Soil Pb and Cd accumulation and age-dependent biochemical responses in Crocodylus moreletii: Evidence of chronic contamination in a protected urban lagoon

In the quiet corners of the world where cities meet the water, ecosystems often carry a silent burden. Heavy metals like lead and cadmium are invisible contaminants that do not break down over time. Instead, they settle into the mud at the bottom of lakes and lagoons, waiting to be picked up by the creatures living there. Because these metals persist, they can move up the food chain, accumulating in the bodies of animals that eat other animals. Scientists have long known that certain large predators, particularly those that live for a long time and stay in one place, are excellent indicators of environmental health. If these top predators show signs of stress, it often means the entire ecosystem is struggling. This is especially true in tropical regions where rapid city growth can outpace environmental protection, leaving wetlands vulnerable to pollution from runoff, wastewater, and industrial activity.

In a protected urban lagoon in Tabasco, Mexico, researchers turned their attention to the Morelet's crocodile, a species that calls these waters home. They wanted to understand if the crocodiles living in this busy, human-dominated landscape were showing signs of chronic exposure to heavy metals. The study focused on Laguna de las Ilusions, a large, shallow body of water surrounded by a growing city. Despite its status as a protected reserve, the lagoon receives constant inputs from the surrounding urban area. The team set out to measure the levels of lead and cadmium in the soil and water, and then to see how these chemicals affected the crocodiles' internal chemistry. They compared wild crocodiles from the lagoon with a group of captive crocodiles living in a controlled environment, using the latter as a baseline to understand what a healthy, unstressed population looks like.

The researchers collected samples from the lagoon and found a landscape heavily loaded with metals. The soil, which acts as a sponge for these pollutants, contained significantly higher amounts of lead and cadmium than the water itself. In fact, the levels of cadmium in the soil were far above international safety guidelines, and a large portion of the sampling sites exceeded the limits for lead. The study revealed that the lagoon's muddy bottom is essentially a storage tank for these toxins, holding onto them tightly due to the specific chemistry of the water and the high amount of organic matter in the mud. While the water contained lower concentrations, the soil remained a persistent reservoir, capable of releasing these metals back into the environment under changing conditions.

When the team analyzed the blood of the wild crocodiles, they found clear signs that these animals were under physiological stress. The wild crocodiles showed significantly higher activity in several key enzymes compared to their captive counterparts. These enzymes, which are produced by the liver and other organs, act as early warning signals when the body is trying to cope with toxins or inflammation. Specifically, the wild crocodiles had elevated levels of enzymes that indicate liver stress and changes in how their bodies process proteins. This suggests that the animals are constantly working to manage the chemical burden in their environment, even if they do not show visible signs of illness on the outside.

Perhaps the most revealing finding was that these biological responses were not the same for every crocodile. The study showed that the age of the animal mattered greatly. Younger crocodiles, particularly the yearlings, displayed the most dramatic changes in their enzyme levels, while the patterns shifted as the animals grew older. This indicates that the way a crocodile reacts to pollution changes as it matures, likely due to differences in diet, growth rates, and how their bodies handle toxins at different life stages. Interestingly, the sex of the crocodile did not seem to play a major role in these responses; the environment and the animal's age were the primary drivers of the stress observed.

The researchers were careful to note that while the soil was full of metals and the crocodiles showed signs of stress, they could not prove a direct, one-to-one cause-and-effect link in this specific study. They did not measure the actual amount of metal inside the crocodiles' tissues, and the animals in the wild are exposed to a complex mix of pollutants, not just lead and cadmium. It is possible that other contaminants, such as hydrocarbons from nearby oil activities, are also contributing to the stress. However, the strong coincidence of high metal levels in the soil and altered enzyme profiles in the crocodiles points to a clear picture of chronic environmental pressure. The lagoon, despite being a protected area, functions as a system where contaminants accumulate over time, affecting the health of its top predators.

This study highlights the value of using crocodiles as sentinels for environmental health in tropical cities. Because these animals live for a long time and stay in one place, they integrate the effects of pollution over many years, providing a long-term record of ecosystem health. The findings suggest that even in protected areas, the invisible accumulation of heavy metals in the soil can lead to measurable biological stress in wildlife. The research underscores the need to look beyond just the water quality and consider the soil as a major source of contamination. It also emphasizes that understanding how pollution affects wildlife requires looking at the specific age and life stage of the animals, as a young crocodile may react very differently to the same environment than an adult.

Ultimately, the work serves as a reminder that urban expansion and industrial activity leave a lasting mark on the natural world, even in places set aside for conservation. The crocodiles of Laguna de las Ilusions are living with a legacy of contamination that is written in their blood chemistry. While the study does not claim to have solved the problem or identified every single cause, it provides compelling evidence that the ecosystem is under strain. Future research will need to dig deeper, looking at the specific tissues of the animals and the complex mix of chemicals they face, to fully understand the long-term consequences for these ancient reptiles and the wetlands they call home.