DDT levels in Antarctic penguins present a complex mystery

For the past 30 years, DDT's derivatives have persisted in Antarctic penguins at a constant level, and researchers say glacial meltwater may be the source.

The use of DDT peaked several decades ago at more than 36,000 metric tons per year (t/yr). Today, less than 1000 t of the organochlorine pesticide—banned in most countries since the 1980s—is applied annually for mosquito control and farming, mainly in the Southern Hemisphere. Despite this drop, Adélie penguins in the Antarctic continue to have the same levels of total DDT in their bodies as they did 30 years ago. New research published in ES&T (DOI: 10.1021/es702919n) identifies Antarctic meltwater as the continued source of total DDT, and possibly other pollutants, in the southern continent's ecosystems.

DDT and other persistent organic pollutants (POPs) follow atmospheric paths to the Antarctic and the Arctic and eventually are deposited there in snow and ice. Animals at both poles sequester the derivatives p,p′-DDT and p,p′-DDE in their fat. But while Arctic-dwelling creatures such as birds, whales, and seals have shown a dramatic drop in total DDT levels during the past decade, levels in Adélie penguins in the Antarctic have remained steady, according to the new results.

Adélie penguins, which are protected under international conservation rules, live across the continent and overwinter there. The birds' stationary behavior makes them apt subjects for studying Antarctic sources of DDT and possibly other POPs; migratory birds such as skuas, which winter in South America or closer to the equator, could pick up these chemicals on their travels.

Focusing on DDT, Heidi Geisz of the Virginia Institute of Marine Science and her colleagues sampled dead Adélie penguins and abandoned eggs from two different populations. They measured total DDT in the breastplate fat sac of each bird as well as in the addled or frozen eggs. The researchers compared their findings with past measurements reported in the literature as far back as 1964. They found that the ratio of p,p′-DDT to p,p′-DDE declined over time; this shift indicates that the birds are exposed to the remnants of older DDT deposition, not new sources.

To pinpoint where the older DDT might be coming from, the researchers used measurements of glacial outwash taken in the past few years. (Directly measuring ice and snow remains technically difficult.) From those data, they estimated that melted snow and ice could be providing about 1–4 kilograms per year of DDT to offshore Antarctic ecosystems.

The short food chain in the Antarctic, from krill to penguins, for example, means that glacial meltwater is an almost direct delivery system of DDT to birds and other large animals, says Geisz. The measured DDT burdens are too low to be harmful to the animals, but continued exposure to mixtures of persistent pollutants at low levels could eventually prove to be problematic, she adds. "We can never really know where these chemicals are hiding," she says. "They show up in places that have no point source."

The team also suggests that climate change could accelerate the release of these chemicals into the environment. However, says David Vaughan of the British Antarctic Survey, climate change on the Antarctic Peninsula" is only increasing the melt by a relatively small percentage," about 10% during the past decade. "I'd be surprised if climate change can be blamed for a wholesale change in the mechanism of DDT transfer," he says. Nevertheless, "the release of this stored DDT into the marine environment is inevitable as glaciers go about their normal business of producing icebergs."

The team has "developed a strong case" for steady and even increasing total DDT concentrations in Adélie penguins, notes Derek Muir of Environment Canada. "The authors have gone to a lot of effort to examine the reliability of the older data," he comments, some of which were based on packed-column gas chromatography. That method could be confounded by PCBs and other persistent organochlorines, but the DDT measurements remain more reliable over time.

The "remarkable" differences between Arctic and Antarctic trends for DDT in seabirds, ringed seals, and other animals present a complex mystery, Muir suggests. The Arctic ice holds more DDT because of its past use in the Northern Hemisphere, but declining levels in animals there "suggest it is not having a large impact," he says. The new results "point [out] the need to examine this in more detail," particularly in the Arctic, where researchers have collected more high-resolution data over time on DDT in animals. —NAOMI LUBICK