To Understand How Warming is Driving Harmful Algal Blooms, Look to Regional Patterns, Not Global Trends

Scientists have widely assumed that climate change is increasing algal blooms around the world. But a new study suggests a more nuanced relationship.

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Algal bloom, Ukraine. Credit: Universal Images Group via Getty Images
Algal bloom, Ukraine. Credit: Universal Images Group via Getty Images

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Last month, a red tint began to spread across the surface of the glacial waters of Kachemak Bay, Alaska. It was a signal: another algal bloom had arrived.

The blooms, caused by the rapid growth of plankton in a short amount of time, can spell trouble for marine life, local communities and fisheries. While some algal blooms are harmless, others create neurological toxins that can sicken or even kill humans and wildlife.

Many researchers have asserted that climate change and coastal development are increasing the worldwide frequency of blooms—sometimes called red tides because the excessive growth discolors the water. But the findings of a new, large-scale study, published June 8 in the journal Nature Communications: Earth & Environment, paint a more nuanced picture. 

The researchers found that algal blooms are present around the world, causing problems wherever they develop. But rather than an increase in algal blooms globally, the study found that over the last three decades, the patterns in blooms differed among regions. Some places, like Arctic Alaska, experienced increases. In others, like Hawaii, algal blooms decreased.

“This is the first time we have scientifically documented that harmful algal blooms are worldwide and persistent with significant human economic impacts,” said Henrik Enevoldsen, head of the UNESCO IOC Science and Communication Centre on Harmful Algae at the University of Copenhagen and a co-author on the study.

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Accounts of harmful algal growths have increased over time. So has monitoring, however, making it difficult to tell whether the rise in observations is simply because there is greater awareness of their occurence or if it truly represents a growing ocean threat. 

“This whole concept of looking for a uniform increase in algal blooms all over the world didn’t make sense, but we really put the dogma to rest,” said the study’s lead author, Gustaaf Hallegraeff, emeritus professor at the University of Tasmania, who specializes in plankton and algal blooms. 

Drawing from over 9,500 observations, Hallegraeff and others reported that after adjusting for the expanded monitoring, there was no uniform increase in the frequency of harmful algal blooms globally from 1985 to 2018. Instead, changes were highly dependent on region and on the type of algae. In Central and South America, the Caribbean, Europe, the Mediterranean and northern Asia, blooms generally increased. In contrast, decreases occurred along the West Coast of North America and in Australia and New Zealand. The East Coast of North America and Southeast Asia displayed no change.

Enevoldsen said these findings were not that surprising. About 200 species of microalgae can underlie harmful algal blooms—about the number of species of animals found at a local zoo. To assume that they would all follow the same trajectory would ignore what is known about the diversity of the microscopic plankton.

To test if the perceived increases in algal blooms held up to a global-scale statistical analysis, Hallegraeff led a team of 109 scientists from 35 countries with UNESCO’s Intergovernmental Oceanographic Commission (IOC). They paired 33 years of reports from two databases. One was the Harmful Algal Event Database (HAEDAT), which only lists algal blooms that threaten the health of humans and the environment. The other was HABMAP-OBIS, a database on the geographic range of harmful species that includes all observations from algal monitoring, harmful or not. By correcting the recorded number of harmful events in a location with total observations over time, researchers were able to separate real trends from perceived ones.

The next steps, Enevoldsen explained, are to focus on better understanding the dynamics not just in specific systems or species, but in future climates as well.

Still a Scientific Puzzle

There are many hypotheses about what is driving the changes in algal bloom frequency. Warming temperatures, changes in ocean acidity and added nutrient pollution all potentially play a role. Yet the exact mechanisms remain unknown, largely because scientists do not have robust enough data sets.

For instance, aquaculture has grown 16-fold since 1985, and the study reported a strong correlation between regional patterns in aquaculture growth and harmful blooms. Researchers suspect the increase is a result of heightened nutrient pollution, which occurs when fertilizer waste from fish farms act as an overdose of food for microorganisms in the water. It’s likely that this stimulated algal activity. But the scientists cannot say for sure. The jump might just represent the increased effort to watch for algal blooms.

Part of the trouble lies in the lack of data about blooms and global change. Take the most recent bloom in Kachemak Bay. Rosie Masui, coordinator for the Harmful Species Community Monitoring Program at Kachemak Bay National Estuarine Research Reserve (KBRR), readily identified that plankton that caused the bloom as Mesodinium rubrum, a harmless species found all over the world. Yet without more frequent and detailed sampling, it’s difficult to pinpoint the exact cause of the bloom or whether it is part of an increasing pattern.

One of the biggest successes of the KBRR’s program are its community ties. “Our monitors are so passionate,” Masui said. “They act as advocates to help share information more broadly throughout the community.” She coordinates the collection of nearly 300 plankton samples every summer, sending sampling kits that consist of nets, thermometers and reusable water bottles to 35 community members. But even with all this testing, it’s not sufficient to build models that can forecast future outbreaks.

Thomas Farrugia, a coordinator for the Alaska Harmful Algal Blooms Network at the Alaska Ocean Observing System, who was not involved in the study, said a key takeaway from Hallegraeff and others’ research was the need for increased funds for detailed monitoring of algal growth. Farrugia uses data Masui and other communities collect to find statewide trends in blooms.

“I’m really happy this study came out,” he said. “This report really shows the importance of monitoring. In Alaska, we’re kind of resolved to the fact that climate change is going to happen here more than in lots of other places. The earlier we start getting a baseline set of information before change happens, the better it’ll be.” 

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Enevoldsen agreed. He said he hoped the new report would be used to argue for increased support of early warning systems and monitoring programs, not unlike the ones Masui and Farrugia coordinate. In the past, some programs have been hesitant to share data for fear that any association of a toxic bloom with an area could cut sales of shellfish that are safe to eat and hurt fisheries economically. “After this report, we have at least a handful of countries who now want to join,” Enevoldsen said. “It’s boosted recognition about how an openness regarding these data is much better for protecting the market.”

Alaska’s Rising Concern

Despite the difficulty in identifying broad patterns across blooms, the report highlighted some hotspot locations that are of increasing concern, such as Alaska. There, climate change is front and center in any future scenario. Farrugia facilitates collaboration among researchers, land managers and stakeholders across the state to keep a close eye on three specific types of plankton that may increase as waters warm: the species that can cause Diarrhetic, Amnesic and Paralytic Shellfish Poisoning. The latter two are often fatal.

“There’s a clear sense from everybody involved that it’s important to understand things happening at a statewide level,” he said. “People are really interested in contributing to a larger effort and to see how their piece fits in with all these other pieces.” 

One of the main areas of current research is investigating how toxic algal blooms form in a warmer world. Don Anderson, a senior scientist at Woods Hole Oceanographic Institute and Director of the U.S. National Office for Harmful Algal Blooms, who was another co-author on the UNESCO IOC report, will lead several new projects in the coming years to track changes in algal toxins in Alaskan waters, especially in the Arctic.

“It’s a whole new world for us,” Anderson said. In the eastern United States, he and others have been studying the impacts of algal blooms on human poisonings from shellfish consumption for a long time. Yet in Alaska, where Native communities rely on hunting and fishing of culturally important food sources, things are different. “Up in those waters,” Anderson said, “you’ve got the subsistence harvesting of everything—seabirds, walrus, marine mammals, and fish—in addition to shellfish.”

Anderson’s new research seeks to address many of the unknowns about where and how these harmful algae will bloom. But Alaska’s coastline is expansive and sometimes hard to reach, so Anderson and his research team are partnering with Indigenous villages and rural towns to establish community monitoring in a wider area than they could sample on their own. The program is similar to the one Masui manages in Southcentral Alaska.

Masui said she hopes that as the KBRR monitoring program, now in its 16th year, expands, it stays true to its goal of serving the coastal communities involved.

“It’s amazing to be able to harvest wild resources in a sustainable manner,” she said, but it can also be scary if someone falls ill or dies. Villages and towns are often so isolated, Masui said, that “if these communities experience a harmful event, it means getting on a plane or two planes or a boat for medical help.”

According to Farrugia, that’s why more monitoring and testing is key, before harmful blooms become a more widespread problem.

“Our power in Alaska is the people that live on the coast,” he said. “So we would like to harness that power by doing as much community sampling as possible. Hopefully that will lead to safe and reliable predictive models and forecasts.”

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