In the previous lesson, students investigated how conductivity and chloride can reveal the presence of road salt in streams. This warm-up activity shifts the focus from the chemistry of the water to the organisms that live in it. Students begin by comparing three aquatic ecosystems—freshwater, estuaries, and saltwater—to discover that different organisms are adapted to different levels of salinity. This provides the foundation for understanding why increasing salt concentrations can affect freshwater organisms and why macroinvertebrates are valuable indicators of stream health.

Freshwater, Estuaries, and Oceans

Distribute the Freshwater, Estuarine, and Saltwater Species fact sheet showing freshwater, estuarine, and marine ecosystems. Ask students to compare the organisms that live in each environment and the typical specific conductance of each type of water.

Guide students to consider:

• What differences do you notice among these three aquatic environments?
• Why do you think different organisms live in each one?
• Can some organisms survive in more than one environment?
• What might happen if a freshwater organism were placed in saltwater?

Explain that every aquatic organism has a range of salinity it can tolerate. As salt concentrations increase, some organisms become stressed or are no longer able to survive. This idea provides the foundation for understanding how road salt can affect freshwater ecosystems.

Interview with a Scientist

After exploring the fact sheet, students will hear from Dr. John Jackson, Senior Research Scientist at Stroud Water Research Center, who explains how increasing salt concentrations affect freshwater streams. Play the interview (9:25–15:45) and encourage students to listen for evidence that road salt can change not only the chemistry of a stream, but also the organisms that live there.

After the interview, discuss:

• Why might freshwater organisms be affected by increasing salinity?
• Would all organisms respond the same way to salt?
• If chemistry provides a snapshot of stream health, what might living organisms tell us?

What Are Aquatic Benthic Macroinvertebrates?

Introduce the term aquatic benthic macroinvertebrate by breaking it into its four parts:

• Aquatic – lives in water
• Benthic – lives on or near the bottom of a stream or river
• Macro – large enough to see without a microscope
• Invertebrate – an animal without a backbone

Most aquatic macroinvertebrates are immature insects that spend months or even years living in freshwater before becoming adults. Because they remain in the same stream for much of their lives, they experience changes in water quality over time. Macroinvertebrates are one group of organisms that are particularly sensitive to increases in salinity. While some species can tolerate elevated salt concentrations, others cannot. As a result, the types of macroinvertebrates living in a stream provide valuable evidence about its long-term health.

Macroinvertebrates are also an important part of the freshwater food web, providing food for fish, amphibians, birds, and other wildlife. When road salt or other pollutants reduce populations of sensitive macroinvertebrates, the effects can ripple throughout the ecosystem. In the next two activities, students will become stream ecologists, identifying aquatic macroinvertebrates and using them as biological indicators to evaluate the health of their local stream.

Students now shift from learning why macroinvertebrates are useful indicators of stream health to learning how scientists identify them. Using the Leaf Pack Simulation, students practice identifying common aquatic macroinvertebrates and calculating a Pollution Tolerance Index (PTI) before working with live organisms in the next activity.

Working individually or with a partner, have students visit the Leaf Pack Simulation and select Stream A. Begin with the Habitat tab and use the stream photograph to answer the habitat questions. Then select the Macroinvertebrates tab to complete the virtual investigation. Students will deploy a leaf pack, sort and identify the collected macroinvertebrates, and calculate the stream’s PTI score.

Encourage students to discuss how the types of macroinvertebrates they found influenced the PTI score. Explain that they will use these same identification skills in the next activity when they investigate macroinvertebrates collected from a local stream.

Students now apply the skills they practiced in the simulation to a real stream investigation. Using leaf packs collected from a local stream, they will identify aquatic macroinvertebrates and use a Pollution Tolerance Index (PTI) to evaluate the stream’s biological health. What looks like a simple bag of leaves is actually a tiny stream habitat—after just 3–4 weeks underwater, the leaf pack becomes colonized by a remarkable variety of organisms. This makes leaf packs an effective way for scientists to sample stream life and allows students to connect biological evidence with the chemical evidence from the previous lesson.

Making a Prediction

Before students begin, ask them to predict the health of the stream. Based on what they have learned so far, do they expect the stream to be Poor, Fair, Good, or Excellent? Encourage students to explain the evidence behind their prediction. Remind them that pollution-sensitive macroinvertebrates contribute more points to the PTI than pollution-tolerant species, so streams with a greater diversity of sensitive organisms generally receive higher scores.

Investigating the Leaf Pack

Working in small groups, provide each group with a leaf pack, a shallow sorting tub, an ice cube tray, stream water, hand lenses, paintbrushes, spoons, a Macroinvertebrate Identification Dichotomous Key, and a Biotic Index Data Sheet. Add stream water to both the sorting tub and the ice cube tray before students begin. Place a small handful of leaves into the sorting tub and remind students to keep both the leaves and macroinvertebrates submerged throughout the investigation.

Students should carefully examine the leaves and gently remove each macroinvertebrate using a paintbrush or spoon. As organisms are found, place them into separate compartments of the ice cube tray, grouping together organisms that appear to be the same type. White ice cube trays make the organisms much easier to see and compare.

Once students have sorted the organisms, they should use the Dichotomous Key to identify each type and record the number of each on the Biotic Index Data Sheet. After all groups have completed their identifications, combine the class data to calculate the stream’s overall PTI and determine whether the stream’s biological health is Excellent, Good, Fair, or Poor.

Making Sense of the Evidence

Bring the class together to discuss the results:

• Did the biological evidence support your original prediction?
• How did the biological assessment compare with the conductivity and chloride data collected in the previous lesson?
• What does combining chemical and biological evidence tell us that either one alone cannot?

A Scientist’s Challenge

Present students with the following scenario:

Suppose the chemistry data suggested the stream was healthy, but the macroinvertebrates suggested it was unhealthy. What questions would you investigate next?

Allow students time to discuss possible explanations. They may suggest that water quality has changed over time, that pollution occurred before sampling, that another pollutant besides road salt is affecting the stream, or that additional data should be collected. Emphasize that scientists rarely rely on a single measurement when evaluating ecosystem health—they combine multiple lines of evidence, ask new questions, and collect additional data before drawing conclusions.