Top 10 Worst Toxic Algal Blooms of 2023

January 1, 2024

“2023 will go down as a terrible year for harmful algal blooms in the U.S. and around the globe…The good news is that this problem is preventable.”        

                                               - Jan Spin, President of Americas

Photo Credit: NOAA

Top 10 Worst Toxic Algal Blooms of 2023

As 2023 comes to a close, BlueGreen Water Technologies releases its list of the Top 10 Worst Toxic Algal Blooms of 2023. Outbreaks of toxic algae are growing in intensity and frequency, choking the life out of water bodies worldwide. In 2023, harmful algal blooms suffocated aquatic ecosystems, endangered human health, and proved fatal for pets and wildlife. 

“2023 will go down as a terrible year for harmful algal blooms in the U.S. and around the globe,” said Jan Spin, President of Americas, BlueGreen. “Red tide poisoned marine life in California, Texas, and Florida, and toxic blue-green algae choked water bodies from Kansas to the UK.”

BlueGreen water scientists track the progression of harmful algal blooms year-round using data from satellites, artificial intelligence, deep learning analysis, and drones. 

“The 2023 season was impacted by the climate pattern El Niño and marked by life-threatening extreme heat waves,” said Spin. “The good news is that this problem is preventable. With effective, targeted treatment, harmful algal blooms can be controlled.” 

BlueGreen deploys its technologies across multiple continents to improve water quality and availability by remediating harmful algal blooms which, if left untreated, can result in severe health, environmental, and economic consequences. BlueGreen’s Net Blue™ methodology is the first nature-based climate solution to reduce algal blooms and sequester carbon at the same time.

“Our Top 10 list should serve as a wake-up call,” said Eyal Harel, CEO, BlueGreen. “Water nourishes our planet. Clean water supports life, protects ecosystems, and ensures the diversity of species. It is our most precious natural resource and we must protect it. That is our mission and our focus every day at BlueGreen.”

Top 10 Worst Toxic Algal Blooms of 2023 


Photo Credit: David McNew/Getty Images

California’s summer of aquatic carnage looked like a scene out of a horror movie: hundreds of sick and dying sea lions and dolphins washed up along the coast, many suffering seizures and appearing disoriented with bulging eyes and foaming at the mouth. Scientists with NOAA Fisheries blamed the mass casualty event on a toxic algal bloom that produced high concentrations of domoic acid, a neurotoxin linked to the algae Pseudo-nitzcschia. The scale and severity of this year’s outbreak was unprecedented and will go down as Southern California’s largest harmful algal bloom on record.


Photo Credit: Sky News

The largest freshwater lake in the UK, spanning some 154 square miles in Ireland, suffered a severe environmental crisis and ecological emergency as toxic blue-green algae overtook the lake, fueled by excess nutrients from sewage, agricultural runoff, and invasive zebra mussels. The outbreak killed hundreds of fish and birds and proved fatal for several dogs who came into contact with the infected water. It also threatened drinking water, as Lough Neagh supplies nearly half of all drinking water in Ireland, and produced a powerful stench that kept people away.

  1. Utah Dog Deaths

Photo Credit: Rick Egan/AP

One dog died and three others were sickened after being exposed to a harmful algal bloom on the Virgin River. Water samples detected high levels of antitoxin-a, a dangerous toxin produced by cyanobacteria, also known as blue-green algae. Separately, a toxic algal bloom in Utah Lake is blamed for the death of a beloved family pet after the family went for a walk on the shore. The dog died hours later. Utah Lake and the Virgin River have been plagued by toxic algal blooms for years.


Photo Credit: NASA

Dead fish washed ashore as red tide returned to southwest Florida this year with a vengeance. Caused by large amounts of toxic algae known as Karenia brevis, the outbreak arrived earlier and in higher than normal concentrations. The toxic blooms were fueled, in part, by Hurricane Ian, which inundated the Gulf waters with millions of gallons of polluted runoff. 

  1. Lake Okeechobee

                                         Photo Credit: NASA Earth Observatory

The largest freshwater lake in Florida, and the tenth largest in the U.S., Lake Okeechobee provides an important habitat for fish and wildlife and is a drinking water supply for nearby communities. It also serves as an irrigation source for the agriculture industry surrounding it. Occasionally referred to as “Florida's inland sea”, the Central Florida water body has grappled with harmful algal blooms for years. In June and July of 2023, a toxic bloom covered almost half of the 730-square-mile lake. The 440 square-mile bloom resulted in warning advisories, preventing recreational use of the water body. The cyanobacteria bloom problem received national coverage and attention.


The eleventh-largest lake in the world and fourth-biggest of the five Great Lakes, Lake Erie is bordered by Indiana, Michigan, Ohio, Pennsylvania, New York, and Ontario, CA. Another water body that has historically faced toxic algal outbreaks, Western Lake Erie’s algal bloom outbreaks have intensified in the last decade. While NOAA had predicted a smaller-than-average harmful algal bloom for western Lake Erie in 2023, blooms appeared on the lake earlier than usual and the 2023 season wound up being more severe than forecast. At its peak, a toxic algal bloom in the western basin covered 312 square miles


Photo Credit: Ted Harris/Kansas Biological Survey

This year, Kansas' algal bloom season extended from summer into the fall, and according to researchers, the blooms became more toxic and spread farther north. During the peak summer season when residents visit lakes the most, nine lakes across the state were under advisory at the same time. In October, the BlueGreen team declared Kansas as a hot zone for toxic algal blooms.

  1. Lady Bird Lake and Lake Austin, Austin, TX

Photo Credit: Austin American-Statesman

Lady Bird Lake is a popular recreational destination for Austin residents, the river-like reservoir has previously had outbreaks of toxic algae so severe that it has killed dogs. This year’s toxic algae season began early in March and continued throughout the summer, spreading to Lake Austin, which is a part of the Colorado River.


An outbreak of red tide produced by Karenia brevis algae killed fish, sharks, and sea snakes along the Texas Gulf coast and prompted warnings to stay away. Beachgoers who did venture to the coast reported eye irritation, coughing, and other respiratory symptoms


Photo Credit: Dr. Jennifer L. Graham / USGS

A major tributary of the Columbia River, the Willamette River is the thirteenth-largest in the U.S., located in northwestern Oregon. Flowing from north to south, it is 187 miles long. This past summer, starting in July, a massive toxic algal bloom covered several miles of the river, prompting an advisory for people to stay away. Algal blooms on the water body have consistently appeared every year since toxic algae monitoring began.  

Mountains above a clear lake

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