Lesson 4: Carbon Pools and Fluxes in Changing Ecosystems

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Overview

NOTE: This lesson introduces a quantitative model of carbon pools and fluxes, then uses that model to explain how ecosystems change over time. This is challenging content that addresses NGSS High School Performance Expectations. You may want to skip it with middle school classes.

In Lesson 2, students identified the pattern of organic matter distribution (the organic matter pyramid) in a meadow ecosystem. In Lesson 3, they explained why that pattern exists by tracing matter and energy and connecting scales. In Lesson 4, students explain changes in ecosystems by keeping track of carbon pools and carbon fluxes. Carbon fluxes are the rates (mass/time) at which carbon transforming processes occur.

Guiding Question

How and why do carbon pools in ecosystems change over time?

Activities in this Lesson

Activity 4.1: Tiny Pool and Flux Game (30 min)
Activity 4.2: Carbon Pools and Constant Flux Simulation (30 min)
Activity 4.3: How Fluxes Change and Photosynthesis Limits (40 min)
Activity 4.4: Seasonal Changes and Ecosystem Disturbances (40 min)

Unit Map

Ecosystems Unit Map

Target Student Performance

Lesson 4 – Carbon Pools and Fluxes in Changing Ecosystems (students as explainers)
Activity	Target Performance
Activity 4.1: Tiny Pool and Flux Game	Students describe the relationship between pools and fluxes in a physical model: changes in pool sizes depend on balance among fluxes.
Activity 4.2: Carbon Pools and Constant Flux Simulation	Students describe the relationship between pools and fluxes in an online computer model: changes in pool sizes depend on balance among fluxes.
Activity 4.3: How Fluxes Change and Photosynthesis Limits	Students use an online computer model to describe how changes in carbon pools over time depend on the maximum possible rate of photosynthesis in an ecosystem.
Activity 4.4: Seasonal Changes and Ecosystem Disturbances	Students use an online computer model to describe how seasons and disturbances affect an ecosystem.

NGSS Performance Expectations

High School

Chemical Reactions. HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
Ecosystems: Interactions, Energy, and Dynamics. HS-LS2-1. Use mathematical and or computational representations to support explanations of factors that affect carrying capacity of ecosystems at different scales.
Ecosystems: Interactions, Energy, and Dynamics. HS-LS2-2. Use mathematical representations to support and revise explanations based on evidence about factors affecting biodiversity and populations in ecosystems at different scales.
Ecosystems: Interactions, Energy, and Dynamics. HS-LS2-4. Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem.
Ecosystems: Interactions, Energy, and Dynamics. HS-LS2-5: Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.
Ecosystems: Interactions, Energy, and Dynamics. HS-LS2-6: Evaluate claims, evidence, and reasoning that the complex interactions in ecosystems maintain relatively consistent numbers and types of organisms in stable conditions, but changing conditions may result in a new ecosystem.
Earth’s Systems. HS-ESS2-6. Develop a quantitative model to describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere.

Middle School

Matter and Energy in Organisms and Ecosystems. MS-LS2-1. Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
Interdependent Relationships in Ecosystems. MS-LS2-2. Construct an explanation that predicts patterns of interactions among organisms across multiple ecosystems.
Matter and Energy in Organisms and Ecosystems. MS-LS2-3. Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.
Matter and Energy in Organisms and Ecosystems. MS-LS2-4. Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
Matter and Energy in Organisms and Ecosystems. MS-LS1-6. Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy in and out of organisms.
Earth’s Systems. MS-ESS2-1. Develop a model to describe the cycling of earth’s materials and the flow of energy that drives this process.
Human Impacts. ESS3-4. Construct an argument supported by evidence for how increases in human population and per-capita consumption of natural resources impact Earth’s systems.

Three-dimensional Learning Progression

In Lesson 4, students explain carbon movement in ecosystems in terms of pools and fluxes. A pool size is constant when the fluxes into and out of it are the same or balanced – i.e., when the net flux is zero. Pool sizes change when in and out fluxes are not balanced. High school students explain how photosynthesis limits the maximum, steady state amount of organic matter an ecosystem can support. When an ecosystem reaches its photosynthesis limit, the rate of photosynthesis and cellular respiration are essentially the same. Students explain how different types of disturbances affect pool sizes and flux rates.

Key Ideas and Practices for Each Activity

Activity 4.1 introduces students to a 2-carbon pool model of an ecosystem where organic carbon is in organic matter and inorganic carbon is in the atmosphere in the form of carbon dioxide. The hands-on activity uses a very simple system to introduce students to the effects of fluxes on pool sizes. They discover that balanced fluxes, that is situations where the net flux is zero, result in unchanging pool sizes. Through graphing exercises, students associate the value on the y-axis (the amount of carbon) with pool size and the slope of the line (carbon movement per unit of time) with flux rates.

In Activity 4.2, students use a computer simulation of a larger 2-pool system to find that the rules still hold. When the net flux into and out of a pool is zero, the pool size does not change and conversely, when the net flux is not zero, the pool size will change.

In Activity 4.3, students use a more sophisticated model of carbon pools and fluxes where the ecosystem has a maximum photosynthesis rate. In this model, the flux rates of cellular respiration and photosynthesis, rather than being constant, depend on the size of the organic carbon pool.

In Activity 4.4 students use the 2-pool simulation to explore the effects of various disturbances on ecosystems. Press disturbances are long-term changes to the environment that may affect an ecosystem’s photosynthesis limit. Pulse disturbances are one-time events that ecosystems often recover from.

Key carbon-transforming processes: photosynthesis, cellular respiration, combustion

Content Boundaries and Extensions