Assemblage and Population-Level Responses of Stream Fish to Riparian Buffers at Multiple Spatial Scales JESSE R. FISCHER,* MICHAEL C. QUIST,SKYLER L. WIGEN,ANDREW J. SCHAEFER, TIMOTHY W. STEWART, AND THOMAS M. ISENHART Department of Natural Resource Ecology and Management, Iowa State University, 339 Science II, Ames, Iowa 50010, USA Abstract.—Riparian buffers can improve stream water and habitat quality by reducing non-point-source pollution (e.g., nutrients and sediment), increasing canopy cover and thereby reducing water temperature, and contributing allochthonous organic matter (e.g., leaf litter and woody debris). However, the influence of riparian buffers on biotic assemblages in streams is poorly understood, particularly in the Midwestern United States. In this study, we evaluated the effects of riparian buffers on instream habitat, fish assemblage structure, and population characteristics (i.e., the growth of two small-bodied species) in three streams in central Iowa. The streams were surveyed at two spatial scales; specifically, 41 reaches were sampled and 247 macrohabitat types (i.e., pool, riffle, and run) were subsampled in the summer of 2007. Fish assemblage structure data were summarized into separate data sets by the relative abundance of individual species (i.e., fish per minute of electrofishing) and guilds (e.g., trophic and spawning). Multivariate analysis of variance was used to evaluate the differences in instream habitat and fish assemblage structure between areas with and without riparian buffers. The results indicated little to no relationship between the presence of buffers and fish assemblage structure and instream habitat characteristics. Nonmetric multidimensional scaling (NMS) was used to evaluate the patterns of fish assemblage structure for the reaches and macrohabitat types. The NMS further illustrated the similarity in fish assemblages between buffered and unbuffered reaches. However, the growth of central stonerollers Campostoma anomalum was greatest in unbuffered reaches, while that of creek chub Semotilus atromaculatus was greatest in buffered reaches. Differences in food availability associated with riparian buffers (e.g., increased algal production and decreased terrestrial invertebrate contributions in unbuffered reaches) probably resulted in growth disparities. The results of this study suggest that while riparian buffers have minimal local effects on instream habitat and fish assemblage structure in Iowa streams, they influence instream features (e.g., food availability) that affect fish population dynamics. Stream ecosystem integrity has been drastically altered by human disturbances, such as large-scale land use changes, stream channelization, riparian vegetation removal, and destruction of wetlands. According to the National Water Quality Inventory, 44% of rivers and streams in the U.S.A. are considered impaired (EPA 2006). Factors associated with agricul- tural practices contribute to 27% of all impaired rivers and streams, specific causes of degradation being identified as sediment and nutrient inputs, and habitat alterations that include loss of riparian vegetation (EPA 2006). Land use changes in the Midwestern United States have been more dramatic than other regions of North America. For instance, anthropogenic distur- bances in the Great Plains have resulted in losses in area of native terrestrial vegetation as high as 99.9% in the states of Indiana, Illinois, and Iowa (Sampson and Knopf 1994). Replacement of native vegetation with row crops and pasture often has extensive deleterious effects on aquatic ecosystems. Specifically, agricultural practices near streams (i.e., row crops, livestock grazing) reduce streambank vegetation and increase the amount of sediment and nutrients entering streams (Kemp and Dodds 2001). Recognizing consequences of agriculture on water quality in streams and rivers, management agencies are increasingly using conserva- tion practices, such as riparian buffers, to reduce nutrient and sediment inputs to streams (Peterjohn and Correl 1984; Rabeni and Smale 1995; Naiman and De ´camps 1997). Riparian zones play an essential ecosystem role in the physical and biological structuring of streams (Gregory et al. 1991; Naiman and De ´camps 1997). As such, the use of riparian buffers to mediate the effects of disturbances on water quality is particularly important because riparian zones are the interface between terrestrial and aquatic ecosystems. Therefore, riparian buffers have been installed and managed most extensively in areas where increases in sedimentation, nutrients, and water temperatures negatively influence fish assemblages throughout North America. It has been demonstrated that vegetated riparian zones (i.e., * Corresponding author:fischer@iastate.edu Received March 10, 2009; accepted August 12, 2009 Published online December 1, 2009 185 Transactions of the American Fisheries Society 139:185–200, 2010 Ó Copyright by the American Fisheries Society 2009 DOI: 10.1577/T09-050.1 [Article]