Aquatic Macroinvertebrates of a Spring-Fed Ecosystem in Hays County, Texas, USA

Abstract

The San Marcos River is fed by the second largest spring system in the state of Texas and is home to multiple endemic and federally listed endangered and threatened plant and animal species. Due to a dam originally created to facilitate a gristmill in 1849, the spring orifices are inundated by up to 12 m of water. The area upstream of the dam is referred to as Spring Lake. Directly below the dam, the river is clear and rapidly moving, ranging from about 5 m to 15 m wide, flowing over a predominantly firm gravel bottom with many shallow riffles alternating with deep pools (Lemke, 1989). The historic mean spring flow for this system is 4.5 m3/sec (Ogden et al., 1986). The headwater stretch of the San Marcos River (the upper 7.2 km) stays at a relatively constant temperature at around 22°C (Hannan and Dorris, 1970). The conductivity ranges from ~600 – 700 µS/cm for the majority of this upper portion of the river, but then increases to a range of ~625 - 850 µS/cm downstream of the San Marcos Wastewater Treat ment Plant (Groeger et al., 1997), which is located about 5 km downstream of the springs. The pH in the artesian well located near the headwaters on Texas State University property and Spring Lake is lower (range = 6.3 – 7.4) than sites down stream (range = 6.6 – 7.8), which indicates that aquifer CO 2 concentrations are higher than those in equilibrium with the atmosphere (Groeger et al., 1997). Human-induced impacts to the Edwards Aquifer which feeds the springs (e.g. water pumping) as well as to the river itself (e.g. urbanization) may have negative effects on the aquatic macroinvertebrate community in the San Marcos River. There have been few peer-reviewed manuscripts on the aquatic macroinvertebrate community of the San Marcos River (Edwards and Arnold, 1961; Bowles et al., 2007). Given the described concerns and the hydrologic consistency of the upper San Marcos River (Fries and Bowles, 2002) there is a need for detailed information on macroinvertebrate community composition in the river. The purpose of this study is to create a baseline species list to evaluate future changes in San Marcos River macroinvertebrate community composition. We collected aquatic macroinvertebrates from multiple locations throughout Spring Lake and the San Marcos River in a wide variety of microhabitats in an attempt to compile a macroinvertebrate taxa list that was as complete as currently possible (Fig. 1). We set 0.25 m2 quadrats in 188 locations along the river bottom and 147 locations within the lake. We placed a 0.5 m x 0.5 m modified dip net (500-µm mesh) downstream of each quadrat. We agitated the substrate inside