Urban stream health: Macroinvertebrates and periphyton as indicators

Background/Question/Methods

Changes to stream ecosystems due to urbanization are known to degrade riparian ecosystems through multiple stressors including increased erosion and sedimentation, expansion of impervious surface leading to altered flow regimes, and degradation of riparian vegetation and habitat.  Ultimately, such degradation can inhibit ecosystem services including contaminant filtration and nutrient cycling. This study examined the effects of urbanization on the structure and function of riparian communities in several streams in the Salt Lake Valley watershed.  We attempted to characterize these changes using monthly monitoring in urban and non-urban stretches of the same streams.  Specifically, we measured macroinvertebrate biodiversity and community composition, and periphyton biomass (as measured by chlorophyll-a), since both are known indicators of riparian ecosystem health.  In addition, we monitored water quality parameters including temperature, dissolved oxygen, nitrate and ammonium concentrations.

Results/Conclusions

While the total number of individuals in the macroinvertebrate community was greater in urban reaches than in the non-urban reaches of the same streams, biodiversity was greater in non-urban reaches of the streams.  In addition, the proportion of the macroinvertebrate community that was sensitive to pollution, thereby an indicator of good water quality, was greater in non-urban reaches of the streams than in their urban counterparts.  Simultaneously, we found an increase in pollution tolerant species of macroinvertebrates in urban reaches compared to the non-urban reaches.  Preliminary data suggest that periphyton biomass was 35 times greater in urban reaches when compared to non-urban reaches of the same streams, as mean periphyton biomass in urban reaches was 455 ± 114mg/m², while in non-urban reaches it was 13 ± 6 mg/m². These findings were correlated with higher temperatures, lower dissolved oxygen, and higher levels of nitrate and ammonium in urban reaches compared to non-urban reaches of the same streams.  Overall, these findings point to urbanization as a potential source of ecosystem degradation in the Salt Lake Valley.  They suggest that long-term monitoring is warranted, along with an in-depth investigation into the ultimate mechanisms responsible for the degradation.