Impacts of urbanization and aquifer recharge on Spokane river macroinvertebrates

Background/Question/Methods: This study addresses the impairment of ecological communities of the Spokane River as a result of pollution, and includes two components.  First, macroinvertebrate communities were compared from 6 reaches upstream and 6 reaches downstream of the Spokane urban core. This 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 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. 

Results/Conclusions: We found that Spokane river study reaches vary in characteristics indicative of ecosystem health and this is reflected in the communities of macroinvertebrates found in each reach.  Mean water temperatures clearly demarcate aquifer influence; upstream and downstream gaining reaches were 14.9 C and 14.8 C, respectively.  Mean water temperatures for upstream losing reaches were 27.7 C.  The most distinct macroinvertebrate differences were also found between ”gaining” reaches and “losing” reaches. All gaining reaches were characterized by an abundance of species representative of high water quality such as Antocha craneflies, Pteronarcys stoneflies and Calineuria stoneflies. Warmer “losing” reaches had higher densities of multivolting dipterans, such as Simuliidae and Chironomidae. The unique Spokane river macroinvertebrate community is dominated in all three reaches by heavy metal tolerant species. Metal tolerant Hydropsyche caddisflies, and Baetid mayflies were among the most abundant invertebrates in all study reaches, while metal intolerant Heptageniid mayflies were absent (though commonly found in local streams with otherwise similar fauna), suggesting that metal pollution from the Coeur d’ Alene Basin upstream is influencing Spokane River communities. 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. The direct correlation between stream health and the observable macroinvertebrate communities can be easily comprehended by the community and used to generate interest and encourage stewardship.