Preparation: Aerial photos are a great way for students to compare land use types. Local extension offices or a university GIS department may have maps you can use, although you can also print aerial photos directly from a web application like Google Maps. Depending on the level of your students, you may want to identify the test watersheds ahead of time. Determining watershed boundaries is easy to do using contour lines, which are shown in the terrain feature within Google Maps (the online version, not Google Earth, which does not have contour lines). Google Earth has a tremendous 3D view of terrain, which students can also use to delineate watershed boundaries. Once the watersheds are identified, print them out for students to calculate the different types of land use.
Engage: Ask students to identify the major land use type in their neighborhood. How does land use change when they drive in different directions? Is land use the same everywhere?
Provide students with the historic and current aerial photos, and have them quantify the land use types according to “Step 1” in the worksheet. Students will count the number of squares that are in different land use types and then be able to discuss the change over time.
Explore: Once students understand how to calculate percent land use, they can begin working on their experimental set-up.
1) Students identify test watersheds A & B (unless you do this beforehand).
2) Students determine % land cover of their test watersheds.
3) Students make a prediction, based on the graph shown below in the Explain section, about the results of a watershed comparison study.
4) Students collect macroinvertebrates. There are a variety of methods for collecting these organisms, which can be found in the accompanying document produced by Hudson Basin River Watch. Alternatively, you can set out leaf packs in the different watersheds and collect them back in 2-3 weeks. For leaf pack methods, visit the Stroud Center’s Leaf Pack website: www.stroudcenter.org/lpn/index.htm . The Leaf Pack Network has a large range of resources available for use.
5) Students compile their data and identify the numbers of groups that they found in the two different watersheds. Students calculate “species” evenness and richness, although what they’re calculating is actually group evenness and richness, since this lesson does not identify to species level.
6) Students calculate the % EPT abundance, which is the numbers of mayflies, stoneflies, and caddisflies in each sample.
7) Students graph class EPT data in relation to % impervious surface. If you have different watersheds for each student group, you will see a better trend than if the groups all did the same watersheds.
8) Students compare their data with data from other scientific sources.
Explain: After you return to the classroom, discuss student findings. What did students notice? Ask students to think about the connections between the organisms that live in/near the aquatic ecosystem with the land use in the ecosystem’s watershed.
Urbanization can be a problem for various reasons, since impervious surfaces change the volume and the timing of runoff, and there may be contaminants in the water. In urban areas surface runoff carries pollutants from substances that have leaked or spilled onto the ground, such as oil or salt. Other pollutants such as nitrogen or phosphorus also accumulate in waterways. If you are interested in this topic, have students complete the “Land Use and Water Quality” lesson, which includes data on the increasing amount of nitrogen in suburban waterways. In addition to contamination, the amount of runoff increases with urbanization. In many cities and towns, rainwater goes into a sewage treatment plant along with sewage. When there is a heavy storm, however, and the sewage system becomes overwhelmed, it cannot treat the water, and it often releases all the water into the nearest water body. This is called “combined sewage overflow” (CSO). For more information on CSOs, use the “History of Wastewater” reading.
Streams and rivers across the country have been artificially channeled, straightened, or otherwise altered. Construction or poor land planning causes excess sediment to wash into streams and rivers, causing them to fill up prematurely, which adds to the flood threat. Streams normally flood, so trying to stop them from flooding in one place can cause ecological damage as well as increase flood damage in another. Urbanized streams that receive a large amount of water in a short amount of time during a storm are called “flashy” streams.
Macroinvertebrates are an important indicator of the health of an aquatic ecosystem. Immature insects such as stoneflies, mayflies, and water pennies (a type of beetle larvae) require a high amount of dissolved oxygen (DO), while aquatic worms, leeches and pond snails can survive in water with low DO. Oxygen-loving species like mayflies and stoneflies “indicator species” because they provide important clues about the water they are living in. If you only find animals like leeches, snails, and aquatic worms, then you know that there is a problem with water quality, and you should do additional studies to determine the cause.
One of the most useful indicators is the diversity or numbers of kinds of organisms. If you find only one or two kinds of animals, no matter what kind they are, you should perform other water quality tests to determine what might be wrong with your aquatic ecosystem. Sometimes, low diversity can indicate a pollution problem or other habitat change that is affecting the ecosystem.
A number of factors besides imperviousness can influence the diversity and density of macroinvertebrates present in an aquatic ecosystem. Seasons, life cycles, types of substrate, food sources, water velocity, and sampling techniques can all affect the diversity in your sample. For example, if you are testing the water in the spring, you might find fewer animals after a flood or heavy rain.
It is also important to know the animals’ life cycles. Many larvae emerge as adults in late spring and are present only as eggs during other parts of the year. The substrate on the river bottom can affect your results as well. A rocky bottom provides more habitat than a silty or muddy bottom. You should also take into consideration the surrounding habitat: a forest often provides more food (in the form of plant material) than a meadow. Finally, you need to decide what kind of sampling technique you are going to use. A screen or net that is too large will cause you to miss some animals, while inappropriate equipment use means you won’t collect a good sample of all the animals living in the ecosystem.
Extend: Students can create a presentation of their research for community members or another audience within the school, and discuss ways of improving water quality through land use change or specific mitigation strategies (pervious asphalt, rain gardens, riparian zones, etc).
Evaluate: Students should be able to explain how their diversity index calculation relates to water quality and land use.