The San Marcos River, Hays County, Texas 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 openings are inundated by up to 12 m of water. Directly below the dam, the river is clear and rapidly moving, ranging from about 5 to 15 m wide, flowing over a predominantly firm gravel bottom with many shallow riffles alternating with deep pools. We set out to examine the temporal and spatial variation in macroinvertebrate communities within this aquatic ecosystem.
Between Fall 2005 through to Spring 2009, we sampled for aquatic macroinvertebrates in a variety of microhabitats that ranged in distance from springs, substrate composition, and macrophyte cover. We used non-metric multidimensional scaling (NMS) ordination to assess the multivariate relationships among the invertebrate communities. In order to delineate invertebrate communities, we used a site-level cluster analysis along with an indicator species analysis.
We identified 17 orders, 68 families and 135 genera living in the San Marcos River. We trimmed the cluster analysis dendrogram at four groups which broke into units that correlated with both location in the river as well as the timing of the sampling. The indicator species analysis showed that the four groups delineated through the cluster analysis differed in functional groups of invertebrates. The final configuration for the NMS ordination had two dimensions. Dissolved oxygen, temperature, conductivity, and macrophyte percent cover showed strong (r > 0.5; p < 0.001) correlations with axis one and pH significantly (p < 0.001) correlated with axis two.
The results from this work allowed us to attain insight on the variability of macroinvertebrates in a spring-fed river that is impacted by human development (e.g., dams and sedimentation). This information can help us in planning future monitoring work and utilizing functional invertebrate groups as bio-indicators for the effects of development surrounding a river ecosystem.