Cenotes, natural sinkholes in Mexico’s Yucatán Peninsula, fascinate with their clear waters and depths. These formations often spark questions about their water type. While often associated with clear pools, their water is more complex than a simple classification. Unraveling the nuances of cenote water reveals a dynamic interplay of geological processes and hydrological connections.
The Simple Answer: Mostly Freshwater
Most cenotes contain freshwater. This connects them to underground rivers and the regional water table, recharged by rainfall. The water is cool and clear, making them popular for swimming and diving. Their freshwater nature defines most of these formations, supplying a resource in regions without surface rivers.
How Cenotes Form and Get Their Water
Cenotes originate from karstification, prevalent in regions with porous limestone bedrock, like the Yucatán Peninsula. Millions of years ago, the area was submerged, forming thick limestone layers from marine organisms. When sea levels receded, this limestone became exposed.
Rainwater, slightly acidic from absorbing carbon dioxide, percolates through limestone cracks. Over time, this acidic water dissolves calcium carbonate, carving out extensive underground cave systems. These networks are underground rivers, fed by rainfall. As these voids expand, cave ceilings can collapse, creating an opening to the surface that exposes groundwater. This connection to the rain-fed water table is why most cenotes are filled with freshwater.
The Saltwater Exception: Haloclines and Coastal Cenotes
Not all cenotes are exclusively freshwater; some, particularly those closer to the coastline, exhibit a mix of fresh and saltwater. This occurs because the porous limestone allows for an underground connection between the cenote systems and the ocean. Denser, cooler saltwater from the sea can intrude into the deeper sections of these subterranean cave networks.
Where the lighter freshwater layer meets the heavier saltwater layer, a distinct boundary forms, known as a halocline. This visible layer appears as a shimmering, blurry effect in the water due to the differing densities and refractive indices of the two water types. Divers often experience a noticeable temperature change when crossing a halocline, as the saltwater layer can be slightly warmer than the freshwater above. The depth of a halocline can vary, typically found between 10 to 30 meters below the surface.
What This Means for Visitors and Ecosystems
The water characteristics of cenotes significantly influence both the visitor experience and the unique ecosystems they support. The predominantly freshwater nature of most cenotes contributes to their exceptional clarity, often allowing visibility for many meters. Water temperatures typically range from 21 to 27 degrees Celsius (70-80 degrees Fahrenheit), offering a refreshing experience for swimmers and divers.
The aquatic life within cenotes adapts to these specific conditions. Freshwater cenotes support fish like mollies, guppies, catfish, and Mayan tetras, along with crustaceans and amphibians. Some plants, such as algae and submerged grasses, thrive near cenote entrances where sunlight penetrates, providing oxygen and habitat. In coastal cenotes with haloclines, organisms like certain fish species and specialized invertebrates can tolerate the brackish conditions. Cave-dwelling species, such as blind fish and shrimp, have evolved in the darker, more isolated sections of these systems.