Effects of hydrologic variability on biological assessments in streams in Austin, TX

Rapid Biological Assessments (RBA’s) are used across the United States to evaluate the condition of stream health, particularly as a means to assess the effects of non-point source pollution that are not apparent in traditional water chemistry analyses. The City of Austin, Texas has been using RBA’s in urban and non-urban streams for the last six years in an effort to expand and improve its environmental monitoring programs. Central Texas weather, which is characterized by flashy spates and long dry periods, is distinct from the temperate climates where the RBA's were developed. In addition to naturally dramatic hydrological cycles, urbanization and its high levels of impervious cover further exaggerate stream flow patterns, producing greater runoff volumes, higher peak flows and less baseflow. Documentation of recent and/or long-term hydrologic variability is not a common practice in the municipal, state or national agencies that conduct RBA’s. Our hypothesis is that hydrologic variability in this region will have a significant relationship to benthic macroinvertebrate community structure (as expressed by an index of univariate metrics). Hydrologic parameters were calculated using historical USGS flow data for study streams and compared to available City of Austin biological data. Results show that hydrologic regimes in Austin have a significant relationship to benthic community metrics. Hydrologic variability is an important structural mechanism in this region and should be utilized as a template in interpreting biological assessments using RBA metrics. INTRODUCTION: Rapid Biological Assessments (RBA’s) are used across the United States to evaluate stream health, particularly as a means to assess the effects of non point source pollution that may not be apparent in traditional water chemistry analysis (Karr and Chu, 1999; Barbour et al., 1998; Merrit & Cummins, 1996; Resh and McElravy, 1993; Plafkin et al., 1989). Benthic macroinvertebrates, the community used most often in RBA’s, provide a sensitive measure of cumulative or low-level chronic contamination and also may reflect the physical or structural degradation of aquatic habitats which can occur in urbanized watersheds (Karr and Chu, 1999; Barbour et al., 1998; Rosenberg and Resh, 1996; Plafkin et al., 1989; Hynes, 1970). Benthic macroinvertebrate community structure data are transformed into metrics and compared to reference conditions to establish a qualitative scoring gradient (Karr and Chu, 1999; Hughes, 1994; Barbour et al., 1994) that reflects the main aspects of community structure (taxonomic richness, composition, tolerance). Assessments using these metrics and indices are often used as water quality management tools and recently as regulatory criteria in water quality monitoring programs. Although RBA’s are intended to assess the effects of point and non-point source pollution, they have not generally been used to distinguish between disturbed and undisturbed hydrologic regimes. Every effort is made during the RBA process to minimize variability in conditions outside of the changes caused by pollution sources. For example, habitat quality assessment has become an integral part of RBA’s (Barbour and Stribling, 1993), normalizing the physical variability attributed to habitat, such as substrate size, riffle development, habitat heterogeneity, and embeddedness. Seasonal and ecoregion variation are also an important consideration in most biological sampling programs. Stream hydrology, although recognized by ecologists as integral in defining ecosystem structure and function (Clausen and Biggs, 1997; Gordon, 1992; Poff and Ward, 1989; Hynes, 1970) is only superficially considered in the interpretation of RBA scores. However, the amount of variability introduced by the hydrologic regime and the unique preceding hydrologic conditions may be significant, especially in urbanized streams where impervious cover has greatly altered natural flow characteristics. The biological response to hydrologic disturbance has been well documented in studies of relatively pristine systems (Clausen and Biggs, 1997; Dole-Olivier et al., 1997; Quinn and Hickey, 1994; Poff and Ward, 1989), as has the biological, physical and chemical response of streams to urbanization (Gordon et al., 1992; Baker, 1977; Poff et al., 1997; Britton et al., 1993; USEPA, 1997; Lenat and Crawford, 1994; Tikkanen et al., 1994; Pratt et al., 1981). However, the effects of hydrologic variability and antecedent hydrologic conditions on biological assessments used in monitoring programs are not well known. Urban streams in Austin, Texas present the typical hydrological problems encountered in densely developed areas all over the world. Impervious cover reduces baseflow by limiting the amount of infiltration in a watershed. Flow volumes and velocities in streams generally increase during storms due to the higher quantity of water that runs off impervious cover and into the stream channel. This creates a very unstable system that goes from destructive floods to total de-watering in very short time intervals (Fig. 1). The resulting biological communities are under constant stress and adjustment. Due to the short duration, high intensity nature of rain patterns in central Texas, hydrologic regimes of streams tend to be more variable and dramatic than in more temperate regions, where bioassessment protocols were developed (Fig. 2,3). Consequently, understanding the effects of hydrology and other factors on the biological communities of urban streams in this region is critical to the correct interpretation of bioassessment data. Figure 1. Hydrograph of mean daily discharge on Shoal Creek from 1993 1998, an urban drainage (12.2 sq. mi.) in Austin, TX. Vertical lines are storms followed by no flow. 0.01 0.1 1 10 10