Dinoflagellate vertical migration fuels an intense red tide

Harmful algal blooms are rising in frequency and intensity, yet the mechanisms behind their extreme biomass remain unclear. While prior research has shown that these blooms can disrupt ecosystems and human health, the specific biological and chemical processes that allow them to thrive are not fully understood. A long-standing hypothesis suggests that dinoflagellates may use vertical migration to access nutrients and outcompete other species. This idea has not been conclusively tested in real-world conditions. Observational tools have improved, offering new ways to study these phenomena in situ. However, the connection between dinoflagellate behavior and nutrient cycling has remained speculative. The role of nitrate in sustaining blooms has been noted, but the exact pathways of its uptake are still debated. Coastal ecosystems face significant challenges from these events, yet the underlying drivers are not well quantified. This uncertainty has limited the ability to predict and manage harmful algal blooms effectively.

This study aimed to test a 50-year-old hypothesis about how dinoflagellates sustain high biomass during harmful algal blooms. The researchers focused on the role of vertical migration in nutrient acquisition and competitive advantage. They sought to determine if downward movement at night allows dinoflagellates to access deep nitrate. The study used an autonomous ocean profiling system to collect in situ data. The goal was to link observed migration patterns to changes in nutrient and chlorophyll levels. The researchers also aimed to compare their findings to historical biogeochemical data. This comparison would help assess the impact of blooms on coastal ecosystems. The study's results could provide new insights into the dynamics of harmful algal events.

The researchers deployed an autonomous, ocean-wave-powered profiling system to collect in situ data. This device measured vertical movement, nitrate levels, and chlorophyll concentrations. They focused on the dinoflagellate *Lingulodinium polyedra* during a 2020 red tide event. The system recorded changes in nitrate and particle load over time. Data were analyzed to determine the timing and depth of downward migration. The maximum migration depth was linked to local nitrate concentrations. The team compared their findings to 70 years of climatological data. This comparison helped establish the uniqueness of the observed biogeochemical conditions.

The study found that *Lingulodinium polyedra* migrated downward at dusk during the red tide event. This movement coincided with a depletion of deep nitrate levels. The maximum depth of migration was determined by the availability of nitrate. As nitrate levels dropped, chlorophyll concentrations and particle load increased. These changes indicated that the dinoflagellates were assimilating deep nutrients. The observed patterns were consistent with the 50-year-old hypothesis. The red tide created biogeochemical conditions not seen in 70 years of historical data. This suggests that vertical migration significantly alters coastal ecosystems during blooms.

The researchers concluded that vertical migration by *Lingulodinium polyedra* plays a key role in sustaining red tide events. The downward movement at night allows access to deep nitrate, which supports high biomass. This process outcompetes nonmotile phytoplankton species. The study confirmed a long-standing hypothesis using in situ data. The observed changes in nitrate and chlorophyll levels were directly linked to migration. The red tide created anomalous biogeochemical conditions compared to historical records. These findings highlight the impact of dinoflagellate behavior on coastal ecosystems. Improved understanding of these dynamics may aid in predicting and managing harmful algal blooms.