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.”