By Adriana Cortés-Gómez, Camila Miguel, and Marc Girondot

Pollution is considered to be one of the top global threats to sea turtles and has been the focus of growing concern in recent years. Although much attention has been given to ocean plastic pollution and other forms of visible waste, invisible forms of pollution have been growing in scale and can now be found in virtually all of Earth’s waters, where they affect the well-being of marine creatures great and small.

One particularly insidious type of contamination is that of heavy metals, such as lead and cadmium; such contamination typically originates from industrial activities. When unnaturally high amounts of such metals enter an environment, they can accumulate in animal tissues and biomagnify as they are passed up the food chain. In many animals as well as in people, even slightly elevated levels of some heavy metals are known to cause cancers, developmental malformations, and numerous pathologies—including reproductive problems and kidney failure. In one well-studied population of killer whales (orcas) in Scotland, researchers observed over many years that the accumulation of such pollutants led to the deaths of several individuals and is now causing many animals to become infertile.

Cadmium in Ridley Turtles from the Mexican Pacific

Heavy metal pollution is specifically implicated as a threat to the health of olive ridley turtles at La Escobilla in Oaxaca, Mexico. This 15-kilometer (9.3-mile) stretch of beach is the site of one of the most remarkable success stories in global sea turtle conservation, and it now sees in excess of one million nesting ridleys annually—more than anywhere else in the world (see “Olive Ridleys: The Quirky Turtles that Conquered the World” in SWOT Report, vol. XVI). On some nights during mass nesting arribadas, miles of shoreline are covered in an unbroken vista of flying sand and heaving shells as turtles lay their eggs. It wasn’t always like this at La Escobilla. Nesting had declined significantly following decades of egg and turtle harvest, and rebounded only after harvesting was outlawed in the 1990s. Today, with tens of millions of hatchlings emerging each year, La Escobilla is key to the future of this once declining species.

However, research that has been under way since 2012 has revealed shockingly high levels of accumulated cadmium in turtle tissues, a contamination that may have a drastic impact on current and future generations of turtles. Samples collected from 46 animals in 2012 showed mean cadmium levels of 150 ppm/wet weight, the highest levels ever reported for a large marine animal. Researchers had hoped this finding was caused by a one-time pollution event, but further sampling showed that by 2019, cadmium had grown to 243 ppm in turtles’ kidneys. To put this observation in perspective, levels above 15 ppm in humans can cause chronic problems that, if not treated, can lead to death.

Although there are no such established thresholds in turtles, biomarkers such as oxidative stress, stress proteins, and asymmetry, as well as histopathological studies, indicate that the sampled animals experienced significant health effects. In some cases, cadmium can be implicated as the direct cause of death. Other species sampled near La Escobilla have also been affected; record cadmium levels were found in tissue samples from an adult leatherback, an immature dolphin, and a baby sea lion, urging researchers to sound the alarm and call for immediate actions to reverse this threat.

It has been difficult to identify a source for this contaminant in Pacific Mexico. Cadmium, mercury, and other heavy metals can occur naturally in the environment from erosion, weathering, movement of dust, and volcanic activities. But rapid urban development, increases in fertilizer and pesticide use, mining and smelting, fossil fuel residues, and industrial discharge have drastically raised the amounts of such elements in ecosystems worldwide.

Mining Disaster Continues to Impact Green Turtles in Brazil

In November 2015, a dam collapsed in Minas Gerais, Brazil, releasing an estimated 43 million cubic meters of iron ore tailings into the Doce River in what is considered one of the global mining industry’s biggest environmental disasters. The resulting mudflow spread 668 kilometers (415 miles) downstream, killing dozens of people and sweeping away everything in its path before reaching the Atlantic Ocean, where it contaminated coastal areas important for sea turtle nesting and feeding.

Since the disaster, researchers from the Chelonia Mydas Project at the Marcos Daniel Institute have been monitoring the impacted areas and focusing on the health of juvenile green turtles that inhabit the Santa Cruz district in the state of Espírito Santo. That area was most directly affected by the tailing plume. During the two-year period from 2018 to 2020, the health of the turtles there was compared with that of green turtles residing at Coroa Vermelha Island in the state of Bahía, the nearest green turtle feeding area that was not directly impacted by the contaminant plume.

Results showed that the Santa Cruz green turtles suffered from notably worse nutritional conditions, as well as a higher incidence of anemia, immunosuppression, fibropapillomatosis tumors, and ectoparasite load, in addition to possible hepatorenal pathologies. There is strong evidence to show that the pollution caused by the chemical tailings is responsible for the observed health differences, and a high priority is to continue monitoring in order to observe the contamination’s longer-term impacts on the animals and to better understand the health impacts they experience.

A Desperate Need for More Data

Sea turtles are surprisingly susceptible to ocean-borne pollutants in a variety of natural forms (such as red tides) and manmade forms. Extended exposure to contaminants can compromise turtles’ physiology, cause abnormalities in their embryos, impair their reproductive success and immune function, disrupt their endocrine systems, cause them chronic stress, and increase their susceptibility to diseases. Although the impacts of heavy metal exposure on sea turtles are just beginning to be understood, findings from the sea turtle case studies outlined here as well as elsewhere (see “Green Turtles as Silent Sentinels of Pollution in the Great Barrier Reef” in SWOT Report, vol. XIII) suggest that there is much reason for alarm. Because sea turtles are long-lived and slow to mature, it may take years of continuous study before researchers are able to understand the full impacts of such exposure. More and longer-term research is critically needed to comprehend the scale of this problem and to implement solutions. Sea turtles are an excellent flagship species to draw attention to this important issue, which has drastic impacts on countless other species—including humans. It is imperative that researchers worldwide begin at once to conduct more and broader sampling of the tissues of sea turtles and other species and that they pay greater attention to the sources of heavy metal and other contaminants in order to find ways to stop this invisible threat.

This article originally appeared in SWOT Report, vol. 17 (2022). Click here to download the complete article as a PDF.

Author Affiliations

ADRIANA CORTÉS-GÓMEZ, Conservation des Populations et des Communautés group, París-Saclay University, France, and SEE Turtles, Oregon, U.S.A.

CAMILA MIGUEL, Chelonia mydas Project, Instituto Marcos Daniel, Brazil

MARC GIRONDOT, Conservation des Populations et des Communautés group, París-Saclay University, France