A recent study identified an average concentration of 0.25 particles per litre of microplastics in the waters of Prato Lake, located in Anavilhanas National Park in the state of Amazonas. Approximately eight out of every ten fish analysed contained microplastics, with an average of 3.3 particles per individual, despite living in a protected area far from urban centres.
Adalberto Val, a biologist at the National Institute of Amazonian Research and co-author of the study, stated that "as a result of the complex interconnection between aquatic ecosystems in the Amazon, plastic is found even in the most remote places". The particles found consisted mainly of polyamide filaments — nylon — along with PET fragments from clothing, abandoned fishing nets, containers and bottles.
The material enters the environment after its use in cities and boats, decomposing into fragments that are then transported to other water bodies by rain, forest streams known as igarapés and rivers. During seasonal flooding, the increase in water levels connects riverbanks, forests, channels and lakes, distributing these particles even in remote areas such as Prato Lake.
An independent researcher found concentrations of 5 to 8 particles per cubic metre in Anavilhanas. Andreu Rico noted that these concentrations "are lower than those found in Amazon cities and do not pose a risk of poisoning for animals", but added that "they demonstrate how plastics travel through water or air and that there are microplastics in any sample that is taken".
The igarapés, which directly receive untreated wastewater and uncollected rubbish in cities such as Manaus and Belém, have microplastic concentrations hundreds of times higher than the main course of the Amazon River. In these small aquatic environments, microplastics create serious stress for organisms that form the base of ecosystems, including aquatic insects that decompose organic matter.
An experiment with Amazon caddisflies revealed high mortality rates in aquatic insects after 15 days of exposure to microplastics. At one of the analysed concentrations, 96 per cent of the insects died. Renato Tavares Martins, researcher at the National Institute of Science and Technology Adapta, stated that "mortality increases approximately seven times compared with an environment free from pollution".
The insects ingest microplastics when shredding leaves that fall on contaminated igarapés. Many species breathe through external gills that can become obstructed when contaminated water passes over these structures, reducing respiratory capacity. Once ingested, microplastics can block the digestive tract and accumulate in excretory organs.
Since these larvae form the base of the food chain in Amazon streams, contaminated insects can transfer microplastics to other animals and terrestrial ecosystems. "If a predator, such as a fish, eats a shredding insect, it will also ingest the microplastics it contains," Tavares explained.
The conidial fungi that colonise fallen leaves in streams are also affected. In an experiment that combined microplastics with warming induced by climate change, these organisms showed a decrease in spore production and a slower decomposition rate, further compromising the vital processes of the aquatic ecosystem.
