Exercise 4: Aquatic Ecology

Fundamentals of Biology / Science and Children







Ecology is the study of an organism and how it interacts with its home. In this case, the home is primarily water and we will be looking at several types of different aquatic environments. An environment is everything around the organism that it interacts with. While we will primarily concentrate on the easily measured, non-living factors, or abiotics, of the environment, we will also look at some of the living, or biotic. elements of the environment. All of these factors exist together in a "cycle" that has material and energy flowing from one factor to another.


The abiotic factors usually start the cycle as they are the raw materials that certain organisms can use to build their bodies. The non-living things are all the things around an organism that are not alive. This includes solids, liquids and gases that the living organism comes into contact with. The solids would include things like the soil and all the nutrients it contains. The liquids would be the water with all the material in it and the gases would be the oxygen, carbon dioxide, water vapor and other gases in the atmosphere. Other abiotics include things like temperature, pH, light, etc.


All biotics will fall into one of three categories: producers, consumers or decomposers. As the name implies, producers are those organisms capable of producing their own food. A more technical term for these organisms is autotrophs where "auto" means self and "troph" means feeding. We usually think of these as the plants and other green things that can take in sunlight, water and carbon dioxide and produce glucose and oxygen. Consumers are those organisms that consume, or eat, other organisms in order to get their food. The scientific term used to describe them is heterotrophs which means "other feeders". In common terminology, this group can be further broken down into "herbivores" which each plants, "carnivores" which each animals, and "omnivores" which are capable of eating both plants and animals. The more scientific way to categorize who is eating who is to talk about "primary", "secondary", "tertiary", etc. types of consumers. Finally, in order to complete the cycle, the bodies of the producers and consumers need to be turned back into the abiotics from which they originally came. The decomposers are primarily responsible for doing this.


As can be seen in the diagram above, the cycle is not just an outer circle in which material flows from abiotics to producers to consumers to decomposers and back to abiotics. Transfers are also happening between the various components as, for example, consumers breath oxygen in and carbon dioxide out, or producers are decomposed without having been eaten by a consumer.


In an aquatic ecosystem, most of the interaction, obviously, will be between the organisms and the water. However, all the abiotic factors that a land organism finds in the air and soil are dissolved in the water for the animals use. Therefore, we can easily study and measure these factors simply by analyzing the water with special instruments and kits.


We will be studying two types of aquatic ecosystems in this exercise. The first will be a typical type of seasonal ecosystem that is a dry riverbed most of the year and a running stream only during the rainy season. These ecosystems are common throughout the Southern California area. During the typical spring, summer and fall seasons they are dry and accumulate material that falls from the surrounding areas. In the winter wet season, they fill with water and the accumulated material is dissolved in the water and carried downstream. Aquatic animals in such a seasonal stream tend to be small insects, crustaceans and microscopic organisms with very short life spans that allow them to be born, live and die during the time the stream is flowing.


The second type of ecosystem studied will be a pond. The APU pond is actually comprised of three microcosms, or small ecosystems, as the pond has a deep end, a shallow end and a marsh area used as a biological filter. Each area is home to different types of plants and organisms because they have different abiotic factors that are suitable for different types of abiotics.


The abiotic factors to be studied will be temperature, pH, phosphate, nitrate and dissolved oxygen. You will be asked to create a hypothesis regarding the relative amounts of each of these factors as compared to the other areas. As you do this, keep the following in mind regarding these factors:


Temperature is affected by sunlight in both the pond and the stream while shallow water will heat up faster than deep water in the pond. Finally, the faster the water is moving in a stream the cooler it tends to be.


pH varies according to a variety of factors that will be difficult to just eyeball.


Phosphate and nitrate are the fertilizers of the ecosystem. They tend to be greater where there is an accumulation of organic matter like leaves, algae, etc. Since this material tends to settle to the bottom of a pond, the closer the samples are to the bottom the higher this levels tend to be.


Dissolved oxygen is a factor of aeration, production by plants and use by all aerobic organisms.





Materials needed:


Thermometer pH paper

Nitrate test kit Phosphate test kit

Dissolved oxygen kit Large aquarium fish net

Microscope Slides and cover slips

Eye droppers Fine cheese cloth

Water bottles 500 ml beakers



Methods to be followed:


1. Place a hose at the top of the stream and turn it on full force. Be sure that it is placed so that the water is running downstream and not out of the stream bed. After twenty minutes sampling can begin.


2. Divide the class into five or six teams of three to four students. Assign each an aquatic ecosystem to check. Omit the middle of the stream if there are too few students.:


a. Deep end of pond

b. Shallow end of pond

c. Marsh area of pond

d. Beginning of stream

e. Middle of stream

f. End of stream


3. As a class, examine all the areas in light of the material at the very end of the "Introduction". Each team is to describe their area and create a hypothesis regarding the relative levels of each factor in their areas compared to the others. Record in the "results" section.


4. All teams will be performing the same analysis of their area using the equipment provided with the idea of characterizing their own area in order to compare it with all other areas. Teams will conduct their analysis, enter their results on the board and be prepared to explain their data for the class.


5. Analysis instructions:


a. Using the thermometer, take the temperature of the water in at least five spots in your area and average them together. Compare to the air temperature provided by your instructor.


b. Dip the pH paper into the water and compare the resulting color change to the chart provided. Repeat at five spots and average.


c. For the dissolved oxygen, nitrate and phosphate kits follow the instructions provided with each kit. Do only one sample.


d. Using the aquarium net, take a single sample. In the pond, move the net through a five foot sweep of the pond. In the stream hold the bottom edge of the net on the bottom of the stream, with the handle straight up, so the water flows through the net. Leave it in place for five minutes. Use a water bottle to rinse the material accumulated into a beaker and describe the material found.


e. Repeat the above process in each area using the same net. However, cover the net with several layers of the cheesecloth so that it catches finer particles. Examine the material found under the microscope by preparing a wet mount of the water in the beaker.





1. Which area did your team study:


2. Describe the area that your team sampled:










3. On the basis of the material at the end of the "Introduction", create a hypothesis as to the relative levels of each factor in your area versus the others:






Dissolved oxygen:


4. Record the data from the class:



LOCATION Temperature pH Nitrate Phosphate Dissolved O2


Shallow Pond

Deep Pond

Stream Start

Stream Middle

Stream End


5. Describe the material trapped inside the unmodified net:





6. Diagram the organisms found in the modified net:














1. Based upon your understanding of each of these factors, does it seem like your area had the relative values you expected?

2. Compare your results with those from the other teams regarding your hypothesis. In the space below compare your hypothesized ranking to what actually was true regarding the relative position of your ecosystem for each of the factors studied.









Dissolved oxygen:



3. Using the class data, compare the factor levels in either the three areas of the pond or the three parts of the stream. Attempt to explain why