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Teacher Notes for Classroom Implementation
This experiment looks at one of several ways agar-filled pots can be used to study factors which influence aquatic systems: in this case, the pots are used to look at the effects of two nutrients, nitrogen and
phosphorus, in a stream. It would be appropriate to carry out this activity before studying nutrient cycling through an ecosystem, allowing the students to determine that the two nutrients affect algal
growth. This inquiry could lead into a unit on biogeochemical cycles. It could also be carried out during or after this unit to demonstrate, reinforce, or extend the students’ understanding of the role
of certain aspects of ecosystems, for example: biotic and abiotic elements, point and non-point source pollution, and influences on biodiversity in aquatic systems. The students set up this experiment to
look at the environmental impact of nitrogen and phosphorus in aquatic systems. This can be related to nutrient overloading, a situation which probably occurs in their own communities due to run off from
fertilization on lawns or agricultural crops.
Depending on the class time available, the teacher can vary the amount of student involvement the preparation work and still retain the essential parts of the experiment. Make sure to allow enough advance
preparation time to soak the pots. New pots should be soaked at least overnight, though as many as 15 days has been suggested. This soaking removes any phosphorus or nitrogen that might be on the pots,
which would invalidate measured amounts of these nutrients in the agar (Scrimgeour & Chambers, 1997).
After soaking, the pots are wiped dry around the edge or air dried so that caulking which attaches the petri dish to the pot will hold tightly. This can be done ahead of the time with the students or just the
teacher. This part will take approximately 1 hour.
Another aspect of the experiment the teacher may do without the students to save class time is preparing the agar for the pots. Preparing the 1.5% agar is time consuming because of the time it takes to make
these relatively large volumes of the four different agars: plain agar for the control, agar with 0.5 M NaNO3, agar with 0.5M KH2PO4, and agar with both 0.5 M NaNO3 and 0.5M KH2PO4. The students could measure
out the amounts of the nutrients if practicing the preparation of molar solutions was a goal, and these could be saved until the agar is prepared. Regardless of who prepares the agar, pouring the agar is an
activity that would make the experiment more meaningful to the students. Measuring, mixing and heating the agar to get it into solution takes from 1 to 2 hours, depending on the equipment available. The agar
can be kept warm and liquid in a water bath of 45-50 C. Each student team can pour the agar into its allotment of 4 pots. The clay pots absorb agar, so after 5 to 10 minutes, additional agar should be
added to completely fill the pot. Then the hole is plugged with a pencil eraser, cork or rubber stopper. Placing the pots in a pond or stream can be done another day, storing them in the refrigerator
until use. .
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