Plankton are one of the most important organisms in a reservoir or pond. Primary producers in the pond ecosystem, these tiny aquatic plants serve as a food source for a variety of other organisms. Photosynthesizing plankton, called phytoplankton, are analogous to grass in a land ecosystem. A variety of zooplankton herbivores such as the brine shrimp "graze" on the phytoplankton and provide a food source for higher organisms.
Phytoplankton vary from single celled diatoms to the multi-celled Volvox. They generally lack means of propulsion and thus are found primarily in slack water areas (ponds and lakes). Utilizing sunlight and the nutrients of the surrounding water, phytoplankton can grow quickly in numbers when the conditions are right. A single celled can form a population of 1015 individuals in a single season!!
Blue-green algae(another type of phytoplankton) are often responsible for algal blooms that often occur in the spring and fall when nutrient conditions are high. Nutrient-rich water from either pollution or natural sources stimulates growth rates and the rapidly reproducing blue-green algae give lakes a characteristic pea-soup appearance. As these organism respire during the night (when photosynthesis is not possible due to a lack of sunlight), or decay, they use up oxygen that may result in fish kills and death of other organisms in the pond.
Because phytoplankton populations are dependent on nutrients, they are good indicators of the health and age of a lake. Newer, naturally formed lakes are often limited in nutrients and will only support a sparse plant community; these types of lakes are called oligotrophic (meaning unproductive). Waters high in nutrients are called eutrophic, and support a large algal community.
Eutrophication is a term often used to describe the predominance of blue-green algae and low oxygen content in the water.
microscope of hand lens
Several plankton sieves must be constructed to collect phytoplankton samples. These can be easily constructed out of old panty hose looped over a small section of 6" PVC pipe, a thick plastic bottle cut in the middle (or something similar). The trailing panty hose (the sieve) should be about 1 foot in length, and connect to a baby food jar (or similar). A 10 foot rope or nylon cord should be securely tied to the PVC pipe in such a manner that the sieve can be thrown out toward the center of the lake and retrieved while the sieve filters phytoplankton and zooplankton from the surface into the jar.
Visit a local pond or reservoir to collect water samples for laboratory analysis. Plankton migrate from the surface to depths, depending on the intensity of the sunlight, time of year, etc., so have your students collects samples from several different depths. This can be accomplished by varying the retrieval rate of the sieve.
Drain off the majority of the water in the jar and keep the water that should have a high concentration of organisms. Methanol can be used to anesthetize organisms to aid in viewing under the microscope.
Return to your laboratory and analyze the samples under the microscope. Consult an appropriate reference (see reference section) for organism classification. Draw your results.
Make other observations about the pond that may help explain your findings when you return to the lab.
You may also want to collect samples from other ponds near their homes for comparison.
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