Dangerous blooms of algae in freshwater lakes — like those that have caused environmental emergencies in Florida in recent years and in Toledo, Ohio in 2014 — are worsening as the Earth warms.
A new study shows that the blooms have become larger and more frequent in many of the world's largest lakes since the 1980s, with lakes that have warmed the least over the past 30 years showing less intense algal blooms.
"That suggests that temperature — and therefore climate change — has a role to play in allowing lakes to recover or not" [from algal blooms], said study co-author Anna Michalak, an environmental engineer at the Carnegie Institution for Science in Stanford, California. "When temperatures get warmer, it can get in the way of management strategies that would otherwise have improved conditions in the lake."
Algal, or phytoplankton, blooms are caused by the rapid growth of microscopic organisms in the water — often tiny plants but also some types of bacteria, such as blue-green algae (cyanobacteria).
For the study, Michalak and Jeff Ho, an environmental engineer at the Carnegie Institution, and Nima Pahlevan, a remote sensing expert at NASA's Goddard Space Flight Center in Greenbelt, Maryland, studied high-resolution satellite photographs of 71 large lakes in 33 countries on six continents.
A comparison of photos taken between 1984 and 2013 showed that algal blooms had increased in intensity in at least 22 lakes, including Florida's Lake Okeechobee. Only six lakes worldwide showed a statistically significant decrease in blooms over that time.
There's nothing abnormal about occasional algal blooms. They can occur naturally when a lake's water warms in summer, and can be triggered when fertilizer applied to nearby land washes into lakes. Most disappear on their own within a few weeks. But under certain conditions they can grow out of control, spreading across hundreds of square miles and lasting for many months, suffocating fish and creating what's known as a dead zone.
Algal blooms can also create high levels of toxins in the water, such as the cyanotoxins produced by blue-green algae. People and animals exposed to algal toxins by drinking contaminated water, bathing in it or even being splashed can suffer skin rashes, fevers, breathing problems and liver and kidney damage.
Algal blooms in coastal waters — like Florida's annual red tides — cost the U.S. economy about $82 million each year, mainly from losses in fisheries and tourism. But algal blooms in lakes, which can be sources of drinking water as well as places for recreation and tourism, cause annual losses of more than $2 billion in the United States, according to a 2009 study.
Since 2016, toxic blooms of blue-green algae in Lake Okeechobee have caused an environmental crisis in Florida. A bloom in Lake Erie in 2014 forced Toledo to declare its water undrinkable for several days.
Thomas Frankovich, a biologist at Florida International University in Key Largo who wasn't involved in the new study, praised it for its use of satellite imagery to assess algal blooms, instead of manually testing water samples from each lake. "It has a global perspective on phytoplankton blooms in lakes, something we really can't get using land-based measurements," he said.
When algal blooms do occur, they can be curbed by the application of chemical herbicides and other techniques. But often, it's a matter of waiting until the algae die out.
As algal blooms become more common, water managers will need to take greater account of changes in lake temperatures and rainfall patterns, Ho said.
For Michalak, the best thing that can be done to prevent freshwater algal blooms is to limit climate change by cutting carbon emissions. But she agrees that lake management must become a higher priority. "We will need to be much more aggressive in how we manage lakes in order to achieve the same outcome as we would have had climate change not been occurring," she said.
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