Plankton Super Swimmers Fuel Historic Red Tide Event in Southern California

In the spring of 2020, a major red tide event occurred in waters off Southern California, resulting in dazzling displays of bioluminescence along the coast.

Extremely high densities of Lingulodinium polyedra, a plankton species famous for emitting a neon blue glow, were responsible for the spectacle.

While the red tide captured the public's attention and made international headlines, it was also a dangerous algal bloom. 

Toxins with the potential to harm marine life were detected at the peak of the bloom, and dissolved oxygen levels dropped to near zero as the red tide's extreme biomass decomposed.

This lack of oxygen resulted in fish deaths and other negative effects on local ecosystems.

For the first time, scientists at UC San Diego's Scripps Institution of Oceanography and Jacobs School of Engineering have pinpointed how this plankton species-a dinoflagellate-was able to produce such a dense bloom.

The answer lies in dinoflagellates' remarkable swimming ability, which gives them a competitive advantage over other phytoplankton species.

This swimming ability, according to the authors, may contribute to the development of dense blooms, involving bioluminescent blooms. 

How dinoflagellates swim and outcompete other plankton

(Photo : VALERIE MACON/AFP via Getty Images)

Dinoflagellates are a group of microscopic organisms that belong to the phytoplankton category, meaning they are photosynthetic and use sunlight to produce energy.

However, unlike most phytoplankton, dinoflagellates have two whip-like appendages called flagella that allow them to swim in the water column.

This gives them an edge over other phytoplankton that are unable to propel themselves against the current and drift along with the ocean.

The study showed that L. polyedra dinoflagellates are highly mobile, swimming upward during the day to photosynthesize and downward at night to access a deep nutrient pool.

This resulted in an intensified ruddy coloration of the water at the surface of the water, giving rise to the term "red tide," which was most visible in the afternoon.

A large population of dinoflagellates was observed descending at night, though a portion remaining near the surface waters, resulting in nighttime bioluminescence displays.

The authors discovered that it was due to this vertical migration that the dinoflagellates outgrew their non-mobile competitors, which include other phytoplankton species. 

By accessing both light and nutrients, they were able to sustain high growth rates and dominate the phytoplankton community.

The study also revealed that L. polyedra dinoflagellates have a high affinity for nitrate, a key nutrient that is often limiting in coastal waters

By swimming down to deeper waters where nitrate is more abundant, they were able to take up more nitrate than other phytoplankton and use it efficiently for their growth.

Also Read: "Dead Fishes Everywhere": Red Tide Turned Florida Coast Toxic, Threatening Marine Ecosystem

Implications and Future Directions

The study supported a 50-year-old hypothesis proposed by Scripps Oceanography biological oceanographer Richard "Dick" Eppley, who proposed that the upward movement of dinoflagellates was linked to harmful algal blooms, a phenomenon that has been observed off the coast of Southern California for at least 120 years.

The study also demonstrated the use of novel ocean technologies, such as autonomous underwater vehicles and optical sensors, that allowed for unprecedented measurements of how phytoplankton respond to small-scale changes in the coastal ocean.

The authors hoped that their findings will help improve the understanding and prediction of harmful algal blooms, which have significant ecological and economic impacts on coastal regions around the world.

They also suggested that more research is needed to explore how climate change and human activities may affect the occurrence and intensity of these events in the future.

Related article: Persistent Toxic Red Tide Causes Rapid Die-Off for Seagrass in Florida