Ephemeral Pools

Ephemeral pool on a large boulder

NPS

Throughout much of the Colorado Plateau, rain is sporadic during the summer months. With surface temperatures reaching above 100 degrees in the southwest, obtaining moisture sometimes becomes top priority for the animals that make this region their home. The little rain that does fall tends to run off over rock or gets absorbed quickly into the dry soil. Some moisture can get trapped in shallow depressions in the rock. This water collects in “ephemeral pools” that may only last a few days. These pools are essential for the survival of many desert creatures.

Most animals use these ephemeral pools as a wayside stop in their daily journeys. Some species of aquatic life make these ephemeral pools their permanent home. Since potholes have several wet-dry phases, the organisms that live in them have special adaptations to survive in this ever-changing environment.

Not all the rock at Black Canyon readily forms potholes. Sandstone and igneous rocks weather away into pits or pools that gather water and support life. Because the rim of the canyon lies between 7,700 ft (2,347 m) and 8,300 ft (2,530 m), the temperature ranges can be extreme from season to season. The organisms that live in these potholes must deal with often insufficient amounts of water and a short window of time for growth and development. The high elevations and cold temperatures found at the Black Canyon provide a more hostile environment for pothole creatures when compared to the rest of the Colorado Plateau.

 

The biggest challenge facing any of the organisms living in the pools is desiccation, or drying out, after the water has evaporated. These animals have three main ways of dealing with drought: resist, escape, and tolerate.

Drought Resisters
Some organisms have a tough, waterproof exoskeleton that prevents dehydration. Burrowing into the fine mud that lines the bottom of the pool further reduces their exposure to the sun. After the pool has dried completely, the surface of the mud may be baked solid. This helps to seal in the moisture the animal will need to survive until the pool fills again.

Drought Escapers
Other organisms use potholes to lay eggs and develop. Once they are adults, they cannot tolerate desiccation. After their drought-resistant eggs are laid, the adults must move on to a permanent water source, or more typically, they die as the pool dries. However, the next generation is now ready to hatch during the next fill cycle.

Drought Tolerators
Tolerant organisms have developed ways to lose up to 90 percent of their total body moisture for long periods of time. When a pool fills again, they rehydrate and become fully functional. This process is called cryptobiosis. It is carried out by a command center located in the nervous system that remains hydrated and can carry out the basic life functions of the dehydrated cells. Other tolerators have only one stage in life, such as egg or larva that can survive desiccation, but will die if the pool dries out during another phase.

 

A Difficult Life

Even when water is present, these organisms have difficulty reaching adulthood and typically live a very short life span. Tadpole shrimp, one of these pothole organisms, can reach maturity in as little as 24 to 36 hours, mate, and lay eggs. Another method of survival employed by pothole organisms is delayed hatching. To compensate for varying levels of water, not all eggs hatch at the same time. Some will remain dormant even after several wet-dry cycles. This spacing apart insures, by random chance, that at least one hatch will receive enough moisture to reach adulthood.

Pothole ecosystems and the species that live within them are extremely sensitive and can be greatly affected by climate change or any disturbance to their environment. These pools do not have the ability to counteract sudden shifts in their pH levels. These changes can be brought about industrial pollution, acid rain, or using soap for handwashing and dishes too close to potholes. Even walking on a dry pothole can be detrimental. The animals living in these shallow pools may not be able to adjust to sudden changes and perish. Extra care should be taken around these pool to help ensure the continued survival of these unique and fragile ecosystems.