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Ephemeral pools can be found in many parts of the world. These pools range in size from small rock basins holding no more than 1-2 liters (Figure 1), to large vernal lakes covering hundreds of hectares (Figure 2). They occur at high elevations, below sea level, on bedrock, and on very old soils. They fill in different seasons depending on climatic patterns, and may have a single annual wet phase, or fill and dry many times a year. Most pools are heterotrophic, much of the energy passing through them comes from detritus, not direct photosynthetic production (Kuller and Gasith 1996). Pools supporting a wetland/terrestrial plant community can be considered autochthonous in that vascular plant production during the dry phase provides detritus that supports the aquatic system during the next wet phase. Some systems (e.g., rock pools and playas, Figure 3) lack significant vascular plant production, most of their energy comes from allochthonous detritus blown in or carried into the basin from the surrounding watershed, with primary production by algae in the basin varying in significance.
Figure 1. Very small pothole on the Colorado Plateau. Figure 2. Hog lake, a large vernal lake in Tehama County, Calif.
By definition ephemeral pools dry up periodically, typically holding water for only a few days to months, yet these may represent one of the most permanent kinds of aquatic environments on Earth, judging from the age of some species that inhabit them (Fryer 1996). For example, lower Triassic tadpole shrimp fossils were assigned to the living species Triops cancriformis (Kerfoot and Lynch 1987), and Walossek (1993) recently reported finding branchiopod fossils in Cambrian rock. Despite the age of some of these groups, many branchiopods appear to be closely adapted to current climatic conditions in their pools. Cues for hatching, time to maturity, temperature tolerances, and other aspects of their ecologies are all relatively closely matched to current conditions within pools each species inhabits.
Figure 3. Aerial view of potholes in Navajo sandstone, Grand County, Utah.
Because ephemeral pools form under different climatic regimes, environmental conditions found in pools vary considerably from place to place. In a Mediterranean climate, pools are relatively cool, long-lasting environments, while under a summer monsoonal climate, pools may be fairly short-lived, warm water systems with multiple wet/dry cycles each year. Playas fill and dry slowly; the long history of mineral accumulation in these closed basins exerts a major influence on the aquatic environment in these systems. The predictability of the aquatic environment within an ephemeral pool varies considerably across climatic patterns, substrates, and geologic/geomorphic history; organisms inhabiting each pool type are well adapted to particular pool conditions. Significant changes in climate will have different impacts on different kinds of ephemeral pools, but as a group, ephemeral pools may be more sensitive to climate change than the broader landscape because they are tightly coupled to precipitation and temperature patterns.
Seasonal precipitation patterns and temperature regimes, the predictability of precipitation (intra- and inter-annual variation), chemical and physical properties of the substrate, and whether there is overland flow of precipitation before accumulating in the pools all affect the temporary pool environment. Unlike permanent bodies of water, there is no capacity to dampen out climatic fluctuations, e.g., storing water from wetter than average years that would allow continued survival of aquatic species through drier periods. Any large shift in filling and drying patterns will result in a different system, probably with retention of some species, but significant changes in community and ecosystem properties are also likely.