Urban streams commonly have low abundances and poor diversity of many freshwater insects that fish and other aquatic animals eat. Even after people have restored these stream habitats, the waterways often remain biologically impoverished, with diminished numbers of fish. New research reported Nov. 8 at the annual meeting of the Society of Environmental Toxicology and Chemistry (SETAC) in Portland, Oregon, has identified a probable culprit: pollutants in stormwater runoff that flushes through the streams.
Scientists have demonstrated that increasing urbanization leads to declining stream health, but they have struggled to pin down the causes. Urban stormwater runoff is a leading suspect. The force of runoff water rushing into these streams can cause physical damage to the streams' habitats. In addition, stormwater carries pollutants from numerous sources including vehicle exhaust, industrial plants, homes, and construction sites.
Kate Macneale and her colleagues at the National Oceanic & Atmospheric Administration's Northwest Fisheries Science Center decided to focus on water quality's effects on the biological habitat of urban streams. They ran experiments to observe how runoff affected 35 types of insects that generally indicate good stream health and serve as valuable prey for juvenile salmon.
Alongside a creek in the Seattle area, the researchers set up a series of experimental channels, similar to rain gutters. They collected rocks that had been colonized by the insects from a relatively pristine watershed nearby and added them to the channels. Some channels received creek water that the scientists had filtered to remove most of its contaminants; the remaining channels carried unfiltered water. Analysis of the unfiltered creek water showed that it contained hydrocarbons, metals, and other pollutants.
The researchers placed fine-mesh nets at the channel ends to capture insects drifting downstream—a behavior that signals avoidance of some stressor. After a three-week period, they counted the numbers and type of insects that had drifted into the nets and those that had remained in the channels.
The first series of experiments, conducted during the autumn of 2007, showed no significant differences in insect survival between the filtered and unfiltered water, possibly because weak fall storms produced little runoff. However, in a second set of experiments during spring 2009, the researchers found that the overall abundance of insects remaining in the channels exposed to unfiltered water declined by about 26%. They also discovered that the species diversity in those channels had changed.
"Even relatively brief exposures to urban runoff can alter the diversity and abundance of stream communities," Macneale says. "We need to consider water quality in restoring fish habitat."
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