Monitoring benthic macroinvertebrates—the little critters living in streams and rivers—is a powerful tool when trying to gauge healthy waterways
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 peered 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 that can be seen 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 stream in search of benthic macroinvertebrates, which are the primary food source for fish.
Why should we be interested in these benthic macroinvertebrates, BMI for short? Good question. Two words: fish food. That might be oversimplifying it, but BMI are the primary food source for _q_tweetable:The benthic macroinvertebrate community also serves as the perfect tool for closely monitoring and assessing streams─even better than fish or water quality._q_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.
Because of this critical importance as “the middlemen” in the stream’s ecosystem, the BMI community also serves as the perfect tool for closely monitoring and assessing streams─even 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 and are therefore better at indicating the stream’s health and condition at the site where they were collected, often within one year’s time.
Author Tim Nightengale checking flow velocity in a stream.
In contrast, fish can simply escape most disturbances altogether, and many salmon have life cycles of 5 to 7 years, meaning you must wait that long just to see if your efforts had any effect on that population that reared there five years previous (assuming harvest and other external factors are not influencing survival and recovery).
Best of all, BMI are relatively small enough to be easily collected, and the sampling process itself is simple, inexpensive, and minimal in disturbance.
Sadly, the BMI community also remains one of the most overlooked tools and is frequently left out of monitoring efforts for many of the types of in-water projects we conduct. Stream restorations, fish passage projects, dam removals, and bridge construction projects could all benefit from incorporating BMI sampling into the monitoring program.
For instance, a stream with restored fish passage can appear healthy and in good condition, but 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 upstream and downstream from the site, could easily describe how effective the restoration efforts were at improving the stream’s condition.
A stream sample in the lab.
As a bonus, many state and federal agencies incorporate BMI sampling into their own biomonitoring programs. Matching methodology with those agencies brings them on-board and shows our clients we are knowledgeable with the current protocols. This further standardizes monitoring efforts, so we can compare results with other streams and watersheds helping to determine stream improvement and project success.
Sampling BMI communities as part of a project’s monitoring program is a simple, yet powerful, tool available to us. The inclusion of BMI samples alongside water quality, fish surveying, and habitat surveying helps to “connect the dots” in terms of how a stream’s ecosystem is functioning, responding, and hopefully recovering. By getting the full suite of information, we can describe what is happening in a stream, gain the ability to explain why it’s happening, and gain insight to what else needs to be done.
About the AuthorMore Content by Tim Nightengale