At the south end of Seattle’s densely populated Rainier Valley, Taylor Creek meanders its way through Dead Horse Canyon, passes between rows of houses, and enters Lake Washington. Neighbors often walk the creekside trail, which lies within Seattle’s Lakeridge Park. Many hope for a day when healthy runs of salmon will return to the urban stream.

Twice within the last three years, hikers on the trail were caught off guard by hard-working strangers toting baskets of rocks down and into Taylor Creek. A few curious observers were bold enough to query the rock-carrying crews, led by King County researcher Kate Macneale, who keeps searching for a simple answer.

“We could say, ‘We’re here to put bugs in the stream,’ but that doesn’t translate very well,” said Macneale, an environmental scientist. “We try to find ways to tell them that this is about improving the stream.”

While rocks are the most visible feature of this research project, the real focus is on the tiny aquatic creatures that hide among the rocks and sticks or attach themselves to hard surfaces. These are the organisms in a healthy stream that feed young salmon and support the entire food web.

Thanks to Seattle Public Utilities and scores of volunteers, Taylor Creek is making a comeback from a degraded condition. But the urban stream still lacks, for unknown reasons, some of the insects and other invertebrates usually seen in a healthy stream. So Macneale and her crews are engaged in “bug seeding,” an experimental approach of transplanting invertebrates back into Taylor Creek.

If the healthy bugs can grow and multiply, they will become prey for salmon, help rebuild the food web, and provide evidence that the restoration efforts are paying off. 

“There is a lot of love for this stream, as well as other urban streams,” Macneale said. “People really want a stream that supports salmon.”

Bugs can tell a story

For more than 50 years, aquatic biologists have been assessing the health of streams by measuring the diversity of benthic macroinvertebrates. These are the small, yet visible, assortments of insects, snails, worms and other organisms that live on rocks, sticks and organic debris in a stream. For Puget Sound streams, water-quality scores are frequently assigned by using the Benthic Index of Biotic Integrity, or B-IBI, which focuses on the diversity of macroinvertebrates common to our region.

Healthy streams tend to have more groups of invertebrates, including some that rarely survive in areas with significant human disturbance. Mayflies, stoneflies and caddisflies — all important to salmon — are among the most vulnerable, so their abundance generally means good water quality and a high B-IBI score. Healthy streams also tend to have smaller proportions of “tolerant” species, known to survive under poor conditions.

Macneale and other biologists monitoring water quality in King County became interested in Taylor Creek and several other urban streams undergoing restoration when they realized that the B-IBI scores weren’t rising as much as hoped, considering the habitat improvements.

Seattle, like most cities, developed over a period of time. For Taylor Creek, something killed off the healthy bugs during years of development when streams got less attention than they do today. Likely problems included sedimentation caused by erosion of the stream banks, pollution from stormwater runoff and sewer overflows, and spills of toxic chemicals. 

In most streams undergoing restoration, recolonization of the food web — from microorganisms to salmon — occurs naturally as species find their own way back to more hospitable conditions. Some may move downstream to restored areas from healthy waters upstream. Flying insects that begin their lives in the water may flutter over land from nearby streams to establish new populations.

In the case of Taylor Creek, Macneale wondered if the urban stream might be too isolated for some aquatic species to find their way back. After all, the stream is less than 3 miles long; its headwaters lie in an urban area; and it is miles from the nearest healthy stream.

“What we know about the life stages of most insects is that they don’t fly very far,” said Macneale, who studied the travels and dispersal of stoneflies for her doctorate degree in entomology at Cornell University.

Moving day involves rocks and debris

With grants totaling $100,000 from the Environmental Protection Agency’s National Estuary Program, Macneale and her crew designed a bug-seeding project, and they seeded four King County streams in the summer of 2018. In addition to Taylor Creek, they were Gold Creek near Woodinville, Miller Creek in Normandy Park, and a tributary of Yarrow Creek in Bellevue.

The work began by placing about 28 pounds of rocks and woody debris into plastic flow-through baskets — 73 baskets into the Cedar River and 73 into Webster Creek, a tributary of the Cedar River. The streams were chosen for their healthy conditions and high B-IBI scores. The rocks, which came out of those streams, remained in the baskets for six weeks to accumulate a variety of organisms. The waters contained no known pathogens or invasive species that could spread to the transplanted streams.

In the Northwest, some of the key invertebrate species missing from urban streams are those most sensitive to disturbance.

After the six weeks were up, the research team returned to the donor streams to collect the baskets of rocks. Each of the four seeded streams received 34 baskets —17 from each of the two donor streams. Five other baskets were selected randomly from each donor stream for laboratory analysis, which provided an estimate of the type and number of invertebrates transplanted. The researchers believe that 46,000 individuals were moved into each stream.

Based on long-term sampling, Macneale had a good idea which invertebrates — listed in groups called “taxa” — were already living in the four streams where the seeding took place. She estimated that they were adding 15 new mayfly taxa, 9 new stonefly taxa and 13 new caddisfly taxa to each of the streams.

A year of waiting followed before Macneale and her team conducted a full assessment of all four streams to determine which invertebrates had survived. It turns out that each stream had a few taxa that had not been seen there before, suggesting that they survived the transplant, but there was no evidence of burgeoning new populations.

Samples gathered from Taylor Creek, for example, showed evidence of a new caddisfly, a new beetle and a new midge after the first year. None of those three were found two years after seeding. But a different transplanted caddisfly showed up in three cutthroat trout whose stomachs were emptied as part of a separate diet study. The caddisfly, not found in Taylor Creek trout before the seeding project, were still being eaten by trout two years after the seeding.

Similar mixed results were also seen in the other streams involved in the project. In Gold and Yarrow creeks, two-year-old stoneflies were collected in 2019, probably survivors from the transplant, Macneale said, but then they were not found the following year.

“That tells me that the water quality was OK,” Macneale said, “but we don’t know if they mated and laid eggs. Maybe there were not enough of them, or maybe they were unable to survive as adults.”

Individual insects must search and find a mate for reproduction. Given the size of a stream, a major question still hangs over the project: How many transplanted individuals of a given species does it take to establish a new population?

In August of this year, Macneale and her team returned to Taylor Creek to seed the stream again in similar fashion by moving 45 baskets of rocks. All were from Webster Creek, which was found to be more similar to Taylor Creek than the larger Cedar River. The search for survivors will begin next summer.

Studies improve knowledge of stream ecology

Transplanting bugs from one stream to another remains an experimental effort, according to Sarah Morley, aquatic ecologist with NOAA’s Northwest Fisheries Science Center. Morley was involved in her own bug-seeding effort as part of a major restoration of Seattle’s Thornton Creek, which flows into Lake Washington north of Sand Point.

The Thornton Creek restoration, directed by Seattle Public Utilities, involved the reconstruction of two major floodplains. The work included the complete removal of the embedded and hardened stream channel, followed by the addition of gravel up to 8 feet deep. The project was designed specifically to increase the flow of water through the deep gravel underlying the stream — something seen in nature but generally ignored in stream-restoration efforts.

This hyporheic flow, as it is called, has been associated with increased biological productivity, greater oxygen levels and moderating effects on stream temperature. Spawning salmon often seek out the hyporheic zones, where subterranean flows reconnect with water in the stream.

At the time of placement, the imported gravel in the restored floodplains had no aquatic invertebrates, and the stream microorganisms that feed them were largely missing as well. Extensive research efforts were undertaken to measure changes in physical, chemical and biological processes before and after the restoration.

To assist colonization in one of the restored floodplains, the researchers first sterilized gravel of the type used in the new streambed. The gravel, placed in baskets made of corrugated plastic pipe, was then positioned into tributaries of the Cedar River to collect bugs and microbes. After several weeks, the baskets were moved to the restored Thornton Creek floodplain in the Kingfisher Natural Area near Northgate.

For the next three years, using specialized techniques, macroinvertebrates and microbes were sampled separately from the benthic zone on the stream bottom and from the hyporheic zone within the gravel that lies beneath.

In the hyporheic zone, the study found higher numbers of macroinvertebrates as well as more variety of taxa, as compared to unrestored sections of Thornton Creek. Microbial activity was greater in restored areas, and the makeup of the microscopic community was notably different. Changes also included significant decreases in chemical contaminants. In all, the placement of gravel to rebuild the hyporheic zone is considered a success.

On the other hand, restoration did not greatly improve the diversity of benthic invertebrates living on the stream bottom, as there was no increase in B-IBI scores. Nevertheless, four new benthic invertebrate taxa were found in the seeded area. Two of them — a mayfly and a nonbiting midge — had not been seen in Thornton Creek for more than a decade, suggesting that the new arrivals could be a result of the seeding project.

“We need more evidence that it (bug seeding) can work and how it will work,” Morley said. “We need to keep testing the idea of recolonization. I’m excited to see what Kate (Macneale) finds.”

Because Thornton Creek remains a highly impacted urban stream, it may not yet be the best place to test the effects of bug seeding, Morley said. Still, progress is being made.

“There is so much disturbance in Thornton Creek and so many factors involved,” she said. “Even though restoration projects improve the conditions — and we were able to show that — it is but one small patch in a large watershed.”

Just as aquatic macroinvertebrates are important to salmon, algae and diverse microbial communities are important to invertebrates, she said. A consideration of healthy microbes may need to be incorporated into future restoration strategies.

Scientists find hope among challenges

A few years ago, German ecologist Jonas Jourdan and 13 colleagues surveyed the scientific literature to find writeups on bug-seeding projects throughout the world. Of the 40 projects they found, most involved the transplant of single species for specific restoration purposes, and most were in Europe. Of those reported, about one-third were failures, mostly because of the complexity of invertebrate life cycles and remaining habitat problems, they concluded.  (The actual failure rate could be higher, because scientists are less likely to write about the failures.)

In the Northwest, some of the key invertebrate species missing from urban streams are those most sensitive to disturbance. As such, they cannot be easily grown in large numbers in a laboratory, which would be one way to increase the chances of transplant success, Macneale said. The approach used in the King County projects, including the collection of multiple species, offers the hope of restoring whatever links may be missing in the food web.

Restoration projects alone cannot guarantee that a stream will support a healthy population of invertebrates, or salmon, Macneale said. Unidentified pollution, for example, could be stifling the growth of stream bugs with consequences for young salmon.

 “I think of bug seeding as one tool in the restoration toolbox,” she said. “I definitely think it should be considered for urban streams where we know that there are no refuges to allow bugs to colonize themselves.”

Restoring urban streams remains a major challenge, Macneale noted. From a strictly financial standpoint, it might be better to invest in rural waterways, where damage is often less, and project costs are lower. But finances don’t tell the entire story.

“We’re talking about where people live,” she said. “These are the streams that people know and care about. There is so much good will and interest in having these streams restored. People just want to know that things are getting better.”