- Scientists assessed harmful effects on Amazonian fish after a barge capsized in 2013 with approximately 60,000 liters of petroleum asphalt cement in Manaus.
- Although the concentration of the contaminant decreased with rising water levels, the fish continued to exhibit effects of exposure three months after the accident, including DNA damage and neurotoxicity.
- The Negro River’s waters are rich in dissolved organic matter, which may increase the toxicity of the chemical compound.
- This research adds to other studies that attest to the harmful effects of oil on vertebrates, aquatic insects and plants in the Amazon, given the pressure for greater exploration in the region.
On the morning of March 26, 2013, approximately 60,000 liters (16,000 gallons) of an oil byproduct used to produce asphalt spilled from a barge on the Negro River, near the São Raimundo Harbor in Manaus. When researcher Helen Sadauskas-Henrique saw the news on TV, she did not hesitate to cancel plans to spend Easter with her family in São Paulo and rushed to the site to collect samples.
“I had already conducted lab tests on the effects of oil exposure on fish in the Negro River, but I’d never observed anything like this in the natural environment,” recalls Sadauskas-Henrique, now a professor at Santa Cecília University and a researcher at INCT Adapta, headquartered at the National Institute for Amazonian Research (INPA). “It was all very fast and intense. The samples had to be collected as quickly as possible to assess the impacts, both immediately and over time.”
The results of this effort were published in the journal Environmental Research. The team evaluated the effects of oil over 10, 45 and 90 days on two species that are common in the Negro, the threadfin acara (Acarichthys heckelii) and the demon eartheater (Satanoperca jurupari), using samples taken from a clean stretch of the river in the Tupé Sustainable Development Reserve as references — that is, a place without significant influence from human activities or the spill.
The study shows that the concentration of the contaminant in the water decreased with rising water levels, but the fish continued to exhibit effects of exposure months later, including DNA damage. The characteristics of the Negro River’s waters — naturally acidic and rich in dissolved organic matter — increased the chemical compound’s toxicity.
In the Amazon, incidents like the one at the São Raimundo Harbor are not isolated. In countries with more intense oil exploration in the forest such as Peru and Ecuador, accidents are frequent: A 2020 Oxfam survey found 474 spills in the Peruvian Amazon from 2000-19. In Brazil, agencies such as IBAMA, Brazil’s federal environmental agency, and ANP, the National Petroleum Agency, keep reports on these accidents but have no consolidated figures specific to the Amazon.
In Amazonas state, data on oil product spills are scattered throughout violation reports at the Amazonas Environmental Protection Institute (IPAAM). Mongabay conducted a search with the agency under Brazil’s Access to Information Act and found 19 oil spills from various sources, which were fined between 2016 and 2020, without details on volume.
As pressure mounts over new oil exploration projects in the region, both onshore and along the so-called Equatorial Margin, a consistent body of research warns of its harmful effects on Amazonian aquatic ecosystems.
Oil and black water in the Amazon: A deadly combo
The chemical compound spilled in the Negro River in 2013 was an oil byproduct: petroleum asphalt cement (PAC), used to produce asphalt. It is a highly viscous, usually poorly soluble material, but with a fraction that can dissolve or disperse in water. It contains polycyclic aromatic hydrocarbons (PAHs), contaminants that are harmful to the health of humans, animals and plants. Because it is more viscous than mixtures used in fuels, PAC clumps together. “It turns into blocks or ‘stones,’ which have started to appear on beaches,” she explains.
The portion that passes into the water tends to bind to the river’s organic matter and accumulate in sediments. In the Negro River, this is worse: Its waters contain a lot of dissolved organic carbon (DOC) resulting from the decomposition of leaves, branches and tree trunks, which gives the water a “tea-colored” appearance, as Sadauskas-Henrique explains. DOC can interact with PAHs, increasing their availability for fish to absorb.
“There are few studies on oil contamination specifically in the Negro River’s water,” she says. “Lab tests indicate that dissolved organic carbon present in water can make it easy for oil components such as PAHs to enter fish cells.”
The Amazon poses challenges for science. Although studies indicate that blackwater rivers can increase the toxicity of oil components, there is not enough research to establish this as a general rule. “The composition of organic matter changes with rainfall and depends on the source of the material. Therefore, the interaction of that water with PAHs and cells may also change,” Sadauskas-Henrique explains. “Different sources of dissolved organic carbon must be studied to understand how they may affect permeability in cell membranes and absorption of compounds such as PAHs.”
Amazonian fish may be more susceptible
Once in the sediment or water column, both soluble and particulate oil fractions enter through the gills or are ingested with food. Approximately 10% of the 1,165 fish species in the Negro river feed on the substrate. Because of this, and due to their more sedentary and territorial lifestyle — as is the case with the species studied — their susceptibility to this type of contamination may be greater.
At the São Raimundo Harbor, the concentration of PAHs in the water dropped by approximately 56% between 10 and 90 days after the accident, following the rise of the river. But the effects on the fish persisted. Ten days after the spill, DNA damage was already present, as well as signs of overload in cell membranes and liver. “This indicates that the animal is expending a lot of energy to face the contamination, which impairs other critical functions such as swimming, growth and reproduction,” Sadauskas-Henrique explains.
Within 45 days, both species showed high levels of genetic damage associated with the DOC-PAH combo in the water. At 90 days, the effects were compounded by signs of nervous system intoxication, which can affect swimming and feeding.
The association between PAHs and dissolved organic carbon, on the one hand, and greater stress and damage, on the other, suggests that the effects of oil interact with the characteristics of Amazonian blackwaters. “In this ecosystem, the impacts of an accident can be prolonged,” Sadauskas-Henrique reflects. “Faced with a large contamination load, species like the ones we studied may face greater pressure, at risk of local decline.”
And it’s not just petroleum. In Manaus as in other urban areas of the Amazon, interactions between pollutants, such as other oils, pesticides and microplastics, and extreme weather events such as prolonged droughts, combine in harmful ways, which science is still trying to understand.
Oil in the Amazon: Damage and Spills
Since the 1960s, oil spills have been frequent in Amazonian countries neighboring Brazil such as Peru and Ecuador. From 2011-22, approximately 36% of Peruvian oil was produced in the Amazon. In Ecuador, most of the production takes place in the eastern Amazon region.
Some cases stand out. In 2000, a stranded barge spilled 5,500 barrels of oil into Peru’s Marañón River. In 2007, in a Marañón tributary, a landslide ruptured an oil pipeline, spilling between 4,000 and 6,000 barrels, according to independent experts. In 2013, 11,000 barrels leaked into Ecuador’s Coca River, also as a result of a pipeline rupture. In the Ecuadorian Amazon, between 1972 and 2015, most spills occurred in river basins that connect protected areas and supply local communities, Indigenous peoples and riverine communities.
Although oil exploration in the Brazilian Amazon is not yet on the same scale as in neighboring countries, serious accidents have also occurred. In 1999, a ruptured pipeline released oil into the Cururu Stream in Manaus, reducing its wealth and abundance of aquatic insects. The oil quickly spread across the surface and settled in the sediment along the banks. The pipeline connected the Manaus refinery (Isaac Sabbá, now privatized) to the former Manaus Energia plant.
Currently, the main onshore oil and gas hub in the Brazilian Amazon is Urucu province in Coari, Amazonas state. In 2016, the average daily oil production in the area was approximately 40,000 barrels, in addition to 14 million cubic meters (494 million cubic feet) of natural gas. The 663-km (412-mile) Urucu-Coari-Manaus gas pipeline has been in commercial operation since 2009.
Álvaro Bezerra de Souza Junior, an engineer with a master’s degree in energy planning, conducted a case study in the 1990s on the safety and risks of exploration in Urucu. He says that Urucu oil is light and requires less refining, being primarily used in fuels.
While oil exploration in the Brazilian Amazon is currently considered safe, “the risk of spills during transportation is always significant,” he says. “This is important in a scenario of increasing fragmentation in the distribution market. Since Petrobras divested itself of control over BR Distribuidora and left the distribution business, many operators work under varying levels of control and rigor, and that includes outsourced barges for river transportation.”
In 2013, the company responsible for the petroleum asphalt cement cargo did not have a license to operate at the São Raimundo Harbor, according to IPAAM. In violation reports from 2016-20, Mongabay found 15 other fines related to irregular transportation or storage of oil products, obtained from IPAAM under the Access to Information Act.
Is there a future for oil exploration in the Amazon?
The impacts of oil products on Amazonian biodiversity are numerous and usually harmful. Before studying the 2013 spill, Sadauskas-Henrique was part of INPA teams that documented, in the laboratory, the effects of annatto oil on fish — e.g., the cachama (Colossoma macropomum) — and aquatic plants.
Respiratory disorders and severe changes in gills, such as displacement of their cell cover and necrosis, were observed in cachama exposed to soluble and insoluble fractions of the oil. Similar effects were reported for gold spot pleco (Glyptoperichthys joselimaianus).
In aquatic plants, the oil forms a physical barrier, reduces light penetration and hinders gas exchange, in addition to affecting flowering and germination. Furthermore, these plants are the staple diet of other animals such as manatees, which would make them more vulnerable to contamination.
These are some of the reasons for concern about the expansion of oil in the Amazon, which increases the risk of accidents and contamination. In addition to the attempt to drill an exploratory well on the Equatorial Margin, the ANP included 68 Legal Amazon blocks in the June 17, 2025, round: 47 in the mouth of the Amazon and 21 onshore, in the Parecis Basin. In addition to the environmental risk, the proposal directly or systematically threatens the areas of influence of 19 Indigenous lands, according to a technical note from the Coordination of Indigenous Organizations of the Brazilian Amazon (COIAB).
Petrobras is the leader in oil exploration in the Brazilian Amazon. Brazilian banks such as BTG Pactual and Itaú Unibanco and international ones such as Santander and Citibank have financed the company to the tune of $100 billion over the past 20 years, according to a report by Stand.earth. The study indicates that, on average, 71% of the Amazon is not effectively covered by the socioenvironmental policies of the region’s main oil and gas financiers.
Martyna Dominiak, leader of the Exit Amazon Oil and Gas campaign, says oil and gas exploration in the Amazon is associated with deforestation, pollution and violations of Indigenous peoples’ rights. “Oil infrastructure in other Amazonian countries show that it opens roads for other activities, often illegal, and for other activities that pollute the Amazon and do not benefit local communities,” Dominiak argues. “The entire Amazonia should be protected.”
Banner image: A demon eartheater (Satanoperca jurupari) covered in oil, collected after a vessel accident transporting petroleum asphalt cement near the São Raimundo Harbor in Manaus, Amazonas. Image courtesy of Helen Sadauskas-Henrique.
This article was first published here in Portuguese on Sept. 22, 2025.
Citations:
Sadauskas-Henrique, H., Braz-Mota, S., Campos, D. F., Dos Santos Barroso, H., Kochhann, D., Luis Val, A., & Maria Fonseca de Almeida-Val, V. (2024). Oil spill in an Amazon blackwater environment: Biochemical and physiological responses of local fish species. Environmental Research, 250, 118347. doi:10.1016/j.envres.2024.118347
Rivera-Parra, J. L., Vizcarra, C., Mora, K., Mayorga, H., & Dueñas, J. C. (2020). Spatial distribution of oil spills in the north eastern ecuadorian Amazon: A comprehensive review of possible threats. Biological Conservation, 252, 108820. doi:10.1016/j.biocon.2020.108820
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