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Annual variations in the ecology of a body of water

Annual variations in the ecology of a body of water

Introduction: (Initial Observation)

A body of water such as a lake, river, or pond is a fertile ground for thousands of animals and plant species to grow and reproduce. Often, such waters have a balanced ecosystem in which varieties of plants and animals live together and depend on each other. Certain weather or environmental conditions, however, may change such a balanced system and cause serious damage or destroy certain species.

Scientists often study the life of plants, animals, and micro organisms living in lakes and other bodies of water. Information gathered from such studies can help us manage natural resources, get the most benefit from them, and protect them from destruction. The most common benefit is fishing. Fish is a major source of protein for both humans and wild animals who feed on fish.


If you have any questions, click on the help button at the top of this page to send me your questions. I may respond by email, but often I update this page with the information that you need.

Project Advisor

Do you know why civilizations are formed around rivers, lakes, and oceans? Why people build Villages and cities where there is sufficient access to water resources? What will happen if a lake dries? What happens when a beautiful lake full of fish becomes a swamp with frogs, snakes, and alligators? Do people still build houses around it? What happens to existing houses around the lake? Do they lose value?

Imagine local residents of a lakeside city notice that excessive amounts of plants are growing in the lake, water is drying and population of fish is declining. They hire you as an ecologist and ask you to investigate the cause and give them a solution. What do you do?

Adult supervision and support is required for this project. Wear life jacket even when you walk into shallow waters.

Information Gathering:

Find out about plants and animal life around lakes, ponds, rivers, or any other body of water. Read books, magazines or ask professionals who might know in order to learn about the factors affecting population of certain species in and around water. Keep track of where you got your information from.

What is ecology?

Ecology is the study of how organisms live, interact with other organisms and how they interact with their environment. There are many fields in ecology and in those fields there are many amazing careers to pursue. In general, ecologists try to understand the connection between organisms and their environment. Ecologists can also study humans and how we interact with our environment! The main idea behind being an ecologist is to be really curious about how life works on (mainly) this planet.

What do ecologists do?

Look at ecological impacts of developments and recommend solutions to mitigate (or lessen) those impacts on plants and animals. They also work to solve ecological problems. As environmental consultants, ecologists often check out building sites or places that people want to preserve to figure out what plants and animals live there, how many there are, and what to do about them. They are especially concerned about endangered and threatened species that exist there.

What does it take to be an ecologist?

Because ecology is a link between living things and their habitats, someone who wants to become an ecologist should have a strong background in life sciences (botany, zoology, biology etc.), and, of course, enjoy them. For research purposes, computer skills are also very important. Since communicating is a large part of ecology, it is important to develop good written and verbal skills. For some ecologists it is important to know about economics, social sciences, and/or engineering. This broader background helps ecologists communicate their findings to policy makers, industries, and the general public.

Water is the principal component of the living matter. The embryos’ cells, the very young organisms, the marine animals and many plants contain nearly 90% of it. In the human body water represents about 70% of the total mass. For the majority of the mammals (men included), the loss of 10% of the water weight means death.

Ecology (from Greek meaning “study of the home” and logos “science”) is the science of the habitat. The term was invented in 1866 by Ernst Haeckel, a German pro-darwinist biologist. Ecology is the study of the interactions between living things and the environment. The environment includes both the abiotic environment — non-living things like climate and geology — and the biotic environment — living things like plants and animals. Much of ecological research is concerned with the distribution and abundance of organisms and how distributions are influenced by characteristics of the environment. Organisms influence their environment and the environment influences organisms.


The aquatic environments are differentiated by their salinities: tiny in fresh waters, medium in the estuaries, it reaches an important rate in the sea. One of the characteristics of the deserts is its very low availability in water. In the other hand, there are the tropical forests which its ground is filled with water. In aquatic environments, the photosynthesis can be carried out only at places where there is a sufficient quantity of light.

Fish need oxygen to breathe. There are a number of factors that control the amount of oxygen in water. Oxygen levels increase by wave action or from the photosynthesis of Algae. However, when Algae die they are decomposed by bacteria which consume oxygen. When dissolved oxygen get below about 3 ppm, fish can’t live. Dissolved oxygen levels in water are very hard to measure by chemical methods.

Dissolved Oxygen meter is used for quick measurement of dissolved oxygen in water.

The price of a dissolved oxygen meter is about $200 to $400. I have seen a few online and on ebay.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement. Following are two possible purpose statement:

The purpose of this project is to study the ecology of a body of water. We need to know what animals and plants are living in and around a specific body of water and how physical and environmental factors affect their distribution. (You can think of many specific questions while doing an ecological study. Following are some sample questions: Do fishes live in deep water or shallow water? Do frogs live in warm water or cold water? Do fishes prefer areas crowded with plants or areas with no plants? Do mosquitoes fly over shallow water or deep water? …..)

The purpose of this project is to study the ecology of a specific body of water and determine how does its population of living things change during the year (or any shorter period of your study). (You can think of many specific questions while doing an ecological study. Following are some sample questions: What season is best for fishing? what season you will see more frogs? What season you will see more mosquitoes? )

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Here I offer you two different generalized question or purpose that you may choose from. From the dependent variables, delete what you are not able to do. For example you may not have access to a device that measures the dissolved oxygen, however you may have access to a thermometer that measures the temperature.

For the first question or purpose, independent variable is physical location. Possible values are:

  • Close to water or shore
  • Shallow water about 1 foot
  • Deep water about 3 foot

Dependent variables are what you measure. Possible dependent variables are:

  • Temperature
  • pH (Acidity, Alkalinity)
  • Concentration of planktons
  • Dissolved oxygen (caused by plants photosynthesis)
  • Dissolved carbon dioxide (caused by decaying matter)
  • Number of certain animals (certain insects, fishes, snakes, beavers, frogs, zooplanktons)
  • Number of certain plants, aquatic plants, phytoplankton

Constants and controls are the time of the year, weather conditions, methods of observation and recording.

For the second question or purpose, independent variable is the time of the year.

Dependent variables are what you measure. Possible dependent variables are:

  • Temperature
  • pH (Acidity, Alkalinity)
  • Concentration of planktons
  • Dissolved oxygen (caused by plants photosynthesis)
  • Dissolved carbon dioxide (caused by decaying matter)
  • Number of certain animals (certain insects, fishes, snakes, beavers, frogs, zooplanktons)
  • Number of certain plants, aquatic plants, phytoplankton

Constants and controls are the location and methods of observation and recording.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

If you are required to have a hypothesis, then you need to be more specific in your question. For example your specific purpose may be to determine annual variations in the plankton concentration in a body of water. In this case following is a sample hypothesis:

The concentration of planktons has a direct relation with the weather temperature and daylight. My hypothesis is based on my gathered information that phytoplankton need sunlight for photosynthesis in order to grow and reproduce. Also zooplanktons feed on phytoplankton.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”


What you will do in this project, is the same as what an ecologist does while studying the ecology of a body of water. An ecologist will observe, measure and record the population of plants and animals in different parts of water. He will also measure dissolved oxygen and dissolved carbon dioxide. Ecologist may also record temperature, pH or concentration of certain chemicals such as mercury and phosphates in different parts of a body of water. Experiments and observations that are suggested below are all considered ecological experiments. You may select any combination of these observations or experiments for your project.

If you want to study the annual variations (or seasonal variations), you will need to repeat the same experiments at exact same locations every week for the period of your research.

If you want to study physical variations, you will need to repeat the same experiments at different locations in a short period (1 to 3 days).

Experiment 1:

What is the variation of fish population in a body of water at different times of the year?

Introduction: Studying the population of certain plants and animals is an important part of ecological surveys. Fisherman who do daily fishing in an area know how many catch do they have at every day. If you know a fisherman who is willing to help and a make a note for you, you can just collect date. Otherwise you need to make your own observation. Following experiment is suggested for fish population. You can arrange similar experiments for turtles, water snakes or even birds such as ducks who live on water and around water.


  1. Select two or more safe locations where you can see fishes and visually count them.
  2. Select an hour of the day for each area that you will be back to the sun while making your observation.
  3. Sit for a while (about 5 minutes) because moving can scare the fishes away.
  4. Make your first observation and count the number of fishes that you can see. Record it in your book. Then take a picture of the fishes for later counting.
  5. Wait for 5 minutes and repeat counting, recording and taking picture again.
  6. Wait another 5 minutes and repeat counting, recording and taking picture for the third time.
  7. Repeat this for all other locations.
  8. Record your observations in a table like this.

Fish count at sample locations at (Name of the lake or pond) on (Date)

Location Observation 1 Picture 1 Observation 2 Picture 2 Observation 3 Picture 3

Repeat this experiment once a week for the period of your study (a year or a month). Make one table for each day of your observation. Finally calculate the average of all averages for each day and write the results in a table like this:

Date Average count

Finally use this table to draw a bar graph.

Variations: You may throw some bread in water right before taking the picture. This will encourage fishes to come to the surface. If you do that, you must use the same type and amount of bread every time.

Note 1: A similar study can measure the variation of fish population in different parts of a body of water. In this case you will want to know which physical conditions (shade, sun, deep, shallow, warm, cold, clear, muddy, low plankton, high plankton,….) is favorable for fishes. In this case you will limit your study to a day or maximum a few days.

Note 2: You may repeat the same experiment/ observation for every other animal that lives in, on or around a body of water. Make sure you limit the boundaries of your observation and your observation time. For example you must not count a fish that you observe out of your study boundary.

Experiment 2:

Measure the relative concentration of plankton in a body of water.

Note: You can measure the concentration of planktons in different sections of a body of water or you can measure the concentration of planktons in different times of year. This depends on your question or purpose.

Introduction: Microscopic green plants called algae or “phytoplankton” form the base of the food chain for fish. All green plants need light, proper temperature and nutrients for growth. If sufficient light, proper temperature and necessary nutrients their abundance increases.

As phytoplankton reproduce to form dense communities pond water turns a greenish or brownish color. This is called a phytoplankton bloom.

The diagram in the right, shows shows some phytoplankton in a drop of pond water under a microscope.

As phytoplankton multiply they are eaten directly by some fish or by other mostly microscopic aquatic animals called “zooplankton”.

You may view zooplankton in a jar of pond water through a magnifying glass.


Phytoplankton and zooplankton (collectively called “plankton”) also serve as food for larger aquatic organisms.

Diagram in the right shows larger aquatic organisms consumed by fish.

When phytoplankton is abundant, it makes water a turbid green or brownish color. If the pond water is not very muddy, the turbidity caused by phytoplankton can serve as a measure of phytoplankton abundance.


Secchi Disk Transparency is a measure of the clarity of the water, and a quick, simple, and accurate method for estimating lake water quality. A black and white disk (called a secchi disk) is lowered into the water until it just disappears from sight–this depth measurement is recorded. The deeper the measurement, the clearer the water.

The disk measures 20 cm in diameter and is painted black and white in opposing quarters as shown in the picture. Secchi disk can be made from a round heavy metal or ceramic disk. Hang it to a rope and you are ready to go.

A simple disk can be made from a round can lid. Since the lid is not heavy, instead of rope, you must connect it to a wooden stick marked off in centimeters. Measure plankton density by lowering the disk into the water with your back to the sun while viewing the disk from directly above. The depth at which the disk just disappears from sight is the Secchi disk reading.

If you are comparing the concentration of plankton in different areas of a body of water, then you can make multiple readings in a day and record your results over a map of the area.

If you want to know the variations in plankton concentration at different times of the year, then you need to repeat your observations once a week or once a month for the period of your study.

Following is a sample result table for multiple years, but only 6 months of the year.

Average Monthly Secchi Disk Readings For Indian Lake

Month 1992 1993 1994 1995 Average
Per Month
May 17.1 26.2 18.3 17.3 19.72
June 12.9 18.5 13.5 17.8 15.67
July 11.5 14.2 11.9 12.9 12.62
August 11.2 10.2 11.1 12.2 11.17
September 11.7 11.6 12.2 12.5 13.5
October 14.9 13.6 13.3 13.3 13.77

Experiment 3:

Annual variations of temperature in a body of water

Introduction: Proper temperature is required for the growth and reproduction of all aquatic plants and animals. By recording the water temperature at different times of the year, we can have data that may help us to forecast the growth and activities of different plants and animals at different times of the year.


Use a thermometer and record the water temperature at different parts of a body of water. Insert the thermometer in water and read it while still in water.

Make recording from multiple locations and then calculate the average. Record the results in a table like this:

Water temperature at …… lake:

Date Location1 Location2 Location3 Average

You may later use the date column and the average column to produce a graph.

Experiment 4:

Effects of different pollutants on the ecology of a body of water.

Pollutants such as runoff of factories, sewer system or chemicals washed by rain may end up to a local lake, pond or river. Such pollutants may affect certain organisms in water. Some pollutants may kill plants, some others may cause overgrowing plants, some may kill plankton and some may kill fish. Ecologist may need to conduct experiments to determine the effects of every possible pollution on every possible organism in a body of water. Since they don’t want to cause damage or cause imbalance in the water, they perform such experiments in the lab. We can perform many of such experiments at home. For example we may test to see how do detergents affect plankton or how do oils affect fish.

Here is an experiment for testing the effects of different pollutants on aquatic plants. With this experiment, you will be able to compare and contrast differences in water quality by observing and recording changes to water during an experiment in control and polluted environments.

Materials Needed:

  • Several clear plastic 16-20 oz. drinking cups or jars
  • Several small aquatic plants (can be collected from a pond or purchased at the local pet store)
  • Two teaspoons motor oil*
  • Two teaspoons sugar
  • Small amounts of various household detergents*
  • Non water-soluble paint*
  • One or two crushed charcoal briquettes*
    (must NOT be treated with any formula for quick lighting)
  • Several straws
  • Peel and stick labels

*Adult supervision and support is required while handling harmful material.


Before beginning this experiment, study about the importance of water quality for humans. Make a list of the many ways we use water and then make a list of the pollutants that would poison our water supply.

Then, ask yourself why water quality is so important to fish in lakes, ponds, rivers and oceans. Predict what happens to water that is polluted by chemicals or is stagnant (Not moving or flowing; motionless).

Clothes Detergent





Dish Detergent

First, prepare a “control” cup by placing sand mixed with crushed charcoal as filter on the bottom, pressing the roots of an aquatic plant into the sand/charcoal mix, filling the cup with clean tap water.

For the remaining cups, first place sand and the aquatic plant on the bottom (with no charcoal), fill with clean tap water and…

1. Put two teaspoons of motor oil in one cup.

2. Put a teaspoon of dishwashing detergent in one cup.

3. Put a teaspoon of clothes detergent in one cup.

4. Put some non-water-soluble paint in one cup.

5. Put two teaspoons of sugar in one cup.

All cups can be placed in a windowsill, which will enable you to easily measure the effect of evaporation in addition to the impact that the “pollutant” has on the water quality inside the cup.

At least once a day, you should insert a straw about 1/8 inch into the water and blow a few gentle puffs of air into the water. (Make sure you blow out, not suck in.) This will simulate the filtration and exchange of gases that must take place within a healthy aquarium. Make sure you do this for all samples and control.

Make predictions about what will happen to the water quality in these cups.

Report your Results:

Each day, you can observe the changes in water quality and the condition of all plants and record them in a table. At the end of your experiment period (2 to 4 weeks), report you findings. Explain what your findings mean to the fish and plants that must have clean water to survive in their underwater habitat.

Materials and Equipment:

List of material varies based on your experiments and methods. Prepare it in the last step.

Secchi Disk is available at MiniScience.com.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.



Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Can such studies show the importance of water quality in a fish’s habitat? How?

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.