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Plankton super swimmers fueled the red tide event of 2020

In the spring of 2020, a significant red tide event off the coast of Southern California captured global attention with its stunning displays of bioluminescence.

This phenomenon was caused by exceptionally high densities of Lingulodinium polyedra (L. polyedra), a plankton species known for its ability to emit a neon blue glow. 

Despite the awe-inspiring visuals, the red tide event also marked a harmful algal bloom, with toxins detected at peak bloom levels that could potentially harm marine life. 

Furthermore, the decomposition of the extreme biomass from the red tide led to near-zero dissolved oxygen levels, resulting in fish die-offs and other detrimental impacts on local ecosystems.

What caused the red tide event?

Scientists from UC San Diego’s Scripps Institution of Oceanography and Jacobs School of Engineering have conducted a study that sheds light on how this particular dinoflagellate species managed to create such an exceptionally dense bloom. 

The key lies in the dinoflagellates’ extraordinary swimming ability, which gives them a competitive edge over other phytoplankton species and enables them to form dense, including bioluminescent, blooms.

“The idea that vertical swimming gives dinoflagellates a competitive advantage actually goes back more than half a century, but only now do we have the technology to conclusively prove it in the field,” explained study senior author Drew Lucas, an associate professor at Scripps Oceanography and the Department of Mechanical and Aerospace Engineering at UC San Diego.

Highly mobile dinoflagellates 

Lucas, along with former graduate student Bofu Zheng and several colleagues, seized the opportunity to deploy sophisticated ocean instruments off the San Diego coast during the red tide event in April and May 2020. 

This effort was supported by funding from the Southern California Coastal Ocean Observing System (SCCOOS) through an award by the National Oceanic and Atmospheric Administration (NOAA).

The study revealed that L. polyedra dinoflagellates are highly mobile, swimming upward during the day to photosynthesize and downward at night to access deep nutrient pools. This movement intensified the water’s ruddy coloration at the surface, leading to the term “red tide,” most prominently observed in the afternoon. 

Researchers documented a significant population of dinoflagellates making the downward journey at night. However, some remained near the surface, resulting in nighttime displays of bioluminescence. 

The authors concluded that this vertical migration enabled the dinoflagellates to outgrow their non-mobile competitors, including other phytoplankton species.

50-year-old theory confirmed 

This study validates a 50-year-old hypothesis by Scripps Oceanography biological oceanographer Richard “Dick” Eppley, who suggested that the vertical migration of dinoflagellates was associated with harmful algal blooms, a phenomenon documented off Southern California for at least 120 years.

Although extensive lab research supported this idea, researchers had not field-tested it until the 2020 event.

L. polyedra, like many dinoflagellate species, possesses a pair of flagella, whip-like appendages that propel the single-celled organism through water. Besides its swimming ability, L. polyedra exhibits remarkable speed, reaching up to 10 body lengths per second for almost 24 hours. 

“In the plankton world, they are Michael Phelps,” said Lucas. “For comparison, fast-burst swimming in species like bluefin tuna or shortfin mako is around 9-10 body lengths per second, but only for very short periods.”

“Their exceptional swimming allows L. polyedra to dive to cold depths where they can take up nutrients, allowing these organisms to really bloom and explode in population.”

Studying red tide events

The researchers used the Wirewalker, an autonomous, ocean-wave-powered vertical profiling system developed at Scripps Oceanography, to continuously measure physical and biochemical conditions from the sea surface to the seafloor, reaching a depth of 100 meters (300 feet). 

The team also captured near-surface images of the bloom using an Imaging FlowCytobot (IFCB), a robotic microscope installed on an offshore mooring, now part of a larger IFCB network managed by SCCOOS.

What the researchers learned 

Data and images collected by these instruments confirmed Eppley’s original theory, showing that L. polyedra descended at dusk, reaching a maximum depth of about 30-40 meters (100-130 feet) after 18 to 24 hours of swimming. 

In the deep, the dinoflagellates would take up nitrate, a growth nutrient for plankton, before returning to the surface around noon to photosynthesize during maximum sunlight.

“These single-celled organisms, namely L. polyedra, are so functionally complex and amazing,” said Zheng.

“In addition to their swimming speed, which is far beyond human limits, they can coordinate their behavior according to the day-night cycle by migrating down at night and coming back to the ocean surface during the day; they can produce spectacular bioluminescence; they can photosynthesize; they can even prey on organisms that are smaller than them.”

Potential consequences of the bloom

The researchers also analyzed long-term ocean monitoring data from the California Cooperative Oceanic Fisheries Investigations (CalCOFI) and long-term mooring data maintained by the Ocean Time-Series Group at Scripps Oceanography to assess other consequences of the bloom. 

Based on over 70 years of climate data, the results indicated that the bloom created physical and chemical conditions in the water column that deviated from the norm, highlighting the potential for massive blooms to alter coastal ocean characteristics.

Study implications 

Study co-author and biological oceanographer Clarissa Anderson said this research stands out for its use of novel ocean technologies, which allowed for unparalleled measurements of how phytoplankton respond to small-scale changes in the coastal ocean, as well as calculations of nutrient uptake by dinoflagellates at such fine scales. 

Anderson also noted the importance of long-term observations as being key to any future efforts to better understand harmful algal blooms.

“The more we understand complex mechanisms that allow a particular species or population of plankton to thrive and persist, the better we can predict runaway events like the 2020 red tide that lasted much longer than theory might dictate,” said Anderson. 

“With longer time series of rapid change in coastal nutrient delivery, circulation, light regimes, and algal toxins, we could build more accurate dynamical models for predicting plankton blooms, including those that turn harmful.”

More about red tide events

“Red tide” refers to a harmful algal bloom (HAB) that can turn coastal waters a reddish color. Large concentrations of microscopic marine plants known as phytoplankton cause these events.

The most common type of phytoplankton associated with red tides in the Gulf of Mexico and along the east coast of the US is the dinoflagellate called Karenia brevis. Here’s a detailed overview.

Origins and causes

Natural phenomena: Red tide events are natural phenomena that have occurred for centuries. Historical records of fish kills and human respiratory irritation from areas around the Gulf of Mexico suggest the occurrence of red tide events for centuries.

Nutrient enrichment: While red tides are naturally occurring, human activities can exacerbate or influence their frequency and intensity. Runoff from agriculture, untreated sewage, and other sources of pollution can introduce excess nutrients into coastal waters. This promotes the growth of algae.


Marine life: K. brevis produces toxins known as brevetoxins. These toxins can kill fish and can accumulate in shellfish, making them unsafe to eat.

Humans: Humans can be affected by red tide events in two main ways. Consuming contaminated shellfish can lead to neurotoxic shellfish poisoning (NSP), which can cause neurological symptoms. Additionally, when waves break and cause the algal cells to rupture, the toxins can become airborne and lead to respiratory irritation. This is especially true in those with respiratory conditions like asthma.

Economic: Red tide events can have significant economic consequences, particularly for communities that rely on tourism and fisheries. Beach closures, fish kills, and seafood harvesting bans can deter tourists and result in financial losses for local businesses.

Detection and monitoring

Satellites: Remote sensing from satellites can detect chlorophyll concentrations in coastal waters, providing an indication of algal bloom activity.

Water sampling: By collecting water samples, scientists can detect and measure the concentration of K. brevis cells and the presence of associated toxins.

Mitigation and management

Reducing nutrient pollution: By managing and reducing sources of nutrient pollution, the risk of intense and prolonged red tide events can be lessened.

Early warning systems: Monitoring networks can provide early warnings to local communities and fisheries, allowing them to take precautionary measures.

Research: Continued research into the ecology and behavior of harmful algal blooms can provide insights into predicting and managing red tide events.

Global distribution

The term “red tide” commonly refers to K. brevis blooms in the Gulf of Mexico, but harmful algal blooms happen worldwide. Different species of algae, producing different toxins, can be responsible for these events in various parts of the world. For instance, in the Pacific Northwest, another dinoflagellate, Alexandrium, can cause paralytic shellfish poisoning (PSP).

Climate change and red tides

The link between climate change and red tide events is an area of active research. Warming waters, sea level rise, and increased stormwater runoff can potentially influence the frequency, duration, and intensity of these blooms.


Not all algal blooms are toxic, and not all turn the water red. The term “red tide” can be misleading. Many harmful algal blooms do not change the color of the water. In addition, not all discolorations of the water are harmful.

Research and monitoring of red tide events continue, as scientists and policymakers seek to better understand these phenomena and develop strategies to minimize their impact on the environment and human health.


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