This report summarizes in table format 32 aquatic vertebrate (primarily fish), 54 aquatic invertebrate, and 13 aquatic plant studies available for the area of the South-Central Texas study unit of the U.S. Geological Survey National Water-Quality Assessment. The studies, published mostly during 1973–97, pertain to the Guadalupe, San Antonio, and Nueces River Basins, the San Antonio-Nueces and Nueces-Rio Grande Coastal Basins, and the Edwards aquifer where it underlies the upper parts of the three river basins. The biology of the study-unit streams is determined mostly by the characteristics of the ecoregions they transect—the Edwards Plateau, Texas Blackland Prairies, East Central Texas Plains, Western Gulf Coastal Plain, and Southern Texas Plains.
About 20 percent of the previous fish and invertebrate studies and about 75 percent of the aquatic plant surveys have centered on Comal Springs in Comal County and San Marcos Springs in Hays County. Although several important studies are available for the San Antonio region, documentation of aquatic biology for the remainder of the study unit is relatively sparse. The streams in the study unit, particularly in the Edwards Plateau, support three dominant biological groups—fish, aquatic invertebrates, and plants. Potential threats to these organisms include 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. Of the 19 aquatic species Federally listed as endangered or threatened in Texas, 8 are associated with springs and spring runs in the 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.
The U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program began in 1991 to assess many of the Nation's major river basins and aquifers. The Program is designed to produce technically sound descriptions regarding the status of and the trends in the resource quality of these aquatic systems. NAWQA also is designed to increase the understanding of the natural and human factors that affect these water resources and to link this understanding with the observed status and trends. The nationally consistent, integrated assessment of chemical, physical, and biological resources will provide water managers and policy makers with information for directing water-quality management programs and for evaluating the effectiveness of these programs. Gilliom and others (1995) present a complete description of the NAWQA objectives and design.
The building blocks of the NAWQA Program are study-unit investigations. The study units selected encompass one or more major river basins and aquifers. When fully implemented, there will be more than 50 study units distributed across the Nation. Combined, they encompass about one-half of the conterminous United States and 60 to 70 percent of the population and national water use. One-third of the study units are intensively studied for 3 years on a rotational basis with each of the other two groups of study units, resulting in each study unit being revisited for intensive study on 9-year cycles. The NAWQA study unit addressed in this report is the South-Central Texas (SCTX) study unit (fig. 1), which includes the Guadalupe, San Antonio, and Nueces River Basins; two minor coastal basins; and the Trinity, Edwards, Carrizo-Wilcox, and Gulf Coast aquifers where they underlie the three river basins.
In addition to intensive field investigations, retrospective reports of existing environmental data are prepared at study-unit and national levels to improve the understanding of historical and present conditions of the water resources and to help interpret results from the intensive field investigations. Retrospective synthesis of existing stream and aquifer biological data for the SCTX study unit is addressed in this report.
Uses of Aquatic Biological Data
Biological monitoring is widely used to assess water resources, both as an integrative assessment tool and as the only direct method to determine instream attainment of State water-quality standards for aquatic life use. These standards are assigned to most U.S. surface waters. To determine instream attainment of these standards, some states have incorporated biological criteria into State water-quality criteria and regulations (Davis and others, 1996). These criteria are either narrative descriptions, such as lists of fish species, or numerical expressions (metrics) of aquatic life variables, including diversity indices and pollution tolerance values (U.S. Environmental Protection Agency, 1996).
Biological monitoring serves as an integrative assessment tool in two ways: (1) The type and condition of organisms reflect the overall health of the aquatic resources (Karr, 1995), and (2) the relatively stationary nature of many aquatic organisms signifies their ability to integrate environmental conditions over time, thereby reducing frequency of sampling needed to detect changes (Hynes, 1960).
Combining biological monitoring with physical and chemical data can be used to develop a comprehensive and efficient approach to water-quality surveillance (Hornig, 1984). Initial surveys of one or more components of the resident biota (typically fish, invertebrates, or plants) provide evaluations of the overall quality of the water resources. When results from these initial surveys indicate biological impairment, follow-up analyses (chemical, habitat, or more intensive biological studies) are done to determine the extent and probable causes. Biological monitoring also can be used to measure the success of restoration and the subsequent attainment of water-quality standards.
Reference-site or paired-site monitoring helps factor out annual area-wide variations in the biota, improving the ability to distinguish localized (typically human-caused) effects from regional (typically climatic) effects. Reference sites are the least impaired sites in a specific geographic region and serve as "benchmarks" for evaluating the stream quality at other sites. State water-quality agencies use biological data at reference sites to develop the biological criteria used to determine attainment of water-quality standards for aquatic life use (Hornig and others, 1995).
The USGS recognizes the critical role of biological and habitat data for comprehensive assessment of aquatic environments by water-resource managers. When complemented with chemical constituent data and land use information, biological and habitat data can be useful for identifying the natural and human factors affecting current conditions and the trends in aquatic-resource quality (Cuffney and others, 1997).
This retrospective report summarizes 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. (Aquatic biological information on the Trinity, Carrizo-Wilcox, and Gulf Coast aquifers is not included.) The report contains lists and distributions of fish and aquatic invertebrates, and lists of aquatic plants, aquifer organisms, non-native aquatic species, and endangered aquatic species. The report identifies sources of information on the biology of the streams and rivers and of the aquifer. Maps are provided to identify studies in specific areas. The report summarizes major publications, serving as a "one-stop" resource for historical aquatic biological data (prior to 1998) for this region.
Primarily Federal, State, and academic organizations have collected biological data from river basins in the SCTX study unit. The studies and reports of the U.S. Fish and Wildlife Service (USFWS), Texas Natural Resource Conservation Commission (TNRCC), Texas Parks and Wildlife Department (TPWD), San Antonio River Authority, Southwest Texas State University, and University of Texas provide most of the information in this report. This report emphasizes biological surveys that mostly were published after 1972. Young and others (1973) provide a compilation of information collected prior to 1973.
The authors thank their colleagues at the U.S. Fish and Wildlife Service, Texas Natural Resource Conservation Commission, Texas Department of Parks and Wildlife, San Antonio River Authority, Southwest Texas State University, and University of Texas, who have been instrumental in providing literature and biological data cited in this report.
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