CHAPEL HILL, N.C. — On Earth Day and every day, being “green” is linked with good things —like lowering the carbon footprint and eating locally-grown foods.
But that is not the case when it comes to drinking or swimming in lakes fouled by algae, according to Hans Paerl, an environmental scientist at the University of North Carolina at Chapel Hill.
In freshwater lakes around the world, such algae blooms often result from an overabundance of what’s known as cyanobacteria, or blue-green algae.
Cyanobacteria form widespread, very visible blooms that look like blue-green paint or scum floating on the water. They may be toxic to humans and other animals. These filamentous bacteria clump into mats that cover a lake’s surface from one shore to the other, using up oxygen in the water and eventually turning the lake’s depths into a dead zone.
Taihu to Erie
It was June 2007, when water coming from kitchen faucets in Wuxi, China, was pea-soup green. The water came from Taihu, China’s third largest lake.
Cyanobacteria obscured the surface of the 900-square-mile lake and quickly overwhelmed the intake plant for the city of Wuxi’s drinking water.
Chinese officials scooped 6,000 tons of algae from Taihu and diverted water from the Yangtze River to flush the lake. However, Paerl said, the bloom persisted into the fall.
Two weeks into the bloom, Paerl visited China and discovered that the algae was similar to that found in blooms in North Carolina, as well as and in many larger bodies of water such as Lake Erie, Lake Victoria and the Baltic Sea.
“Nowhere are the blooms worse than on Taihu, however,” says Paerl, whose work is funded by an NSF Dimensions of Biodiversity grant. “Habitat for fish, crabs and other aquatic species is becoming endangered.”
Ten million people depend on Taihu for drinking water, fisheries and tourism. Lake Erie on the border of the United States and Canada faces the same challenges.
Read more about Ohio’s water quality legislation and farmers’ practices in our ongoing coverage.
In 2011, a record-breaking bloom of similar cyanobacteria to the species that plagued Taihu smothered Lake Erie to the point that it was visible on satellite images. At the bloom’s peak in October, it expanded to more than 1,930 square miles, three times larger than any Lake Erie bloom on record.
The “recipe” for controlling the problem, Paerl said, has been to reduce phosphorus finding its way into lakes from sources on land like fertilizers.
“That’s based on the long-standing paradigm that phosphorus is the key nutrient limiting freshwater algae blooms,” he said.
But another element, nitrogen, flowing into lakes and rivers is increasing more rapidly than phosphorus, leading researchers to question whether both nitrogen and phosphorus should be controlled to stem the tide of proliferating algae blooms, Paerl said.
Long range goals
The goal, Paerl said, is to find new ways of ensuring sustainable uses of lakes prone to blooms.
Simon Malcomber, lead NSF program director for Dimensions of Biodiversity, said harmful cyanobacteria blooms caused by excessive phosphorus and nitrogen are threatening freshwater lakes worldwide.
“This research shows the importance of taking a holistic approach to understanding harmful cyanobacteria blooms,” Malcomber said. “Only with an ecosystems approach can long-term successful sustainability strategies be formulated.”
Begins on land
The complex chain that leads to algae blooms in freshwater begins not in lakes but on land, Paerl said, when farmers fertilize fields. If the fertilizer, laden with nutrients like phosphorus and nitrogen, washes into creeks and rivers, it’s eventually carried to lakes. When the excess eventually reaches freshwater, it fertilizes aquatic algae such as cyanobacteria—just as it encourages plants on land to grow.
The algae proliferate, becoming massive blooms. As the algae die, they fall to the lake’s bottom and are digested by microorganisms. The process removes oxygen from the water, creating low-oxygen “dead zones,” fish kills and tainted waters.
“New nutrient reduction strategies,” Paerl and his colleagues wrote in the journal Science in October 2014, “Should incorporate point and non-point sources, including nitrogen removal in wastewaters, optimization of fertilizer application, and erosion controls.”
It’s the only way, Paerl said, to keep blooms of cyanobacteria and other algae in check.
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