Developing Linkages between Biological Impairment and Stream Siltation: A Case Study in the Northern Great Plains Ecoregion

Loss of biological integrity in streams due to excessive suspended sediment is a major cause of water quality impairment in the United States today. States monitor and assess streams for biotic integrity, and list streams as "impaired" according to Section 303(d) of the Clean Water Act for those that do not meet biological criteria. Biological criteria are set by states based on benthic macroinvertebrates or fish community metrics, developed as indices of biotic integrity (IBI) per Level IV ecoregions. In many cases, causes for biological impairment are not clear, and commonly states list causes as "siltation" and/or habitat alteration without strong evidence. A critical need exists to improve analytical tools to link loss of biological integrity with "siltation", and to estimate total maximum daily loads (TMDLs) for these impaired streams. This study tested an idea using autecological fish data and USGS suspended sediment concentration (SSC) data to better link causes of biological impairment to SSC metrics. The study was located in Northern Great Plains Level III Ecoregion, in which 77 USGS gauging stations with flow and suspended sediment data were co-located with fish data from federal and state agencies. Autecological data was summarized by fish species found in this ecoregion, including attributes on mesohabitat preferences, water quality tolerance, trophic structure, and feeding, diet, and spawning behaviors. An autecological data matrix was created using the species autecological information and site fish presence/ absence data, forming a matrix that included, per site, the number of occurrences an autecological attribute occurred for fish species present. Ecological analysis found significant differences for several autecological attributes between stable and unstable channels. Fish located at unstable sites tended to: 1) prefer open-water mesohabitat, 2) be dominated by herbivores with algae diet, and 3) exhibit non-guarding spawning behavior. Use of fish autecological data and the protocols applied in this study were found useful to improve "strength of evidence" approaches to link biological impairment to excessive SSC.