Vernal pool

Vernal pools, also called vernal ponds or ephemeral pools, are temporary pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe development of natal amphibian and insect species unable to withstand competition or predation by fish. Certain tropical fish lineages (such as killifishes) have however adapted to this habitat specifically.

During most years, a vernal pool basin will experience inundation from local surface runoff, followed by desiccation from evapotranspiration. These conditions are commonly associated with Mediterranean climate. Most pools are dry for at least part of the year, and fill with the winter rains or snow melt. Some pools may remain at least partially filled with water over the course of a year or more, but all vernal pools dry up periodically. Some authorities restrict the definition of vernal pools to exclude seasonal wetlands with defined inlet and outlet channels. Such seasonal wetlands have larger drainage basins contributing higher concentrations of dissolved minerals, and increased probability of periodic scouring flows through the wetland. Low dissolved mineral concentrations of smaller vernal pool basins may be characterized as oligotrophic, and poorly buffered with rapid pH shifts due to carbon dioxide uptake during photosynthesis.