Spring Ecosystems of the North American Deserts: Ecology, Hydrology, and Conservation, Tucson, AZ, 5-6 May 2000.

Springs as unique aquatic environments.

P.J. Unmack & W.L. Minckley

"Springs hold a position of importance as study sites that is out of proportion to their size and number" (E.P. Odum 1953:250).

The significance of springs environments to biologists has long been recognized, although few have utilized their potential. Springs have attracted attention due to their small size relative to most aquatic habitats and somewhat "simple" communities which make them amenable for certain studies. Also, in arid regions springs are often all that remain of once extensive aquatic systems, hence they allow remnant faunas to persist. However, these important characteristics are not related to why springs are unique aquatic habitats. The most unique feature is their water source which comes from deeper aquifers. Because of the depth water rises from, and the time it takes to travel through an aquifer it tends to be largely invariant in physico-chemical characteristics. If an aquifer is sufficiently large spring discharge will also be constant. Once water emerges at the surface, physico-chemical parameters start to change, however, they tend to be somewhat consistent at any given distance from the spring source. The degree of change depends upon discharge as well as atmospheric and biological interactions.

Generalizations about springs are difficult to make due to the very character of their water source. Each aquifer tends to be unique due to differences such as geographic size, volume, mineral composition, water transmission rate, piezometric surface, etc. These differences may also lead to different springs types within large aquifers. Two major factors that influence spring type are discharge volume and geomorphology of the spring outlet. When opposite ends of a range of spring types are examined the differences are clear. Springs with large discharges that form large pools are some of the most constant aquatic systems in existence. In contrast, small shallow seepages can be one of the most variable. These differences disintegrate as intermediate spring types are included.