Banned Decades Ago, DDT and PCBs Still Lurk in Baltic Gull Eggs

More than four decades after DDT was banned across Europe, the insecticide’s chemical fingerprints are still showing up in the eggs of wild birds along Germany’s Baltic coast. A new study of Black-headed Gulls breeding in the southern Baltic Sea has found that every single egg tested contained residues of DDT and its breakdown products, along with polychlorinated biphenyls, the industrial chemicals once used in electrical equipment and flame retardants. The findings, published in the journal Environmental Monitoring and Assessment, offer a sobering reminder that the pollutants we ban do not simply vanish. Instead, they linger in sediments, cycle through food webs, and continue to reach the bodies of animals at the top of the chain, even in ecosystems that have been subject to some of the strictest chemical regulations in the world.

The research team, led by Magdalena Wlodarz and Angela Schmitz Ornés of the University of Greifswald together with Friederike Habedank of the State Office for Agriculture, Food Safety and Fisheries in Rostock, focused on the two largest Black-headed Gull breeding colonies in the southern Baltic region. One colony sits on the island of Boehmke in the Achterwasser Lagoon, a sheltered body of water near the island of Usedom. The other occupies Riether Werder in the Szczecin Lagoon at the German-Polish border, a system heavily influenced by the Oder River, which drains a large mixed agricultural and industrial watershed. These two lagoons differ in hydrology, water exchange, and pollution sources, making them an ideal natural experiment for comparing how legacy contaminants reach coastal birds.

Over two consecutive breeding seasons, 2023 and 2024, the researchers collected fifteen eggs per colony per year, one random egg per nest, for a total of sixty eggs. Sampling took place early in the laying period before incubation began, and the removal of early-season eggs allowed the gulls to initiate replacement clutches, minimizing the impact on colony productivity. At each nest, both attending adults were captured and measured, with skull length used to distinguish females from males, yielding forty-seven identified females for the analysis of body condition. The design allowed the team to test whether contaminant burdens were linked to the physical state of the egg-laying mothers or to characteristics of the eggs themselves.

The analytical work was carried out at an accredited official food surveillance laboratory in Rostock, using a validated gas chromatography-tandem mass spectrometry method. Half a gram of homogenized egg, a one-to-one mix of yolk and white, was extracted with hexane and acetone, cleaned up through automated gel permeation chromatography, and then quantified against matrix-matched calibration curves with linearity coefficients exceeding 0.99. The researchers targeted six indicator PCB congeners, the non-dioxin-like compounds PCB 28, 52, 101, 138, 153, and 180, as well as DDT and its metabolites DDE and DDD, the fungicide hexachlorobenzene, and several isomers of hexachlorocyclohexane. A conservative reporting level of 0.01 milligrams per kilogram was applied, meaning that anything below that threshold was treated as zero in the statistics, an approach the authors acknowledge may underestimate true contaminant levels.

The results were unambiguous. All sixty eggs contained both DDT residues and PCB congeners above the reporting level. Mean total DDT concentrations ranged from 0.42 to 0.63 milligrams per kilogram across the four island-year combinations, while mean total PCB concentrations ranged from 0.135 to 0.268 milligrams per kilogram. The PCB profile was dominated by the heavier, more persistent congeners PCB 138, PCB 153, and PCB 180, each detected in one hundred percent of samples at median concentrations above 0.04 milligrams per kilogram. The lighter congeners appeared less consistently and at lower levels, a classic signature of aged industrial contamination, because heavier PCBs bind to organic particles and sediments while lighter ones remain more dissolved and degrade or disperse more readily.

Within the DDT family, the story was one of old, weathered residues rather than fresh applications. The metabolite DDE, the primary aerobic breakdown product of DDT, was quantified in every egg, with p,p’-DDE alone accounting for a mean of 0.57 milligrams per kilogram. Parent p,p’-DDT itself was present in 55 percent of eggs at concentrations up to 0.13 milligrams per kilogram, but DDD was found in only a single sample. Diagnostic ratios, which chemists use to date contamination, showed a median metabolic ratio of about 24, indicating overwhelmingly aged residues. The authors note that the presence of parent DDT does not by itself prove recent input, since residual DDT can persist in environmental reservoirs such as sediments and be remobilized into aquatic food webs, but the near-exclusive dominance of DDE points firmly to legacy sources from the historical use of technical DDT.

Two other pesticides told a story of relative success. Hexachlorobenzene was quantified in only five eggs, all at low levels of 0.01 to 0.02 milligrams per kilogram, and all from the Riether Werder colony near the Polish border, though the authors caution that such sparse detections do not allow conclusions about site-specific sources. Hexachlorocyclohexane, including the insecticide lindane, was not quantified in any sample at all, consistent with its rapid biotic degradation and the absence of substantial recent inputs. This contrasts sharply with historical data from the Elbe estuary, where Black-headed Gull eggs once showed high lindane concentrations, and suggests that the ban on these compounds in the 1980s has genuinely reduced their presence in the Baltic environment.

The comparison with history is where the study becomes most striking. In 1986, researchers Holz and Starke sampled gull eggs from the same Boehmke island and reported total PCB concentrations exceeding 2 milligrams per kilogram, roughly ten times the current mean of 0.18. That decline represents real progress attributable to the bans of the 1970s and 1980s. Yet DDE tells a different story: the historical mean of 0.59 milligrams per kilogram reported for the region is almost identical to the 0.57 milligrams per kilogram measured today. DDT metabolites, in other words, have barely budged in nearly forty years, a persistence explained by their environmental half-lives, which range from two to fifteen years for DDT and a decade or more for DDE, combined with continuous slow release from contaminated sediments.

Crucially, the researchers found no evidence that these contaminant burdens are harming the gulls today. Eggshell thickness, measured at three positions on each shell with a precision gauge, showed no significant correlations with DDT or PCB concentrations, apart from a single weak negative association at the blunt pole that the authors interpret as providing no evidence of an effect. Egg volume and female body condition, calculated from body mass and skull length, were likewise unrelated to contaminant levels. The measured DDE concentrations remain far below the thresholds above which eggshell thinning has been documented in Herring Gulls, and the colonies themselves have shown stable growth over the past decade. Still, the authors emphasize that acute toxicity was not directly assessed, that sublethal effects cannot be ruled out, and that the two-year window and modest sample size limit the power to detect subtle trends.

The study’s limitations are candidly acknowledged. Eggs were collected at random without recording laying order, even though the first-laid egg in a clutch often carries the highest pollutant load, introducing potential variance. Historical comparisons are complicated by differences in analytical methods and sample composition. Yet the broader message stands: legacy persistent organic pollutants remain an active presence in the Baltic Sea food web, transferred from mother to egg and passed through fish, insects, and molluscs to opportunistic gulls. By establishing a modern baseline for a previously understudied sentinel species, the researchers have given Baltic monitoring programmes a critical reference point, and a reminder that even well-regulated chemicals demand continued surveillance long after their use has ended.

Subject of Research: Legacy persistent organic pollutant contamination in Black-headed Gull eggs from the southern Baltic Sea

Article Title: Still exposed? DDT and PCB levels in Black-headed Gull eggs collected from the Baltic Sea

Article References: Wlodarz, M., Habedank, F., & Ornés, A. S. (2026). Still exposed? DDT and PCB levels in Black-headed Gull eggs collected from the Baltic Sea. Environmental Monitoring and Assessment, 198(11), Article 1156. https://doi.org/10.1007/s10661-026-15987-7

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15987-7

Keywords: DDT, PCBs, Black-headed Gull, Baltic Sea, persistent organic pollutants, DDE, eggshell thickness, bioaccumulation, biomonitoring, organochlorine pesticides, hexachlorobenzene, seabirds