The River Tank

Microhabitats

If there are not too many herbivores in the tank, students will soon see several varieties of algae growing. While advanced students can key out the specific types of algae, the entire class can have direct experience with the concepts of habitat and adaptation using a much less technical approach. By observing colors, growth patterns, and the location in the tank, all students can easily differentiate one type of algae from another. Where did the algae come from in the first place? Why does a particular alga grow in one place but not another? How does the distribution of algae change over time?

Figure 4 The American chameleon adds colorful diversity to the 30- and 40- gallon River Tank systems.

Quite naturally, students will begin to discover how minuscule changes in the environment give one species an advantage over another. Ultimately, rather than conceiving of an ecosystem as comprised of several well defined habitats, students will see that each habitat can itself be divided into many microhabitats.

Identifying the characteristics of the microhabitats is a valuable exercise in problem solving. For instance, one challenge could be to figure out how to observe differences in the flow characteristics of the tank's waterfalls, pools, and rapids (Fig. 5). This might be done by watching the movement of strings, which can be attached to glass rods embedded in the gravel or to suction cups mounted on the glass. Long, flexible plants can also serve as natural, qualitative flow indicators. Students could then create their own "wiggle scale" to describe the amount of turbulence.

Another approach might be to release neutral-buoyancy flow indicators (Cuisenaire(R) cubes used in math classes, for example) and record how they travel and where they go.

In addition to flow rate, the River Tank's microhabitats are influenced by light intensity, moisture levels, and substrate differences. Once the microhabitats have been tentatively identified, students can collect samples from each site to make direct observations and confirm whether the algae, diatoms, and bacteria are indeed different at each site.

Collecting samples can be done by scraping off small amounts of material, by examining what grows on the flow-indicator strings, or by using settling plates. Set the plates on the terrestrial and aquatic surfaces or attach them to the glass with lettuce clips (suction cups with a plastic clip attached - available at pet stores). Settling plates must be left in place at least one week, preferably longer. Once the samples are collected, students can hone their lab skills by observing and describing the samples under the microscope.

Figure 5 Students can observe fish swimming and differences in the flow characteristics of the tank's waterfalls, pools, and rapids.

Chemistry in the River Tank

Of the waste products fish excrete, ammonia, phosphorous, and undigested food impact an aquarium most significantly. Phosphorous occurs principally as dissolved phosphate and, since phosphate is often a limiting factor in plant growth, it is quickly removed by the plants directly. Undigested food and some by-products of digested food, namely amino acids and proteins, are a major source of dissolved organic compounds - a major component of the detritus that accumulates on the gravel bed and in the filters. This detritus is essentially harmless, but it clogs the filters and coats the gravel, limiting the ability of any carbonate particles in the gravel to dissolve and help buffer the water.

Ammonia (NH₃) is toxic to fish and must be removed or converted into benign substances before it builds up to lethal levels (levels above 0.1 ppm are considered dangerous). Some of the ammonia is taken up directly by certain plants (including most algae), but most of it is converted to mildly toxic nitrite (NO₂^-) by Nitrosomonas bacteria. Nitrobacter bacteria then convert the nitrite into nitrate (NO₃^-) a relatively benign, useful compound.

Nitrate is quickly removed from the water by plants, which use it as a source of nitrogen to form proteins and nucleic acids. This processing of ammonia to nitrate is part of what is commonly referred to as the nitrogen cycle. Regular testing of the ammonia, nitrite, and nitrate levels gives a good sense of the robustness of the Nitrosomonas and Nitrobacter populations. Testing also provides insight into how well balanced the tank is in terms of the amount of waste generated and the ability of the bacteria to process that waste.

Go to the next page...