The team, led by Rafael Patrick R. Marcelino of the Oceanographic Institute at the University of São Paulo, measured twenty-six polychlorinated biphenyl congeners, the full suite of DDT-related compounds including p,p’-DDT and its breakdown products DDE and DDD, and an extensive panel of PAHs including the sixteen priority compounds designated by the United States Environmental Protection Agency. The results paint a picture of contamination dominated by legacy chemicals. Total PCB concentrations in liver tissue reached a maximum of 759 nanograms per gram on a wet weight basis, while total DDTs peaked at 242 nanograms per gram. When normalized to the lipid content of the tissue, the maximum values climbed to 20,094 nanograms per gram for PCBs and 10,671 nanograms per gram for DDTs. These figures fall within the range previously reported for liver tissue of this species elsewhere along the Brazilian coast, confirming that industrial chemicals banned or restricted decades ago continue to circulate through coastal food webs.

The chemistry behind this persistence is unforgiving. PCBs and DDTs belong to the class of persistent organic pollutants, molecules whose carbon-chlorine bonds resist degradation and whose strong lipophilicity drives them into fatty tissues, where they bioaccumulate and biomagnify as they move up trophic webs. Their semi-volatile nature also allows long-range transport, which is why these compounds appear everywhere from Antarctic penguins to Arctic whales. Guiana dolphins, as long-lived coastal top predators with high lipid reserves and a limited capacity to eliminate such contaminants, effectively integrate decades of exposure across their home ranges. The species inhabits exactly the estuaries and bays where anthropogenic pressures are most intense, and it faces a gauntlet of stressors including bycatch, habitat degradation, noise, and chemical pollution, which is one reason the International Union for Conservation of Nature lists it as Near Threatened.

Perhaps the most striking chemical fingerprint in the new data is the overwhelming dominance of hexachlorinated PCB congeners, which accounted on average for eighty-eight percent of total PCB concentrations in positive samples. The most abundant congeners were PCB 153, PCB 138, PCB 180, PCB 149, and PCB 101, a pattern that matches previous findings in Guiana dolphins and other toothed whales worldwide. These highly chlorinated structures resist metabolic breakdown, so they persist and accumulate preferentially. The DDT profile told a similarly historical story: p,p’-DDE, the principal degradation product of the insecticide, was detected in nearly all positive samples, while the parent compound p,p’-DDT was rarely observed. That pattern indicates predominantly aged contamination, reflecting environmental weathering and metabolic transformation of DDT that was applied decades ago, whether in agriculture or in vector control campaigns, rather than any recent input.

Geographically, the study found a suggestive north-south contrast. Dolphins stranded in Rio de Janeiro and São Paulo tended to show higher PCB and DDT concentrations and higher quantification frequencies than those recovered in Paraná and Santa Catarina. The ratio of total DDTs to total PCBs generally fell below one in the northern, more industrialized states, indicating PCB predominance, and rose above one in the southern states, where DDTs contributed relatively more. The authors interpret this cautiously: the pattern is compatible with the region’s divergent histories, in which ports, shipyards, petrochemical plants, and electrical equipment handling in places like Guanabara Bay and the Santos estuarine system drove historical PCB inputs, while agriculture and mosquito control shaped the DDT legacy further south. Because sampling depended on opportunistic strandings, group sizes were small and uneven, particularly for Santa Catarina with only five animals, and the researchers deliberately avoided inferential statistics that the data cannot support.

That caution reflects a fundamental limitation of stranding-based studies: a dolphin found dead on a particular beach did not necessarily absorb its contaminants there. Although Guiana dolphins show considerable site fidelity, the movement histories of the sampled individuals were unknown, and regional groups may differ in age structure, diet, and body condition. Stranding location was therefore used purely as a geographic grouping variable, not as a proxy for the precise site of exposure. The same restraint applies to time. Concentrations varied substantially from year to year between 2017 and 2022, but the small and unequal annual sample sizes precluded any claim of an increasing or decreasing trend. What the dataset offers instead is something the region has lacked: a robust baseline against which future monitoring of this species can be judged.

The PAH findings carry their own subtle warning. Parent PAHs were quantified in only twenty of the seventy liver samples, and in fifteen of those positives naphthalene, the lightest of the group, was the only compound detected, at concentrations up to 200 nanograms per gram wet weight. It would be easy to read this near-absence as good news for a basin crisscrossed by oil tankers and petroleum platforms, but the authors are emphatic that it implies no such thing. Unlike the stubborn POPs, PAHs are readily biotransformed in vertebrate livers by cytochrome P450 enzyme systems, so their absence from liver tissue may simply mean they were metabolized before the animals died. The non-detection of alkylated PAHs and dibenzothiophenes, compounds used as indicators of petrogenic inputs, does not exclude petroleum-related exposure either, since those molecules may likewise have been transformed or simply present below detection limits. Measuring PAH metabolites in bile or blood, or enzymatic biomarkers of biotransformation, would be needed to assess exposure directly.

Life history added another layer to the story. Fetuses generally carried lower PCB concentrations than older animals, yet the mere detection of PCBs in fetal liver demonstrated that these chemicals cross the placenta, reaching developing dolphins before birth. Maternal transfer could not be quantified from these data, but the finding matters because it means contamination begins in utero, and females can partly offload their burdens through gestation and lactation. That mechanism may help explain why males in the study showed slightly higher quantification frequencies and concentrations than females, whose pollutant loads are repeatedly reduced each time they bear and nurse a calf. Among calves, juveniles, and adults, concentrations overlapped broadly and varied enormously, a pattern consistent with age-related accumulation over a lifetime of contaminated prey, but also potentially shaped by geography, lipid content, and body condition in ways the opportunistic design could not disentangle.

The study is candid about its boundaries and, in doing so, sets a methodological benchmark for the field. By reviewing ten prior studies of PCBs and DDTs in Sotalia tissues along the Brazilian coast, the authors documented a patchwork of protocols: congener lists ranging from six to fifty-one compounds, divergent extraction and cleanup methods, and results reported inconsistently on wet-weight and lipid-weight bases, sometimes without the information needed to convert between them. Toxicological thresholds established specifically for Guiana dolphin liver do not exist, and thresholds derived from blubber or other species cannot be directly applied, so biological effects cannot be inferred from these concentrations alone. What the researchers can offer, and what makes the dataset valuable, is precisely calibrated, quality-controlled baseline data, validated against certified reference materials, for a coastal species that integrates pollution across space and time. In a basin whose offshore pre-salt fields will be producing oil for decades, and whose shoreline keeps growing denser with people and industry, knowing where Guiana dolphins started is the only way to know whether tomorrow’s management is working.

Subject of Research: Contaminant exposure of Guiana dolphins (Sotalia guianensis) to PCBs, DDTs, and PAHs in the Santos Basin, Brazil

Article Title: Legacy contaminants and low occurrence of PAHs in Sotalia guianensis from a highly impacted coastal region, Santos Basin, Brazil

Article References: Marcelino, R. P. R., da Silva, J., de Lucca Marcello Jarcovis, R., Timoszczuk, C. T., dos Santos, F. R., de Araujo, L. D., da Silva, D. A. M., Taniguchi, S., & Lourenço, R. A. (2026). Legacy contaminants and low occurrence of PAHs in Sotalia guianensis from a highly impacted coastal region, Santos Basin, Brazil. Environmental Monitoring and Assessment, 198(10), Article 1114. https://doi.org/10.1007/s10661-026-15956-0

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15956-0

Keywords: Guiana dolphin, Sotalia guianensis, PCBs, DDTs, PAHs, persistent organic pollutants, Santos Basin, marine mammals, ecotoxicology, liver tissue, Brazil, bioaccumulation