Wirewalker data collected during the 2020 Southern California red tide

Harmful algal blooms (HABs) are globally increasing economic, health, and
ecosystem threats. In spite of the relatively frequent occurrence of HABs,
the mechanisms responsible for their initiation and exceptional abundance
remain imperfectly understood. A 50-year-old hypothesis posits that dense
dinoflagellate blooms derive from motility: swimming upward during the day
to photosynthesize and downward at night to access the deep nutrient pool.
This allows dinoflagellates to outgrow their nonmotile competitors. We
tested this hypothesis using in situ data from an autonomous,
ocean-wave-powered vertical profiling system. We showed that the
dinoflagellate Lingulodinium polyedra’s vertical migration led to
depletion of the deep nitrate pool during a 2020 red tide HAB event.
Downward migration began at dusk, with the maximum migration depth
determined by local nitrate concentrations. Losses of nitrate at depth
were balanced by proportional increases in phytoplankton chlorophyll
concentrations and suspended particle load, conclusively linking vertical
migration to the access and assimilation of deep nitrate in the ocean
environment. Vertical migration during the red tide created distinctly
anomalous biogeochemical conditions compared to 70 years of climatological
data, demonstrating the capacity of these events to temporarily reshape
the coastal ocean’s ecosystem and biogeochemistry. Advances in the
understanding of the physiological, behavioral, and metabolic dynamics of
HAB-forming organisms from cutting-edge observational techniques will
improve our ability to forecast HABs and mitigate their consequences in
the future.