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Cont. Teacher Notes for Classroom Implementation
Some careful thought should be given to site selection. One aspect of site selection is deciding what kind of site to sample. This process could involve class research of local sites and subsequent
discussion, if a site has not already been selected. Some schools may have access to a river or pond on the school property or access to a site on which other work has been done. Convenience and ease of
access should receive strong consideration. Sites could be above and below some point source for a stream. Several sites in a lake could be sampled and compared. Sites where fresh water enters an
estuary could be compared to those with greater salinity. Site selection and subsequent adaptations of the procedure should be based on local conditions.
At the site, each group will place 4 pots in a designated area: a control of plain agar, one with nitrogen enriched agar, one with phosphorus enriched agar, and one with both nitrogen and phosphorus in the
agar. The class size and materials would determine the number of groups. A minimum of three groups allows for 3 replications of each type of pot at a site. Invariably, a pot will get broken or moved
or disturbed in some way. Each group needs to be able to place its pots in one site that is deep enough and large enough to separate the pots so that one nutrient does not diffuse over to another pot’s “area”.
Pots should be at least 1 meter apart at a given site and well below water level, about one third to one half meter deep. The pots need to be attached to the bottom of the site in some way to prevent
them from toppling over or washing away. One method is to tie cord around the pot above the ridge and attach stakes on each side that can then be driven into the river/pond bottom. The pots should
be left for at least 2 weeks, ideally at a site that could be checked on by the students outside of school hours during the experiment.
At the time the pots are placed, some measurements of the environmental conditions can be taken. At a minimum, the students should record air and water temperatures. These could be followed on a regular basis
during the length of the experiment and graphed. The students should note also whether it is a mostly sunny (open water) or shady site (tree overhang). The students should also have some visual landmark
to locate their pots when they come to retrieve them.
To collect the pots, the students need to be careful to disturb the area around the pots as little as possible. In a stream, they should approach from downstream. In a single motion, the pots should be
put into quart or gallon zippered plastic bags, one pot per bag. Collecting the water around the pot allows for sampling invertebrates around the pot whose distribution may be affected by the amount of algal growth
on the pot. Final air temperature and water measurements may be taken.
Once the samples are back in the lab, a variety of data analyses can be done, based on time, grade level, and curricular considerations. First, the students can make a qualitative observation of how much growth
is on the pots, relative to the other pots in their sample. Also, they should look at where the growth is on the pots: is it all on one side, upstream or downstream, near the top or near the bottom? The
teacher may want to provide guidance in the types data that can be collected and how they can be quantified. Using toothbrushes, the alga or "scum" is scraped off the pots onto massed filter paper to be
dried and measured as dry biomass. If there is a lot of silt on the pot, it may be difficult to remove. Some common procedure should be agreed on to gently rinse while retaining the attached algae for
measurement. The teacher and students should work out a way to measure the area on the pot represented by this biomass so that results from different groups can be compared. They also need to decide whether to
combine samples from all the pots of one type, for example the control or measure each pot individually. If there were a significant difference in growth on one of the kinds of pots, students would then try to
figure out why. If there is no significant difference among the 4 types of pots or the sites, the students may want to look at the experimental design first and then at other reasons why there might have been
no difference.
These pots can be reused from year to year. The pots should be marked so that they are always used for the same variable so that cross contamination doesn't occur.
This experiment is designed to be as simple or as extended as the teacher or the student wishes. It provides an inexpensive apparatus and relatively simple protocol that can be modified or extended for
individual student research. It can be relatively qualitative or at upper levels, more quantitative. A quantitative approach might include calculation of the area of the pot measured, a statistical
analysis of the differences among the pots, measurements of water perimeters such as DO, temperature, pH, initial concentrations of nitrogen and phosphorus.
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