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Benthic invertebrates range in size from those easily seen with the naked eye to those that cannot be spotted without the use of a microscope. Photo: Christopher Dunagan
Benthic invertebrates range in size from those easily seen with the naked eye to those that cannot be spotted without the use of a microscope. Photo: Christopher Dunagan

Are some streams in Puget Sound getting cleaner?

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Scientists are reporting some potentially good news about the health of Puget Sound’s streams. Ten years of data from 126 stream sites within King County have shown a slight improvement in water quality, according to the county’s Water and Land Resource Division.

The study examined the variety of insects and other invertebrates that were collected from stream sediments. Twenty-one percent of the streams showed an increase in sensitive stream bugs in 2019, earning them a higher score on what is known as the Benthic Index of Biotic Integrity, or B-IBI for short. B-IBI scores for 79% of the additional study sites remained stable, according to King County.

The index is based on the concept that the healthier a stream is, the more invertebrates it will support. Streams are given a score from 0 to 100 based on the number of invertebrates found in a sample, with higher scores given when especially sensitive invertebrates are found that might not survive in certain polluted conditions.

Not all the news was good as more than half the stream sites scored from “fair” to “very poor,” but scientists call the marginal improvement unexpected because it comes at a time when urban development is increasing and putting more strains on the environment.

“It was quite surprising when you consider the overall trend toward increasing urbanization,” says Kate Macneale, an environmental scientist at King County involved with the studies. Scientists were expecting to see fewer bugs due to pollution from stormwater runoff and other sources that find their way into streams, she said.

“We joke that nobody has spent more time trying to figure out what is wrong with their dataset,“ Macneale said.

Does this mean cleanup efforts are starting to show results?

“We don’t know for sure,” but the trends are welcome, says Macneale. An October 2018 fact sheet about the studies describes similar results last year and says scientists are “cautiously optimistic” about the findings.

You can read more about B-IBI on the Encyclopedia of Puget Sound. The Puget Sound Partnership has designated B-IBI as an indicator of stream health throughout Puget Sound.

— By Jeff Rice, Puget Sound Institute