Use of macroinvertebrates to assess low-level anthropogenic impacts on the Little Luckiamute River, western Oregon, U.S.A

Background/Question/Methods Oregon's rivers have been affected by a range of environmental stressors, including logging, agriculture, urbanization and recreation. While researchers often focus on riverine systems with the most intensive anthropogenic impacts, studies of rivers with low-level impacts are important for determining the intensity of human activity that can be locally sustained without compromising ecosystem function. Early detection of environmental degradation may be a key to maintaining healthy rivers in moderately populated regions. In this five-year study, we (the author and students in her “General Ecology” course) compared macroinvertebrate communities at two sites upstream and one site downstream of multiple, low-level anthropogenic impacts. These included municipal wastes from Falls City (population ~1100), recreational use, and small-scale agriculture. All sites were downstream of logging. Each October, 2004-2008, we collected benthic macroinvertebrates with D-ring nets (500-micron mesh) from multiple riffles per site and keyed them initially to family. We also assessed several physical and chemical parameters. Since student researchers were involved at all stages, I conducted systematic quality control in the field and lab to insure the integrity and accuracy of the data, and further keyed the specimens to genus/species. Results/Conclusions Preliminary (family level) calculations of benthic and diversity indices showed no clear trends among sites or years. The benthic index of biological integrity bordered between “no” and “moderate” impairment for all sites in all years. The Shannon diversity values ranged, with one exception, between 1.9 and 2.2. Despite similar index values, three relatively pollution-sensitive families routinely found at the upstream sites, including Rhyacophilidae and Glossosomatidae (Order Trichoptera) and Ephemerellidae (Order Ephemeroptera) were uncommon at the downstream site and declined over time. In addition, the relatively pollution-tolerant Family Caenidae (Order Ephemeroptera) appeared for the first time in 2007 at the downstream site only. These data suggest that the downstream site may be adversely affected by anthropogenic factors; however, these patterns may be caused by other factors including subtle differences in substrate composition as well as position within the stream continuum. Also, quality control improved over the course of the study, but this would have enhanced the discovery and identification of the sensitive families in later years. Despite uncertainty about causal factors, these initial results suggest that analyses based on pollution tolerance may better detect differences in macroinvertebrate communities than more generalized indices.