Crinoid Bio-Medium Aquaculture: Nutrient Flows in Classroom Aquaria
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Hits the Standard: HS-LS2-3: Construct and revise an explanation based on evidence for the cycling of matter and nutrient flow of energy in aerobic and anaerobic conditions.
Lesson Brief: Producers such as plants and consumers such as fish are important components of many ecosystems. Energy and nutrients flow between them and other components. Providing a substrate, or physical habitat, for another component, decomposers, can increase the efficiency of nutrient recycling. Using a linked fish – plant system, students will add a decomposer element, in the form of nitrate-reducing bacteria, to the system and document how this alters nutrient flow through the system.
Background: Box and arrow diagrams are used in ecological science to show the magnitude and location of different pools of a nutrient as it moves through a system. The boxes represent different locations, different forms of the nutrient, and different biotic groups in the community as they use the nutrient. Arrows show the flow of the nutrient from one box to another. Together, these form a graphical balance sheet for the nutrient in a given ecosystem.
Nitrogen is vital to all life. It is an important especially in proteins. When animals break down nitrogenous compounds, they release waste products such as ammonia, NH4. Plants take up nitrogen in the form of nitrates, compounds containing NO3-. In a natural ecosystem, there are many different routes between these forms of nitrogen. In addition to producers and consumers, a functioning ecosystem has decomposers. These organisms break down complex organic matter into simpler forms, and alter the form of nutrients making it available to producers and different consumers. Many decomposers are bacteria that alter ammonia to nitrite, NO2-, and other bacterial species that alter nitrite to nitrate which is then useable to plants. Bio-media provide a physical substrate for bacteria communities that help change ammonia from fish waste into plant-useable nitrate.
Activity: Print out and assemble several sets of the Crinoid bio-medium model (Figure 1). If two aquaculture set-ups or two aquaria are available, outfit one with a chamber of Crinoids downstream of the existing filter. Leave the other aquarium as it is. Allow the water to circulate as normal for at least several weeks. Using commercially-available test strips, test the nitrogen levels in the tanks. Create a box and arrow diagram for the nitrogen cycles in the two tanks. If only one aquarium or aquaculture set-up is available, test nitrogen levels before using Crinoid media and several weeks after introducing.
Analysis: Did the ammonia, nitrite, or nitrogen levels differ between the treatments with and without the Crinoid media? Was the surface area provided to the nitrogen-altering bacteria sufficient? After examining the box and arrow diagrams, suggest ways that this system could become more self-sustaining, in other words, require less inputs and removals.
Figure 1: Assembly of the Crinoid bio-substrate. Print 1 scolex and 16 arms. Glue arms halves together ensuring careful match of halves of pegs. Insert pegs into holes. Printing can be facilitated by printing enough for 1 Crinoid at once, and by ensuring flat side of arms are down. Holes for 4 arms are not visible in the figure.