The Use of Mayflies, Stoneflies and Caddisflies as Indicators of Fine Sediment Pollution in Salmon-Bearing Streams of the Pacific Northwest

Abstract

Fine sediment (particles < 2 mm in diameter) deposition is a natural component of streams but in excess amounts it is harmful to aquatic biota. Fine sediment is the main cause of impairment in streams and rivers of the Pacific Northwest and it can lead to unsuitable spawning and rearing habitat for Endangered Species Act (ESA) listed Salmon and steelhead. From a regulatory perspective, fine sediment presents a challenge because it is difficult to measure in situ and to establish criteria that would indicate stream impairment. Consequently, there is considerable effort to develop biocriteria using stream macroinvertebrates that would help regulators determine if a stream is impaired by excessive fine sediment. Stream macroinvertebrates are widely used to monitor and assess stream degradation because they are affected by the physical, chemical and biological conditions in a stream. In particular, the stream insects known as mayflies (Ephemeroptera, E), stoneflies (Plecoptera, P) and caddisflies (Trichoptera, T) are commonly used as a metric referred to as EPT richness. EPT taxa are well known as pollutant intolerant insects and EPT richness in known to respond negatively to stream degradation. While the sensitivity of EPT to changing water chemistry, increased land use, and hydrologic alteration has been well established, the use of EPT as biocriteria to monitor fine sediment conditions in streams has not been fully evaluated. Studies on the relationship between EPT and fine sediment have shown mixed results, which is likely due to the challenges of quantifying fine sediment in streams. For example, many studies use reach-scale pebble-counts to quantify sediments and reach-scale macroinvertebrate samples to characterize their condition, but the processes that drive macroinvertebrate-sediment relationships primarily operate at the patch-scale. Another issue is the incomplete sediment gradient often observed in the randomized, reach-scale study designs frequently used to collect stream data for regulatory purposes. The purpose of my thesis is to address the use of stream macroinvertebrates as indicators of excessive fine sediment in the regulatory setting. Specifically, I evaluated which aspect of the macroinvertebrate community should be used as an indicator for fine sediment, explored methods for better characterizing benthic fine sediment at the patch-scale and conducted a spatially oriented study that contained a fine sediment gradient. I accomplished this by: 1) using Random Forest and generalized linear models (GLM) to explore relationships between environmental variables, macroinvertebrate metrics and fine sediment in the Pacific Northwest (PNW), 2) developing and validating a novel