Santos estuary in Brazil has one of the highest levels of microplastic contamination in the world

Researchers sampled oysters and mussels at three locations between the port of Santos and the nearby city of Guarujá. Their aims included assembling data as a basis for public policy on basic sanitation. The law does not currently require removal of microplastic particles from sewage.

By Cristiane Paião  |  Agência FAPESP – A study shows that the estuarine system of Santos, a large city on the coast of São Paulo state and home to Latin America’s largest port, is one of the places in the world where levels of microplastic contamination are highest.

The Santos estuary is a riverine, marine and mangrove ecosystem under intense anthropogenic pressure due to the proximity of the cities of Santos, São Vicente and Cubatão.

An article on the study is published in Science of the Total Environment.

In the study, researchers at the Federal University of São Paulo (UNIFESP) analyzed three portions of the estuary: the ferry route between Santos and Guarujá, Góes beach, and Palmas island. They compared published data from more than 100 studies conducted in 40 countries with samples of oysters and mussels collected in these three areas in July 2021.

Microplastic contamination was worst of all in the area of the ferry route. Samples collected in that area had the lowest nutritional status and health, containing 12-16 particles of plastic per gram of tissue.

“In one mussel, we found more than 300 microplastic particles per gram. It’s important to note that the collection point at Góes was inhabited until very recently by a traditional fishing community. About 300 people live there now. The beach is quite remote. You can only get there by boat or on foot using a goat trail. The locals probably eat shellfish a lot, as the rocky shore and cliffs are easy to reach,” said Victor Vasques Ribeiro, first author of the article and a PhD candidate at the Institute of Marine Sciences (IMar-UNIFESP). The study was conducted during his master’s research with support from FAPESP.

Estuary ecosystems are influenced by tides and contain a mixture of freshwater and seawater. Mangroves are commonly found there and are home to significant biodiversity. The Santos estuary is located in the metropolitan area and receives large amounts of raw domestic sewage as well as industrial waste from the port.

“The results aren’t at all surprising, in my opinion,” Ítalo Braga de Castro, last author of the article and a professor at UNIFESP told Agência FAPESP. “In previous research on other contaminants, I found record levels of contamination by hazardous chemicals in this region. The port is the busiest in Latin America and the metropolitan area is densely populated, with about 1 million inhabitants, most of them in Santos itself. Many dangerous substances and types of waste, both domestic and industrial, are dumped in the estuary, and plastic materials come in from the sea.”

The difference in this study, he added, is that it shows that both oysters and mussels can serve as sentinel species to warn of contamination. The finding was based on experiments involving the mangrove oyster (Crassostrea brasiliana) and the brown mussel (Perna perna). “We can now extend the investigation using these two species to measure the changes that have occurred during the history of our territory,” he said.

The group plan to conduct a similar analysis of estuaries in other states, such as Ceará, Pernambuco, Espírito Santo, Rio de Janeiro, Santa Catarina and Rio Grande do Sul, during Ribeiro’s PhD research with FAPESP’s support.

Bivalve molluscs

Because oysters and mussels filter water for food, the researchers wanted to find out whether they could be used as a tool to measure microplastic contamination in other places along the coast. “They don’t move about but spend their lives on a rocky shore or the hard surface of a bridge or pier, where they’re highly exposed to pollution. They filter water for food and retain microparticles in their tissue,” Castro told Agência FAPESP.

During the study, the team measured the length, width and weight of shells and tissue, as well as assessing nutritional status and general health. “To analyze microplastic particle levels, we chemically digested the tissue with a potassium hydroxide solution, taking due care to avoid cross-contamination in the lab,” he explained.

Castro adds that next steps will include investigating when contamination of these bivalves began, not only in the Santos region but also in other coastal cities, and how it evolves over time as industry expands there. The analysis will extend to animals held in zoological collections.

“Through collaboration with the University of São Paulo’s Zoology Museum [MZ-USP], we’ll analyze samples collected and conserved in Brazil since the 1920s. This approach will enable us to reconstruct the historical events that influenced the rise in contamination,” he said.

Public policy

Contamination and pollution are different concepts, Castro explained. “We use the term pollution when damage is done. The study didn’t assess damage. It merely detected contamination. They’re often considered synonyms, but they have different meanings,” he said.

The researchers were struck by the large number of colorless fibers of all sizes between 10 and 1,000 micrometers (μm) found in the oysters and mussels analyzed, as well as compounds of cellulose and acrylic, probably from laundry wastewater in domestic sewage. “Textile fibers have been shown to be the most common type of microplastic in densely populated urban areas,” Castro said.

Small pieces of plastic fall off ships that transport raw materials for plastic production. “They’re pellets, which come from containers,” he said. “Many pellets escape into the environment during loading and unloading operations, contaminating the estuary and beaches. However, the microplastic particles we found in our oyster and mussel samples didn’t come from pellets, but from textile fibers.”

The probable source is domestic laundry wastewater. “Our clothes are now made mainly of polyester and other kinds of plastic. Washing loosens the fibers, many of which are swept away in the wastewater. Sewage treatment plants don’t remove these particles, which end up in rivers and the sea,” Castro said.

For this reason, the aims of the study, according to the researchers, included contributing to public policy for basic sanitation in order to ensure that laws are passed to require the removal of microplastic particles from sewage and other effluents.

The only legislation governing environmental management in estuaries in effect at present is Law 7,661, dated May 16, 1988, which established rules for the National Coastal Management Plan. The Federal Constitution also contains clauses on environmental protection. “It’s not illegal, but it’s utterly wrong to let the estuary be polluted in this way. The organisms that live there are greatly endangered, and so is human health,” Castro said.

Protected areas

The researchers have also analyzed microplastic contamination of protected marine areas. “In previous studies, we set out to see if these areas, in which biodiversity is supposed to be protected, are threatened by contamination. Two of our lab’s PhD candidates are working on this topic. Yonara Garcia Borges Felipe will focus on protected areas in São Paulo state, in collaboration with Alexander Turra, a professor at USP, and Maria Teresa Castilho Mansor at the Forest Foundation [linked to the São Paulo State Department of the Environment, Infrastructure and Logistics]. Meanwhile, Beatriz Zachello Nunes is studying microplastic contamination on the global and national scales, with the support of environmental institutions in Australia,” Castro said.

The article “Oysters and mussels as equivalent sentinels of microplastics and natural particles in coastal environments” is at : www.sciencedirect.com/science/article/abs/pii/S0048969723010847.