USING BENTHIC MACRO INVERTEBRATES TO ASSESS WATER QUALITY IN 15 WATERSHEDS IN THE PACIFIC NORTHWEST, USA

Abstract

Many government entities have focused on methods to monitor the biological integrity of aquatic ecosystems to address impacts of non-point source pollution. The purpose of this is to determine stream macro invertebrate index (SMI) scores throughout Idaho and to relate these values to observed soil erosion rates and visual estimates of stream water quality. This study took place over a 15-year period. Based on benthic macro invertebrate sampling, water quality ranged from very good to very poor at the 124 sampling sites in 15 Idaho watersheds. Strong, significant relationships were observed between SMI water quality ratings and observed soil erosion rates. Sites with soil erosion rates of <2 mt/ha/yr generally had SMI water quality ratings of very good. Conversely, when soil erosion rates on adjacent landscapes exceeded 15 mt/ha/yr, SMI water quality scores were fair or worse. Strong significant relationships were observed between SMI water quality ratings and observed stream water quality in 13 of the 15 studied watersheds. In general, the land use practices in forestry, range, and agriculture adjacent to streams resulted in SMI water quality ratings of good, good, and poor, respectively. Based on the findings of this study, macro invertebrate sampling is considered the best technique for assessing stream quality and is often more economical than the comparative costs of chemical analysis.