Stream restoration requires working the bugs into your project
January 16, 2024
Monitoring benthic macroinvertebrates—the little critters living in streams and rivers—is a powerful ecosystem-restoration tool
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There is nothing quite as picturesque as a stream or river scene, water flowing over rocks and riffles, thick green vegetation crowding the banks. But have you ever taken a closer look?
You know, waded on in, turned over some rocks, and taken a good look at what is living there? When I point this out to people, few realize that along the bottom of streams, creeks, and rivers is a community of hundreds to thousands of critters: benthic macroinvertebrates.
Bentho-whatsits? Let’s unpack that term. “Benthic” simply means “bottom,” so we are talking about things that live in the substrates of the stream. “Macro” refers to organisms we can see with the naked eye, typically about a half-millimeter or larger in size. “Invertebrates” are animals without a backbone, which includes aquatic insects, worms, snails, leeches, clams, crayfish, and lots of little crustaceans.
Sampling a river’s small aquatic critters, called benthic macroinvertebrates, can help connect the dots in how a stream’s ecosystem is recovering.
Why should we be interested in these benthic macroinvertebrates, BMI for short? Good question. Two words: fish food. I might be oversimplifying it, but BMI are the primary food source for fish. What is doubly important is that they are a key component in a stream’s food web. BMI essentially consume all the leaves and debris that make their way from the surrounding watershed to the stream, locking all that energy into their bodies, which becomes available to fish when they feed on these invertebrates.
A perfect tool for measuring ecosystem restoration
BMI serve as critical “middlemen” in the stream’s ecosystem. As such, I’ve seen over my career that the BMI community serves as the perfect tool for closely monitoring and assessing stream health during and after ecosystem restoration. They are better than fish or water quality.
How so? First, they are common to all aquatic habitats. BMI also have wide range of tolerances and sensitivity to impacts or disturbances, such as pollution, fine sediment, temperature, and even the amount of streamflow. They are less mobile than fish, often living in contact with the substrates within the same stream reach their entire life. This makes them better at indicating the stream’s health and condition at the site where they were collected. This is often within one year’s time.
In contrast, fish are very mobile, and if there is an impact to a stream, the fish can simply avoid it altogether. Additionally, fish lifespans are typically much longer than BMI, so the recovery of a fish population in a stream happens over a much longer span of time.
A closeup of a golden stonefly nymph. Stoneflies are an important food source for a wide range of stream animals.
In the case of Pacific salmon, it’s a delay of at least five years from when you first sampled that species in your stream. That’s how long it takes those fish to return. To further complicate things, external pressures outside of the stream (during migration times and ocean residence) greatly influence those returning salmon.
In short, BMI are a great option for sampling. I’ve sampled dozens of streams, and I know BMI are relatively small enough to be easily collected. Sampling itself is simple, inexpensive, and minimal in disturbance.
Beneficial for fish passage, dam removal projects
Sadly, it’s been my experience that the BMI community remains one of the most overlooked monitoring tools. It is too often left out of monitoring efforts for many of the types of in-water projects we conduct. I believe that stream restoration, fish passage projects, dam removals, and bridge construction projects could all benefit from using BMI sampling as part of the monitoring program.
For instance, when I sample a stream with restored fish passage, it can appear healthy and in good condition. But I’ve learned that if it doesn’t have a healthy BMI community, it won’t be able to support fish populations in the long run. A simple study design with sampling before and after construction, both at or immediately below the site and at an undisturbed site upstream, could easily describe how effective the restoration efforts were at improving the stream’s condition.
Sampling BMI communities as part of a project’s monitoring program is a simple, yet powerful, tool.
We don’t have to limit BMI surveys to restoration projects. I once worked on a BMI survey in the Boise River below the historic Barber Dam as part of its relicensing effort. Our team in California conducted BMI surveys for the Pacheco Reservoir Expansion Project in Santa Clara County that will improve water supply reliability, provide cooler water, and offer year-round habitat for listed steelhead by enlarging the reservoir. BMI surveys have many uses.
A laboratory sample of benthic macroinvertebrates from a Washington stream shows a variety of tiny organisms.
Connecting the dots for the sake of recovery
Many state and federal agencies incorporate BMI sampling into their own biomonitoring programs. I know that matching methodology with those agencies brings them onboard and shows our clients we know the current protocols. This further standardizes monitoring efforts, so we can compare results with other streams and watersheds helping to determine stream improvement and leverage past project successes as well as failures.
Sampling BMI communities as part of a project’s monitoring program is a simple, yet powerful, tool. I think when you include BMI samples—alongside water quality, fish surveying, and habitat surveying—it helps to “connect the dots” in terms of how a stream’s ecosystem is functioning, responding, and recovering.
When we get the full suite of information, we can describe what is happening in a stream. We also gain the ability to explain why it’s happening and expand our insight into other important steps to improve the waterway.