The highly potent pesticide DDT was banned more than a half-century ago, but the toxic chemical persists in lake ecosystems and continues to impact freshwater food chains, according to a new study.

"What was considered yesterday's environmental crisis in the 1950s through 1970s remains today's problem," Josh Kurek, an assistant professor in the department of geography and environment at Mount Allison University, said in a news release. "Decades of intense insecticide applications to our conifer forests have left a lasting mark on these lakes -- and likely many others in eastern North America."

Around the middle of the 20th century, forest stakeholders began using dichlorodiphenyltrichloroethane insecticides to combat pest outbreaks across North American forests. For more than two decades, the chemical was sprayed liberally from airplanes onto tree strands impacted by spruce budworm, a destructive conifer pest. Though applied to trees, DDT persists and eventually washed into watersheds. The chemical accumulated in lakes.

Outcries over DDT's harmful effects on wildlife led to its ban in 1972, but according to new research, the chemical is still hanging around in the environment.

For the new study, scientists collected sediment samples from the bottoms of five remote lakes in north-central New Brunswick, Canada. Analysis of the lake bed sediment cores revealed, as expected, elevated levels of DDT trapped in layers dated to the 1960s and 70s. But researchers also measured significant levels of DDT and its toxic byproducts in modern sediments.

The levels of toxins persisting in modern sediments exceed the threshold for harmful biological effects, according to researchers. Scientists also measured a correlation between greater DDT levels and smaller water flea populations. Previous studies have linked reductions in water fleas to increased algae production and fewer prey for fish.

"We have learned a lot of tough lessons from the heavy use of DDT in agriculture and forestry," said Karen Kidd, professor at McMaster University. "The biggest one is that this pesticide was concentrated through food webs to levels that caused widespread raptor declines in North America. The lesson from our study is that pesticide use can result in persistent and permanent changes in aquatic ecosystems."

Researchers published their survey of DDT's freshwater legacy this week in the journal Environmental Science and Technology.

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