The USGS NAWQA Program is an interdisciplinary program designed to assess water quality across the Nation using chemical, physical, and biological measures. The program is based on investigations of more than 50 study units encompassing one or more major river basins and aquifers. Intensive field investigations and retrospective reports of existing environmental data in the study units increase the understanding of the status of and trends in the resource quality of these aquatic systems. This retrospective report summarizes available information on aquatic biology of the streams and rivers in the Guadalupe, San Antonio, and Nueces River Basins, and the San Antonio-Nueces and Nueces-Rio Grande Coastal Basins, and of the Edwards aquifer where it underlies the three river basins. The biology of the study-unit streams is determined mostly by the characteristics of the ecoregions they transect. The ecoregions are the Edwards Plateau, Texas Blackland Prairies, East Central Texas Plains, Western Gulf Coastal Plain, and Southern Texas Plains.
This report summarizes in table format 32 aquatic vertebrate (primarily fish), 54 aquatic invertebrate, and 13 aquatic plant studies, published mostly during 1973–97. About 20 percent of the previous fish and invertebrate studies and about 75 percent of the aquatic plant studies have centered on Comal Springs in Comal County and San Marcos Springs in Hays County, the two largest springs in Texas. Although several important studies are available for the San Antonio region, documentation of aquatic biology of the remainder of the study unit is relatively sparse.
The SCTX study unit is unique in that it contains a diversity of both surface-water and aquifer habitats. The streams, particularly in the Edwards Plateau, support three dominant biological groups—fish, aquatic invertebrates, and plants. Potential threats to the endemic (native) species exist, such as impoundments and flood-control projects, siltation from erosion, ground-water pumping, recreational activities, wastewater discharge, and introduction of non-native species. More than 30 non-native fish, invertebrate, and plant species have been introduced into the region, including the giant ramshorn snail and blue tilapia of tropical origin that are able to survive the relatively mild and constant temperatures of springs in the study area. About 20 percent of native Texas freshwater species are in potential danger of extirpation (range reduction) or extinction. Of the 19 aquatic species Federally listed as endangered or threatened in Texas, 8 are associated with springs and spring runs in the SCTX study unit. All of the endangered species in the study unit are associated with springs and spring runs.
A large number of endemic species in the study unit are associated with subterranean aquatic ecosystems, most likely a consequence of the unique proximity of the varied topographic and hydrologic conditions of the area and of the geological development of the Edwards aquifer. Ninety-one endemics, including 44 species found solely underground, are associated with the aquatic ecosystems (including springs) of the Edwards aquifer.
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