A benthic macroinvertebrate survey of Secret Ravine : the effects of urbanization on species diversity and abundance

The population in Placer County, California, is growing four times faster than the state of California. With the increase in population comes a large increase in impervious surfaces such as residential developments, strip malls, roads, and a probable decline in local stream water quality. To test whether the recent developments have impacted a local stream, we compared macroinvertebrate populations in an undeveloped (upstream) and a developed (downstream) reach of Secret Ravine. We sampled macroinvertebrates with a Surber sampler, following the EPA Rapid Bioassessment Protocols. The mean number of 55 organisms per sample downstream was significantly higher (p=0.02) than the mean number of 23 organisms per sample upstream. Although there was not any significant difference between the mean %EPT (pollution sensitive organisms) at the upstream and downstream sites, there was a significant difference between moderately sensitive (p=0.01) and tolerant (p=0.01) organisms. The percent moderately sensitive organisms was 32% upstream and 6.8% downstream. The percent tolerant organisms was 52% downstream and 17% upstream. Further indication that the downstream site was impacted by development was the abundance of filamentous algae that indicate a eutrophic (nutrient-rich) stream. Another difference between the two sites was the lack of red Chironomid (midge) larvae upstream, compared to 49% of the downstream organisms as midge larvae. Midge larvae, which tolerate oxygen as low as 20% of saturation, indicated the sediment under the algae and pebbles was anoxic downstream. In addition, the upstream community contained 22% dragonfly larvae, which require high levels of oxygen, while the downstream site was only 4% dragonfly larvae. The abundance of pollution tolerant organisms and filamentous algae indicates the downstream site is receiving nutrient-rich urban runoff, but contained little or no toxins. Further studies should focus on measuring temperature, dissolved oxygen, and nutrient content of the upstream and downstream runoff to determine the extent of eutrophication due to urbanization of the watershed