The potent insecticide DDT still lingers in Canadian lakes nearly 50 years after it was banned, according to a study published Wednesday that highlighted its harmful impact on micro-organisms at the bottom of the local food web.

From 1952 to 1968, planes sprayed more than 6,280 tons of dichlorodiphenyltrichloroethane or DDT on forests in the Atlantic coast province of New Brunswick to control pest outbreaks in commercial forests, according to Environment Canada. It was arguably one of the largest aerial spray programs in North America.

Public awareness of DDT's harmful effects on wildlife and the environment eventually led to curtailed use of the insecticide until it was banned in 1972.

But researchers led by Joshua Kurek at Mount Allison University in New Brunswick recently found traces in lake sediments that "still exceeded levels considered safe for aquatic organisms," they said in a statement.

They also identified a potential risk of the DDT spreading across the local food web, beyond the lakes' shores.

"We believe that the potential for direct transfer of DDT from the aquatic to terrestrial ecosystem exists given the DDT legacy in NB," the researchers concluded in the study.

Their findings were published in the American Chemical Society (ACS)'s Environmental Science and Technology publication.

The team of researchers collected sediment core samples from five remote lakes in New Brunswick, which captured local environmental conditions from about 1890 to 2016.

They analyzed DDT concentrations as well as the partially fossilized remains of water fleas (Cladocera) and found that most of the lakes showed a shift from large-bodied to small-bodied zooplankton species -- which are generally more tolerant of contaminants -- beginning in the 1950s when DDT was widely applied in New Brunswick.

No historic measures of fish populations exist for the remote lakes studied, so the researchers could not account for shifts in predation that may have co-occurred with the environmental changes.

But they speculated that aquatic organisms exposed to appreciable amounts of sedimentary DDT "may contain high levels of DDT within their tissues."

Trout that eat invertebrates exposed to sedimentary DDT may be fished by anglers or consumed by wildlife, for example.

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