The Risk Factor: Climate Change Opens Windows of Opportunity for Harmful Algal Blooms

Marine microalgae in the genus Alexandrium produce potent neurotoxins called paralytic shellfish toxins usually when they bloom. These toxins can accumulate in shellfish and cause human illness or even death if contaminated shellfish are consumed. A specific combination of environmental conditions creates a window of opportunity for these harmful algal blooms (HAB-WOO) and can significantly increase the risk for toxic events in Puget Sound. HAB-WOOs of long duration indicate long periods of time when conditions in the marine environment are favorable for the development of toxic blooms that threaten shellfish safety. Past trends and future scenarios for the HAB-WOO are evaluated for the 2020s, 2040s, and 2080s using an innovative modeling approach. Specifically, the model simultaneously calculates time periods when multiple environmental parameters are within a range that has been determined to be favorable for the development of toxic events. Model results show that the HAB-WOO duration increased since 1978, as did the frequency and geographic extent of toxic events. Regionally downscaled projections of climate change for the Pacific Northwest are used to evaluate scenarios for the future HAB-WOO. Under a moderate greenhouse gas emissions scenario (i.e., A1B), the annual HAB-WOO is projected to increase by an average of 13 days by the end of the 21^st century. Furthermore, the annual HAB-WOO may begin up to 2 months earlier in the year and persist for up to 1 month later in the year compared to the present day typical annual HAB-WOO time period. This study demonstrates for the first time how a changing climate alters the marine environment in a way that may increase the risk of human exposure to HAB toxins.