Off the highways and byways in the Riviera Maya, Mexico, everything around you is jungle. A dense and leafy jungle that, towards the continent, extends to the horizon, covering almost the entire Yucatan Peninsula, almost to the coastline. Seen from the height of the Nohoch Mul temple , in the Mayan archaeological complex of Cobá, the flat surface of the green sea that represents the tree canopy of the jungle is dotted here and there by mounds that break the uniformity, they are other ancient Mayan temples that, forgotten by history, the jungle has reclaimed.
But not all treasures are visible from above. Scattered along the coast, a whole series of chambers and galleries are hidden, produced by the water that seeps into the limestone subsoil, opening huge caves. Some vaults, at a given moment, collapse, opening the cavity to the outside, and are flooded thanks to the entry of rainwater and aquifers. These cavities, of different sizes and ages, are known as cenotes . And they make up a unique type of ecosystem.
The cenote as an ecosystem
The cenote is the most peculiar aquatic ecosystem of the Yucatan Peninsula. In this region, the forest floor sits on a huge platform of calcium carbonate. The heavy rains that characterize this area dissolve, slowly but steadily, the limestone rock and generate a system of caves that are called karstic .
Most of the cenotes are filled with fresh water, coming from direct rain or from the infiltration of groundwater. Although, in those closest to the sea, salt water infiltrates from the coast, and under fresh water there is salt water. In these cenotes, called anquihaline because of the mixture of waters, a condition called meromixis occurs , a form of water stratification where the one on the surface does not mix with the one found in depth.
The water in the cenotes, which are well connected to the rest of the underground aquifer, remains clear and crystal clear for most of the year and it is common to find aquatic plants that spread their floating leaves on the surface of the water. There you can find fauna, especially crustaceans and fish . However, although they have their own food webs, these tend to be relatively simple.
However, in those cenotes that are more isolated, the waters are more cloudy, they appear full of green algae , dinoflagellates, diatoms and cyanobacteria. In these more stagnant cenotes, organic matter accumulates, there is hardly any oxygen available in the water, and anaerobic bacteria abound at the bottom.
Endemisms in the cenotes of Yucatan
Although there is a flow of water between different cenotes, this is produced through the infiltration of the rocks, through which animals cannot pass, hence, in practical terms, each cenote functions as an island of biodiversity, with its own endemic species.
In addition to fish and crustaceans, the cenotes have an abundant presence of rotifers , a group of microscopic invertebrates that are part of the zooplankton, and of which only about 2,200 species are known worldwide. In the cenotes of Yucatan, 102 of them have been described.
Other species endemic to these cavities belong to the group of copepods , some very small crustaceans, of which 35 unique species have been discovered, and none of them are present, not even in all the cenotes of the peninsula.
It is also worth mentioning the remipedians , other crustaceans typical of salty caves, completely blind, and one of the most primitive that exists. In Yucatán they are represented by the species Xibalbanus tulumensis , the first known poisonous crustacean. This hermaphrodite animal is typical of the anquihaline cenotes near the ancient Mayan city of Tulum, and whose generic name tells us about Xibalbá , the “hidden world” of Mayan mythology, a supposed underground world or underworld where the gods of death lived. . According to their mythology, the cenotes functioned as doors to that world.
The risks of the cenotes
The great increase in the population of the cities of the peninsula, especially Cancun, together with the increasing influx of tourists, has increased the domestic, agricultural and industrial activities in the area, which has resulted in poor management of the resources. waste, both garbage and sewage.
The consequence of all this is an increase in the concentration of pollutants in the aquifers and the proliferation of faecal bacteria , such as Escherichia coli . This is revealed by a study carried out in 2020 by researchers from the University of Quintana Roo, in Mexico. The analysis of the waters of ten cenotes in Cancun revealed the existence of abundant fecal bacteria in all of them.
In addition, with the massification of tourism in the area, transport also increases. Another study from the same year analyzed hydrocarbon contamination in 11 cenotes in Quintana Roo, finding polycyclic aromatic hydrocarbons associated with diesel, gasoline and asphalt, and whose increase is associated with the increase in tourist traffic.
The authors of these works propose taking immediate measures to fight against these serious problems, which range from increasing the education and ecological awareness of the inhabitants of Yucatan and visitors, and implementing restrictions on the use of cenotes or nearby areas as garbage dumps.
Borbolla-Vazquez, J. et al. 2020. Total and faecal coliforms presence in cenotes of Cancun; Quintana Roo, Mexico. BioRisk, 15, 31-43. DOI: 10.3897/biorisk.15.58455
León-Borges, J. A. et al. 2020. Hydrocarbon Contamination Patterns in the Cenotes of the Mexican Caribbean: The Application of Principal Component Analysis. Bulletin of Environmental Contamination and Toxicology, 105(5), 758-763. DOI: 10.1007/s00128-020-03026-1
Schmitter-Soto, J. J. et al. 2002. Hydrogeochemical and biological characteristics of cenotes in the Yucatan Peninsula (SE Mexico). Hydrobiologia, 467(1), 215-228. DOI: 10.1023/A:1014923217206
von Reumont, B. M. et al. 2014. The First Venomous Crustacean Revealed by Transcriptomics and Functional Morphology: Remipede Venom Glands Express a Unique Toxin Cocktail Dominated by Enzymes and a Neurotoxin. Molecular Biology and Evolution, 31(1), 48-58. DOI: 10.1093/molbev/mst199