While using nanomaterials in pesticides and fungicides has decreased the amount of toxins sprayed on crops and fields, it may also be leading to a surge in algae outbreaks in nearby waterways.

Researchers from Duke University have found that the use of nanomaterials in agrochemicals is being combined with nutrient runoff from fertilized cropland and manure-filled pastures to produce potentially toxic algae outbreaks.

Engineered nanomaterials are used in various commercial products, including targeted pesticides, and are less than 100 nanometers in diameter. They have different chemical and physical properties, including more surface area for reactions and interactions, than their bulk counterparts do.

The interactions could intensify harmful algal blooms in wetlands.   

Between 260,000 and 309,000 metric tons of nanomaterials are produced globally each year, the majority of which is disposed in landfills. However, about 80,400 metric tons of nanomaterials are released into soils and an additional 29,200 metric tons find their way to natural bodies of water, like nearby streams, lakes and wetlands.

“And these emerging contaminants don't end up in water bodies alone,” Marie Simonin, a postdoctoral associate at Duke University, said in a statement. “They probably co-occur with nutrient runoff. There are likely multiple stressors interacting.”

When excess nitrogen and phosphorous pollution end up in wetlands and waterways in the form of agricultural runoff and untreated wastewater in a process called eutrophication, it causes a substantial increase in algae growth, creating a thick mat of green slime on the surface of the water that blocks sunlight from reaching other plants.

Eventually, the algae blooms will reduce oxygen levels to the point where fish and other organisms cannot survive. Some algal blooms also release toxins that can make pets and people who swallow them sick.

In the study, the researchers set up 18 separate 250-liter tanks with sandy-sloped bottoms that was filled with water, soil and a variety of wetland plants and animals, including waterweeds and mosquitofish to mimic small wetlands.

The researchers gave some tanks a weekly dose of algae-promoting nitrates and phosphates over a nine-month period, while some tanks received either copper or gold nanoparticles and some tanks received both.

The team monitored the water chemistry, plant and algae growth and metabolism and nanoparticle accumulation in plant tissues over the nine-month period.

The researchers found that the nanoparticles had little effect individually, but when they were added together with nutrients, low concentrations of gold and copper nanoparticles used in fungicides and other products turned the once clear water a murky green color and the surface became covered with bright green mats of floating algae.

They also found that the big algal blooms were more than three times more frequent and more persistent in tanks where nanoparticles and nutrients were added together than where nutrients were added alone. The algae overgrowths also reduced dissolved oxygen in the water.

It is unknown how nanoparticle exposure shifts the balance between plants and algae as they compete for nutrients and other resources. However, the results suggest that nanoparticles and other metal-based synthetic chemicals could play a larger than anticipated role in the global trends of increasing eutrophication.

The study was published in Ecological Applications.