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.