Mining, aquifer recharge, and urbanization impact Spokane river macroinvertebrates

Background/Question/Methods

This study addresses the impairment of ecological communities of the Spokane River as a result of pollution. Macroinvertebrate communities were compared upstream and downstream of the City of Spokane. Our bioassessment design determines the effects of aquifer recharge as well as urban discharge. Upstream reaches include 3 “gaining” reaches with upwelling from the Spokane Aquifer to the river, and 3 “losing” reaches where water drains from the river to the aquifer; all downstream reaches (6) are “gaining”. We compared macroinvertebrate communities between upstream reaches to resolve aquifer effects and between upstream and downstream reaches to determine urban effects. We hypothesized that input from the Spokane Aquifer may mitigate the effects of urban pollution on the river’s macroinvertebrate community and that the concentrated pollution effects of the urban core of the city of Spokane will negatively affect downstream communities. The effect of mining pollution on the macroinvertebrate community was assessed by determining metal concentrations in invertebrate tissue at each of the 12 sites. Concentrations in invertebrate tissue were hypothesized to decrease with distance downstream, similar to sediment concentrations. 

Results/Conclusions

Total invertebrate diversity of the Spokane River is strongly limited by metal pollution in all study reaches. This indicates metal pollution from past mining in the Coeur d’ Alene Basin, the source of the Spokane River, is continuing to influence invertebrate communities. Invertebrate communities were dominated by Baetidae, Chironomidae, Simuliidae, and Hydropsychidae.  EPT Richness (an indicator of metal pollution) was significantly lower in the most upstream reach (ANOVA, F=4.2, p < 0.05, n =12), where sediments are most polluted with heavy metals.  Metal-intolerant taxa such as Heptageniid mayflies were absent from all reaches, with a single Baetis species the only mayfly collected. Analysis of invertebrate tissue metal concentrations will compliment these data. Aquifer discharge also has a clear effect on macroinvertebrate communities.  Gaining reaches were characterized by species representative of high water quality such as Antocha craneflies, Pteronarcys stoneflies and Calineuria stoneflies. Warmer “losing” reaches had higher densities of multivoltine dipterans, such as Simuliidae and Chironomidae. The results of our study demonstrate that macroinvertebrates can be a useful indicator of ecosystem health in the Spokane River and we recommend an annual monitoring program to document changes in anthropogenic impacts.