Taking the Pulse of a Stream

Written by: Chelsea Gray

How do you tell the health of a river? This is a question that scientist all over the world struggle to answer. A common method for determining stream health is the Stream Bioassessment.

Damselflies lay their eggs in the water. Damselfly nymphs live under the water and are an example of the type of benthic macro-invertebrate found in Virginia streams.

What is a “Stream Bioassment”?

Because you can’t ask a stream how it’s feeling, we use indirect measures of health for an ecosystem. One way we do this for a stream is by identifying what types of organisms live there. In particular, we look at benthic macro-invertebrates, or the bugs that live in the stream bed.

Because we now have decades of data on benthic macro-invertebrates, we know which are “sensitive” to pollutants and which are “tolerant” to pollutants. The number of sensitive versus tolerant macro-invertebrates found in a stream gives us an indirect idea of how polluted the stream is. For instance, if we find many water pennies, a organism we know are sensitive to pollution, we can infer that there aren’t many pollutants present in the stream.

How do you find macro-invertebrates?

George Mason Students prepare to use a kick net to find benthic macro-invertebrates. To get the bugs to come out of hiding, the students must first hold still for one minute before kicking.

Benthic macro-invertebrates, while visible to the naked eye, are still excellent at hiding from giant predators, such as humans. To find them, scientists use a kick net to catch any critters hiding among the rocks and leaves that cover the stream bed.

A kick net is as simple as it sounds: Scientists kick their feet to drive macro-invertebrates into the net. They take the organisms that they find back to the lab, where they carefully count and identify each one. This is one of those rare situations where an individual wants more work- The more organisms there are to identify, the more likely the stream is a healthy ecosystem.

Where do pollutants come from?

Many of the pollutants that make their way into streams come from runoff. Oil leaking from your car, pesticides and fertilizer from you lawn, and sediment from construction sites can all make their way to local streams when it rains.

Dr. Smith helps a student identify an organism caught her group’s kick net.

A strong ecosystem, however, will be able to prevent many of these pollutants from reaching the stream. One of the best defenses against pollutants are buffer zones, or area of vegetation that absorb pollutants.

Strong buffer zones also provide habitat for many adult macro-invertebrates, such as damselflies or dragonflies. In turn, organisms like damselflies provide food for fish and other small animals. The more benthic macro-invertebrates in a stream, the more likely there are fish and frogs as well.

Preparing students for jobs

Because streams are a common feature in many ecosystems, stream bioassessments like this are done all over the US, by both professional and citizen scientists. PEREC Faculty Dr. Cindy Smith and Dr. Chris Jones wanted to ensure that students in their Human Dimensions and Biomes class got real world experience testing local Fairfax County waters. In October, students visited both Accotink Creek and Cub Run, where they gathered benthic macro-invertebrates via kick net. Giving students experience with this type of data collections prepares students for a variety of jobs, whether it be for the government or volunteering in their local community.

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