxic and non-toxic phytoplankton species at a range of temperatures. Non- (or low) toxic diatoms Pseudo-nitzschia fraudulenta, Skeletonema costatum, and Chaetoceros decipiens were isolated from the 2015 bloom, and cultured at eight temperature conditions (5.6, 6.8, 8.7, 10.8, 13.3, 15.2, 17.2, 19.0°C). A total of 48 cultures (6 tubes per condition), with duplicates at each temperature, were grown in a temperature gradient incubator and monitored for 31 days over three complete growth cycles (runs) of exponential and stationary growth. Specific growth rates, determined from daily measures of in vivo fluorescence, indicate that by Run 3, there was no growth at 5.6°C for Chaetoceros decipiens, and a large decline in the growth rate for Skeletonema costatum at 17.2 and 19.0°C. Pseudo-nitzschia fraudulenta demonstrated the greatest growth rates of all species from 10.8 to 19.0°C, and exhibited the broadest range of elevated growth rates. These temperature results indicate that Skeletonema costatum does not thrive in ocean temperatures above 15°C, and is outcompeted by other algae, including both species of Pseudo-nitzschia. Results of this study will greatly aid oceanographers in determining the dominant species in a coastal region as a function of ambient ocean temperature conditions