STREAM INSECTS AS BIOINDICATORS OF FINE SEDIMENT

Abstract

Fine inorganic sediment (≤ 2mm) is a major non-point source pollutant in streams. While some fine sediment in streams is natural, loads in human-impacted streams often exceed their capacity to flush these sediments during high flows. Regardless of the source, the negative effects of increased levels of fine sediment in streams are realized in all biotic components of stream ecosystems from microbes to fish and in functional components such as primary and secondary production and nutrient cycling. Bioindicators sensitive to these negative impacts would be a valuable tool for resource managers. We focus on aquatic insects and their usefulness as bioindicators of increased fine sediment in stream ecosystems. Aquatic biomonitoring typically is used in most stream monitoring protocols. One disadvantage of current applications of aquatic biomonitoring is that it does not allow one to discriminate among pollutants. To address this disadvantage we targeted a specific pollutant, fine inorganic sediment, and examined the relationship between fine inorganic sediment and aquatic insects. A biotic index to be used to detect and monitor changes in stream ecosystem health directly due to increases in fine inorganic sediments is necessary for resource managers as they work to maintain aquatic ecosystem biodiversity and productivity, as well as sustain economic growth. In this study, fine sediment and invertebrate data were analyzed from 562 stream segments from Idaho, Oregon, Washington, and Wyoming. From 661 invertebrate taxa a subset (n=83) of widely-occurring insects was used to develop the fine sediment bioassessment index (FSBI). We found that there are species- specific responses to the amount of fine sediment in the streambed. We also found that traditional metrics such as the ratio of Ephemeroptera (E), Plecoptera (P), Trichoptera (T) to Diptera (D) could not discriminate among streams with varying levels of fine sediment. To test the fine sediment index as a predictor of sediment levels, a subset of streams were scored with the FSBI using an independent data set collected by the Idaho Department of Environmental Quality from the eastern Snake River Basin/ High Desert ecoregion. Using the FSBI score for each stream the percent fine sediment in a the stream was predicted and then compared to the measured value (r 2