BioEd Online

A computer simulation of the environmental fate of DDT (dichlorodiphenyltrichloroethane) has revealed that substantial quantities of the pesticide are still being released from the world's oceans, despite widespread restrictions on its use during the 1970s.

The calculations show that although remaining DDT use today tends to be in the southern hemisphere, its concentrations are actually growing in the northern hemisphere as it moves through the world's oceans and atmosphere.

An estimated 1.5 million tonnes of DDT were used worldwide between the 1940s and 1970s, both as an agricultural insecticide and to control disease-carrying insects such as mosquitoes – the chemical was a key weapon in the war against malaria, for example. But DDT is toxic to a wide range of aquatic life, and its eggshell-thinning effects also had a drastic impact on many bird species. Concerns about its environmental toxicity led to a series of countries banning the agricultural use of DDT through the 1970s.

There has been a drastic decline in DDT's use since then, but its legacy is still very much with us, say Irene Stemmler and Gerhard Lammel of the Max Planck Institute for Chemistry in Mainz, Germany. DDT is continually re-entering the atmosphere from the ocean, before being dissolved again in a recurring cycle. "DDT is simply sitting there, waiting for the next cycle," Lammel says.

Growing concentration

The scientists created a computer model to simulate the circulation of DDT between ocean and atmosphere between 1950 and 2002. This revealed that since the 1970s, the re-emission of DDT from the ocean has become greater than from the three known modern releases of new DDT: its continued use in some countries for malaria control; degrading storage canisters; and other pesticides that contain DDT as a contaminant.

The emissions from the ocean are small enough not to pose a direct risk to humans. "Cruise passengers will never suffer from this," Lammel says. A greater concern, however, is that DDT is migrating towards northern latitudes. That's because the pesticide evaporates more rapidly from warmer, southern waters, thus increasing concentrations in cooler seas over time.

This is of ecological concern because marine organisms concentrate DDT by factors of millions as it moves up the food chain, reaching levels where it can have toxic effects on fish, for example – or the animals that eat them.

The scientists point out, however, that until recently there has been very little monitoring of DDT in the marine environment, which makes it difficult to verify their model.

Long legacy

DDT won't be with us forever. Some of it, Stemmler and Lammel found, settles into the deep ocean where a portion is buried in sediments, while some is destroyed in the atmosphere by sunlight.

But both of these are slow processes. "You have loss mechanisms, but if they're slow, you can't shed it very quickly in the deep ocean or bury it in sediment." says Robbie MacDonald, a geochemical oceanographer at Canada's Institute of Ocean Sciences, in Sidney, British Columbia, who was not involved in the study. "It's going to take the oceans quite some time to turn [emissions] down." The results are published in Geophysical Research Letters¹.

MacDonald likens the persistence of DDT to filling a giant reservoir, only to discover that it takes a long time to empty. "You would think this compound would have gone away by now," he says, "but it's still cascading through these reservoirs. People frequently ignore that. When you control emissions, [people] expect instant gratification, but you don't get instant gratification."