Seasonal and interannual variability of harmful algal blooms along the U.S. West Coast

The neurotoxin domoic acid produced by Pseudo-nitzschia species is the primary cause of detrimental impacts associated with harmful algal blooms (HABs) along the U.S. west coast, threatening marine ecosystems, fisheries, and human health. Here, a Generalized Linear Model (GLM) is used to produce a 26-year (1995-2020) hindcast of HAB occurrence along the southern Oregon and California coast (32-44°N), with predictor variables derived from the output of a regional physical-biogeochemical model. This approach addresses limitations associated with sparse nutrient observations and sets the stage for producing long-term HAB projections that must rely solely on simulated fields for predictor variables. The GLM reproduces observed spatiotemporal variability of HABs and reveals strong latitudinal, seasonal, and interannual patterns, as well as overlapping local and regional impacts of predictor variables on model skill. Predicted HAB occurrence typically peaks during the upwelling season, but with a northward shift from spring to summer. Interannual variability exhibits multi-year periods of enhanced or reduced HAB presence reflecting the periodicity of basin-scale dynamics known to modulate ocean productivity along the U.S. west coast. Over the period considered, the GLM also predicts a decline in HAB probability south of 36°N and an increase to the north, especially from January to June. Overall, the spatiotemporal patterns of increased HAB formation predicted by the GLM typically coincide with times and locations promoting phytoplankton growth. By providing a comprehensive view of recent historical HAB variability at the scale of the U.S. west coast, the GLM offers relevant information for fisheries management, public health, and future forecasting efforts.