Print PageCow Creek (Adams County, WA) has been degraded by livestock grazing for over 100 years, but is currently the subject of a riparian restoration project by the Washington Department of Ecology. Fencing has been installed to remove cattle from the riparian corridor, but land use practices in the watershed have remained stable. We have assessed the ecosystem health of Cow Creek using two independent techniques: analysis of benthic macroinvertebrate communities and measurement of nitrogen uptake rates using short-term additions. Characterization of benthic macroinvertebrate communities is a well-established method for monitoring stream water quality, but provides little information on ecosystem function. In contrast, measurements of nutrient uptake provide information on how efficiently the stream uses and recycles nutrients. The study includes a control site, which has not been grazed for over 10 years, 2 impacted sites, and 2 reference sites on Crab Creek. We collected 10 macroinvertebrate samples from each site during the summer of 2008 and again in summer 2009, and identified at least 300 invertebrates from each sample to the lowest practical taxonomic level (usually genus). Standard macroinvertebrate metrics were applied to evaluate water quality. We measured Nuptake in late summer 2009 using a whole-stream addition of ammonium at three concentrations, with rhodamine as a conservative tracer. Uptake lengths were calculated from the decline in ammonium concentrations with distance downstream.
Uptake lengths ranged from 56 m for Cow Creek below Cow Lake (impacted) to 2800 m for Crab Creek- Hobb Ranch site (reference). Uptake lengths were shorter in streams with lower discharge, and lower N/P ratios. There was no clear relationship between uptake length and our prior expectations of ecosystem health. Uptake velocity, which is thought to provide a more reliable index of nutrient demand relative to supply, was highest at one of the impacted sites with recent cattle grazing (Cow Creek, Benge). Macroinvertebrate communities were more diverse and had lower pollution tolerance values in Crab Creek compared to Cow Creek, but did not indicate higher water quality for the restored Cow Creek site. Current macroinvertebrate community structure does not suggest that riparian cattle exclusion has been successful in improving water quality of Cow Creek to date. Local riparian influences may be overshadowed by historical impacts and ongoing land use in the surrounding watershed. We also suggest that simple measurements of ammonium uptake did not provide a reliable indicator of ecosystem health for our study streams.
