Assemblage and Population-Level Responses of Stream Fish to Riparian Buffers at Multiple Spatial Scales

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.