Susquehanna University student tests stream quality by studying macroinvertebrates | Education


Reprinted from Middle Susquehanna Riverkeeper

During the last school year, I collected and identified macroinvertebrates a lot. Macroinvertebrates are small creatures that live in ponds and streams. Just pick up any rock in a flowing stream and many different species of macroinvertebrates can be found underneath, as long as the water is clean.

Macros can be found under rocks or by hitting the substrate at the bottom of a river or stream and letting the debris flow into a trickle. Some examples of macroinvertebrates are stone flies, dragonflies, mayflies, and snails. In a few of my ecology classes, we went to various streams near the Susquehanna University campus in Selinsgrove to collect and study macroinvertebrates.

In my Community and Ecosystem Ecology course in fall 2021, we took nets into streams and hit the substrate at the bottom. The nets were placed against the current, so the macros drifted straight into the nets. We went to four different streams where macros could be found in the rapids, two in a pasture and two in a forest.

After collection, we returned to the lab to identify the macros we had captured with microscopes and a dichotomous key. Another useful source that we have used to identify macroinvertebrates is This website contains microscope photographs of many types of common macroinvertebrates, and the photos are interactive so they can be viewed from different angles.

After collecting macros for four weeks, we collected a total of 4 stoneflies, 15 beetles, 29 mayflies and over 100 caddisflies. Plus many others like crayfish and snails. After counting the amount of macros for each sample, we found that grazing streams had slightly higher biodiversity than forest streams. This was different from our hypothesis. We had thought that there would be more macroinvertebrates in forest streams because forests tend to have higher biodiversity.

In another course I recently took in the spring of 2022, we also collected macroinvertebrates. During class time, each person waded through a stream with nets and collected macroinvertebrates for approximately 30-45 minutes by kicking the substrate, as well as picking up rocks. Then all the macros were placed in a bucket, identified and counted. We ended up with over 100 stoneflies and nearly 200 mayflies!

There are a few characteristics of macroinvertebrates that can help tell them apart without using a key or microscope. Mayfly larvae usually have three tails, while mayfly larvae have two. Additionally, mayflies have one claw on each foot, while stoneflies have two.

Caddisfly larvae are usually very segmented and have numerous tails that look almost like hair coming out of one end. An interesting fact about caddisfly larvae is that they build cases to hide in. These cases are made from debris at the bottom of a stream, such as tiny stones, leaves, or plants.

According to the EPA, macroinvertebrates are a good indicator of the health of a waterway. “Macroinvertebrates have the ability to integrate the effects of the stressors to which they are exposed, in combination and over time.” Most are very sensitive to pollution, so they cannot survive in a watercourse that has been contaminated. “Water bodies in healthy biological conditions support a wide variety and high number of macroinvertebrate taxa, many of which are intolerant of pollution.”

Finding an abundance of macros in a stream is a good indicator that the water is clean and healthy for the organisms that live there. Mayflies, stoneflies, and caddisflies are very susceptible, so finding these macros is a good sign.

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