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
