Ask students the following questions:
- What did you find in the pools and rifles during your field trip?
- What was different about the pools and riffles (abiotic and biotic differences)
- In what way can we demonstrate these differences? (Graphing!)
Macroinvertebrate count data should be compiled on a spreadsheet or chart that can be passed out to students.Students will use the data to determine if we can answer our field trip question: “Do different macroinvertebrates live in pools versus riffles?” and “What are the characteristics of these microhabitats that might affect what lives there?”. For both the pool and riffle students should have the following totals:
- number of individuals per taxa (mayfly, stonefly, caddisfly)
- number of different taxa found
- total number of individuals
- Students should create visual summaries of the class data by graphing. These graphs can help us answer the questions posed above. Students can determine what graph to use by using the Graph Choice Chart (from our friends at Acadia Learning for Participatory Science.)
- Hint: Bar graphs and frequency plots (dot plots or box plots) are best for comparing the data between riffles and pools, or between sampling dates if other schools data is available.
- Hint: The X axis should be names of taxonomic groups, and Y axis should be number of individuals. Students (individually, or with a partner) should make two graphs- one Riffle graph and one Pool graph. They can use averages from riffles and pools, or counts of taxa from each of the three leaf packs in each microhabitat (a more challenging option, but more accurate if your data has a lot of zeroes).
- Once students have created a graphical summary of the data, ask them to share their observations. Prompting questions: A) Do the types or number of taxonomic groups differ between riffles and pools? How? B) Does the total number of individuals from one taxonomic group (e.g. mayfly) seem higher in one microhabitat than another? C) Do mayfly, caddisfly, stonefly, etc, seem evenly distributed amongst the two habitats?
- See Extend section for terms to help you define what these numbers mean.
Exploring Environmental Conditions: Habitats are the natural environment in which an organism lives. The distinguishing abiotic conditions of riffles, pools and runs result in specialized environments that are known as microhabitats. The abiotic conditions (dissolved oxygen, turbidity, light and temperature) of these microhabitats can influence which aquatic species can survive and reproduce at that given location and time.
- Dissolved oxygen is a source of oxygen for many living organisms and chemical processes. Dissolved oxygen is a measure of how much oxygen is mixed in with molecules of water. Wind, waves and bubbling of riffles can increase the amount of oxygen that enters the stream from surrounding air. Water with high and relatively stable levels of DO is typically considered to be a healthy ecosystem because it can support greater biodiversity.
- Stream flow is greatest in riffles, moderate in runs and slowest in pools. If water flows too quickly some organisms cannot maintain their hold on rocks and vegetation. Current that is too slow results in stagnant water with low aeration.
- Temperature varies depending upon climate, light penetration through surrounding vegetation and groundwater input sources. Stream temperature can affect species composition through biological processes (metabolic rates) and ecosystem processes (leaf breakdown, nutrient uptake). Warmer water holds less oxygen, which means a decrease in dissolved oxygen levels. Colder water temperatures are favored by many fish and macroinvertebrates.
- The conditions and resources provided by the microhabitat can determine which organisms are present in the given community. It is also important to consider how biotic interactions may influence community biodiversity. These interactions include competition for a food source, predation, or mutualism.
- The depths of pools provide refuge during dry conditions, protection from predators or shelter. The water flows a little slower which allows the organic debris to settle out and provides a food source. Another advantage is that you don't have to relocate to another area if the stream level starts to lower.
- Riffles are at once a food source, a shelter from predators, and a conveyor belt that brings food to the animals. Many species of invertebrates reproduce or grow to maturity in riffles. Riffles also hold larger prey items and only animals that cling very well, such as net-winged midges, caddisflies, stoneflies, some mayflies, dace, and sculpins can spend much time here, and plant life is restricted to diatoms and small algae. Riffles are a good place for mayflies, stoneflies, and caddisflies to live because the riffles offer plenty of cobble gravel to hide in.
- Runs are preferred by fishes that are too small to compete in ponds, such as minnows.
- Lead students in a discussion to answer the second part of the guiding question. “What are the characteristics of these microhabitats that might affect what lives there?"
- Refer back to the Venn diagram you created at the end of the field trip lesson and ask students discuss these factors in reference to the pools and riffles. Help students to think specifically about how abiotic factors are interacting with biota.
- Compile the data you collected during the field trip on environmental factors. If you observed that dissolved oxygen (DO) was lower in the pools than the riffles, extend this observation and answer the questions, “Why is the DO lower?” What affect is lower DO having on the macroinvertebrate community? Why can specific macroinvertebrates live in low DO environments and others cannot? What adaptations help them do this? Can the class agree on a conclusion to their experiment? Challenge students to think of ways to make the experiment better, how they might do things differently, and other questions that they might be able to answer using a leaf pack experiment.
Part 1. Assess biodiversity in riffle and pool microhabitats. The level of biodiversity in an ecosystem can be determined using the following values.
Species richness – the number of species in a community
Species evenness – the relative abundance of individuals within each species
While knowing the number of different species in a community is good to know, it is also important to know the abundance of individuals in each species. For example both your riffle and pool samples may have 10 total individuals and five different taxa (A, B, C, D, E). In the diagram below the pool is dominated by one of the five taxa, while the riffle has the same five species but in equal proportions. In this case the riffle would have higher diversity.
Assess the biodiversity of the habitats your class explored by exploring the following:
- Do the taxa differ between the two microhabitats? Have the students hypothesize why there might be differences.
- Look at the total number of individuals in the pools vs. the riffles. Did one microhabitat have more than the other? Why or why not?
- Look at the specific numbers of individuals in each taxon and the numbers of different taxa. How do these differ between microhabitats? Why?
- Calculate a biotic index
- Macroinvertebrates and Water Quality – macroinvertebrate assemblages can give us insight into the water quality of a stream. State and federal organizations use macroinvertebrates data to help them to determine the levels of pollution in streams. You can find more information about this at Environmental Protection Agency website.
- Based on the presence of certain taxa of macroinvertebrates (specifically Mayflies, Stoneflies and Caddis flies) a biotic index can be calculated. This index gives the stream a score and ranks it from poor to excellent condition. Using the data collected during this lesson has students calculate the Biotic Index for their stream.
- The Stroud Water Research Center’s Leak Pack Manual provides worksheets and instructions on how to calculate the biotic index using the data collected with this lesson.
Part 2. Compare your total number of Mayflies to data collected by other schools in 2013 and 2014. Are the numbers similar? Why or why not? Think about what environmental factors might influence the number of mayflies from year to year.