Historic 2020 red tide led to marine mortality, study shows

A surfer rides a wave off Scripps Pier during the historic 2020 red tide. Photo: Michael Latz.

Spring and summer are excellent times to see bioluminescence in the ocean, when marine plankton agitated by waves emit a blue neon glow. But bioluminescence has a dark side, as explored in a new study.

The aggregations of the marine plankton (the dinoflagellate Lingulodinium polyedra), also known as “red tide” for the brownish-red color they lend the water during the day, were at historic numbers and duration in 2020.

An April 30 study by UC San Diego’s Scripps Institution of Oceanography details the negative impacts from that year: the long-lasting red tide resulted in the deaths of many fish and other marine organisms.

Having such a high rate of marine mortality in open ocean water “made that unique,” said lead author Zachary Skelton, SIO graduate and marine biologist at the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center.

The 2020 red tide event occurred off most of the Southern California coast, memorable to many for its concurrence with unprecedented pandemic lockdowns, when watching the bioluminescent waves was one of the few sanctioned activities. 

A photo compilation showing aspects of the 2020 red tide. (A) A surfer in the thick red tide off Scripps Pier, April 24, 2020. (Credit: Michael Latz.) (B) Bioluminescence of L. polyedra off Scripps Pier, May 14, 2020. (Credit: Kevin Key Photography.) (C) Bacterial mats at Agua Hedionda Lagoon after a mass mortality event, May 10, 2020. (Credit: S Anthony.) (D) A dead thornback guitarfish among subtidal bacterial mats in La Jolla Canyon, May 9, 2020. (Credit: Raph Beresh.) (E) A decaying Pacific oyster surrounded by a bacterial mat in Los Peñasquitos Lagoon, May 20, 2020. (Credit: Carlos Neira.) (F) Organic-rich sea foam at Scripps Beach caused by the decaying bloom, May 19, 2020. (Credit: Zach Skelton.)

“It was very attractive to the public to go out and see the bioluminescence at night,” Skelton said.

But under the waves, “that red tide was actually hard for aquariums to deal with,” he said, as places like Birch Aquarium, NOAA Southwest Fisheries’ experimental aquarium and SIO’s two on-campus aquariums for experimental organisms utilize seawater and found the red tide’s organic compounds and suspended solids clogging filters.

Over time, Skelton and his colleagues received “reports in the wild about organisms that were dying off towards the end of [spring 2020],” he said, deaths that were coinciding with mortality within the local facilities.

Surveying the damage

Studying how red tide affected marine mortality poses a challenge, Skelton said, as red tides are difficult to predict. “It’s really hard to create these systematic studies.”

Using existing infrastructure to piece together the correlation, Skelton’s team used data from long-term moorings that monitor environmental specifics such as temperature, pH and oxygen levels across San Diego County.

The researchers also relied on citizen scientists, collecting data from emailed observations, photos and accounts culled from social media platforms like Twitter (now X), Facebook and iNaturalist. 

Examples of fish and invertebrate mortality during the 2020 red tide, documented by local citizens. (A, B) Dead fish at Agua Hedionda Lagoon, May 2, 2020. (A) Thousands of fishes floated to the surface (Credit: S Anthony), and (B) subsequently washed ashore (Credit: Gary Cotter). (C) Dozens of dead octopuses washed ashore in Ensenada, Mexico, May 10, 2020. (Credit: Lydia Ladah, Ensenada Center for Scientific Research and Higher Education.) (D) Scavengers, like this California seagull with a sweet potato sea cucumber in its beak, removed dead organisms from the beach in La Jolla, May 6, 2020. (Credit: Jenny Lisenbee.) The authors said the full extent of the mass mortality was challenging to measure, partly due to these scavengers and human clean-up efforts.

More information was found in long term datasets, Skelton said, from local estuaries where monitoring of benthic macrofauna, or organisms within the sediment of the estuary, happens.

All the assembled puzzle pieces revealed a larger picture: the mortality occurring in the wild largely corresponded with changes in oxygen, including drops in oxygen (hypoxia) and periods of no oxygen (anoxia).

The low oxygen levels led to the deaths of thousands of fishes and invertebrates, as well as several seabirds.

The study’s results are “interesting because it shows that there are other factors that are at play, … likely compounding with stressors such as low oxygen that are leading to mortality,” Skelton said.

Coastal concerns

That there were such high rates of mortality in the open water concerns Skelton.

Red tide can often affect small, concentrated areas, like estuaries and warm water environments found in places like Florida, but “you don’t really see open coast mortality, because there’s often a lot of flushing of the environment through tidal wave action,” he said.

The 2020 red tide seemed to have been a “perfect environment that was cultivated for” such an event, Skelton said: a combination of large winter rainfalls, nutrient runoff and temperatures “optimal to growth… led to this proliferation of the phytoplankton during this time.”

It’s hard to calculate the exact impact and recovery time, he said. “These environments were likely able to repopulate quite quickly. However, we don’t have any data to support that.”

The study highlights the importance of collaboration and communication among scientists, Skelton said, noting that the pandemic’s “stay at home” mandates complicated research efforts.

With 34 authors contributing to this paper across multiple institutions, “it just speaks to how open and willing people were to communicate,” he said. “That goes a long way.”

Picture of Elisabeth Frausto

Elisabeth Frausto

Elisabeth Frausto has been reporting on and writing about La Jolla since 2019. With dozens of local and state journalism awards to her name, Elisabeth knows the industry as well as she knows her community. When she’s not covering all things 92037, you’ll find her with coffee in hand staring at the sea.
Picture of Elisabeth Frausto

Elisabeth Frausto

Elisabeth Frausto has been reporting on and writing about La Jolla since 2019. With dozens of local and state journalism awards to her name, Elisabeth knows the industry as well as she knows her community. When she’s not covering all things 92037, you’ll find her with coffee in hand staring at the sea.

Subscribe to our Newsletter

Everything happening in La Jolla, sent right to your inbox!

Follow & Subscribe

Subscribe to our Newsletter

Everything happening in La Jolla, sent right to your inbox!

Enjoying our content?

Your contribution helps support our small local team of journalists in continuing to bring you quality content and news about La Jolla.
ADVERTISEMENT

Recommended Posts

Most Popular

Follow & Subscribe

Subscribe to our Newsletter

Everything happening in La Jolla, sent right to your inbox!

Enjoying our content?

Your contribution helps support our small local team of journalists in continuing to bring you quality content and news about La Jolla.
ADVERTISEMENT

Most Popular

All News Categories

What's new in La Jolla?

Stay connected with regular updates on all things La Jolla, CA

Join your fellow La Jolla enthusiasts and get the latest local news stories, exciting events, inspiring roundups for the top things to do, the best eats in town, new businesses, plus exclusive subscriber-only deals.

Get the latest local news stories, see upcoming events, top activities and restaurants, plus exclusive subscriber-only deals.

Join your fellow La Jolla enthusiasts and get the latest local news stories, exciting events, inspiring roundups for the top things to do, the best eats in town, new businesses, plus exclusive subscriber-only deals.