The Jar of Pond: Vision & Population Change
Introductory for Non-Majors Biology Laboratory
Learning Objectives:
• Learn about about abiotic factors in freshwater environments
• Develop and test a hypothesis about the influence of an abiotic factor on a
phytoplanktonic protist
• Gain experience using a compound microscope
• Learn how to use sampling to estimate a population.
• Apply mathematical models to predict population growth
• Examine representations of data patterns by making graphs
• Draw conclusions and evaluate hypothesis based on data patterns
Core Concepts:
o Evolution
o Structure & Function
o Pathways & Transformations of Energy & Matter
o Systems
Core Competencies:
o Ability to Apply the Process of Science
o Ability to Use Quantitative Reasoning
o Ability to Use Modeling and Simulation
o Ability to Tap into the Interdisciplinary Nature of Science
Jar of Pond Week 1: Abiotic factors and plankton
(Time needed – 30 minutes)
Your bean simulation examined the influence of the environment on a population by looking at a
biotic (living) selective pressure. Predators are living things that influence the prey populations.
The environment also includes many abiotic (non-living) factors that can also influence
populations. These include factors like the substrate, which is the bottom or ground type; the
amount of available light or water; the temperature; the amount of salt or other types of
chemicals, including nutrients; and even physical forces like wind or waves. You are going to
have a chance to investigate how some living things respond to abiotic factors in a freshwater
environment.
Each lab group will develop an experiment to test a hypothesis about how an abiotic factor
influences a population of phytoplanktonic protists. Phytoplankton are photosynthetic plankton
that drift in the open water and absorb sunlight energy to make food. For purposes of this
experiment, we will focus on abiotic factors. Each table will receive the same amount of
freshwater, the same amount of plankton and all experiments will take place in our lab
classroom. Light is an abiotic factor that is extremely important to photosynthesizers, so we will
not be altering that abiotic factor. You will select one other abiotic factor to change. We will also
maintain a control jar, which we will leave untreated so that we have a basis for comparison.
Next week, your lab group will determine your starting population by taking samples of a
plankton population living in a jar of freshwater and then set up an experiment by adjusting the
abiotic conditions of your jar. Over the next few weeks in lab, you will be sampling the jar to
determine if and how the population is changing over time in comparison to the population of
phytoplankton in the control jar, which is not being changed.
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The abiotic factors you have the opportunity to manipulate include:
Temperature
Temperature refers to the amount of heat in the environment. Many organisms are adapted to a
specific temperature range. The enzymes in their bodies work best at particular temperatures.
Organisms that are able to maintain a constant body temperature, either internally, or because
they live in a stable environment can have more finely tuned enzymes and biological processes,
though. Freshwater systems can be subject to fairly large seasonal temperature changes;
Human activities like logging can remove sources of shade and raise the water temperature.
If you choose to adjust the temperature of your plankton jar, you may decide to raise it a few
degrees with a seed warming mat or lower it a few degrees with a cold pack. Extreme
temperature changes can shock and kill organisms, so make your temperature changes minor
and gradual.
Chemical minerals and nutrients
The availability of key chemical nutrients plays a role in many terrestrial and aquatic systems.
For example, calcium carbonate is a mineral required by shell-building animals like snails and
corals. Nutrients like nitrate and phosphate are needed by all living things, although some living
things like bacteria and algae that reproduce rapidly can take advantage of additional nutrients
in an environment very quickly, which can negatively influence other living things as oxygen
becomes depleted. A limiting chemical mineral or nutrient can be very important in determining
how rapidly or how large a population may be able to grow. Different living things may respond
differently to different minerals and nutrients. For example, some chemicals that are toxic to
certain plants may be required by others for growth.
If you choose to adjust the chemical nutrients in your plankton jar, you may add some plant food
containing nitrogen to your jar. Extremely high amounts of nutrients can become toxic, so do not
exceed the manufacturer’s recommendations for diluting the plant food.
pH
Chemicals influence the pH of water or soil.
pH refers to how acidic or basic a compound
is, which is determined by the relative
concentration of H+ ions. The pH scale ranges
from 0-14, with neutral represented by a pH of
7. Pure water has a neutral pH, which means
there are equal proportions of H+ and OHions. An acidic compound has more H+ ions in
solution and a basic compound has more OHions in solution. Highly acidic compounds are
at the lower range; strong acids have pH
values of 2 or lower. Basic compounds are at
the higher range; strong bases have pH
values of 12 or more. Different types of
organisms are adapted to different pH values.
Most require neutral or near-neutral
conditions, although some thrive under more
acidic or basic conditions. The pH scale is
logarithmic, which means that a pH of 8 is 10
times greater than a pH of 7, and a pH of 9
is100 times greater than a pH of 7.
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pH
Acidic
Basic
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
H+ ion concentration
relative to neutral pH of 7
10000000
1000000
100000
10000
1000
100
10
0
1/10
1/100
1/1000
1/10000
1/100000
1/1000000
1/10000000
2
If you choose to adjust the pH of the water in your plankton jar, be aware that rainwater is
slightly acidic at values of 5.4 – 6. Due to the logarithmic pH scale remember that small
variations in the pH number are actually increased by a factor of 10 and that strong acids and
bases are very dangerous to living things (including humans). You will have weak acids and
bases to work with for adjusting pH, but you should still keep your changes within 1 or 2 pH
factors.
Salinity
Salinity, or saltiness is a specific chemical factor. Salinity is usually measured in parts of sodium
chloride per thousand parts of water (ppt). Seawater is remarkably consistent in salinity; usually
around 33 ppt. Bodies with very high salinities are referred to as brine. The Dead Sea, which is
so named because nothing can live there, has a salinity of approximately 290 ppt. Salinity
impacts osmosis, when water enters and leaves body tissues as an attempt to balance the
internal salt concentration with the external salt concentration. Organisms that live in freshwater
tend to have internal salt concentrations higher than the surrounding water, so water is always
trying to enter their bodies. Organisms that live in saltwater tend to have internal salt
concentrations lower than the surrounding water, so water is always trying to leave their bodies.
If you choose to adjust the salinity of the water in your jar, remember that few organisms can
survive in waters with salinity above that of seawater, and most freshwater organisms cannot
tolerate salinities that approach those of seawater. Many are very sensitive to even small
changes.
As you build your experiment, you should complete the following:
What abiotic factor are you interested in testing?
What is your hypothesis?
What is the independent variable?
What is the dependent variable?
How will you measure the dependent variable?
What steps can you take to determine that the results you get are due to the independent
variable and not to other factors? In other words, what controls will you use?
Other Experimental notes (include diagrams, if needed):
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Discussion Questions
1. Why do you think phytoplankton are important organisms in the environment? How do they
impact other aquatic organisms?
2. What is the difference between a biotic factor and an abiotic factor in the environment?
3. Why do we need to use controls when developing experimental protocols? Can you control
for every possible factor? What should you do if you cannot?
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Jar of Pond Week 2: Sampling
(Time needed – 45 minutes)
We can use a strategy known as sampling as a tool to find out how the populations of
phytoplankton in our jar of pond experiments are changing. Sampling is a powerful tool that can
allow us to describe an area without counting everything found there. It would be timeconsuming and frustrating, if not impossible to count every single fish in the ocean or tree in the
forest. Sampling is the process by which small areas can be counted and quantified, with the
assumption that they represent the larger area as a whole. To ensure that a sample is
representative of the larger area or population, multiple random samples should be collected.
1. Each lab table will be building your “jar of pond” for your experiment on abiotic factors. Since
we are focusing on phytoplankton populations, each table is focusing on one population, and
you will be sampling only that particular organism for this part of the lab.
1a. Subsample your planktonic (pondwater) sample by placing single drops of water onto a
microscope slide and scanning the drop for individuals of the species Volvox globator.
1b. Mark your beaker with a sharpie marker to identify the top of the waterline.
1c. Your sample is the experimental or test sample. Your instructor will provide you with the
the control sample.
2. Record the number of individuals you found in your drop of water in the Week 1 Test row in
Table 1.
2a. Each person at your table will have counted the number of organisms in their drop of
water.
2b. Calculate the average number of organisms per drop.
2c. Each drop of water is approximately 0.125 mL. Your jar contains 50 mL of water. To
determine the approximate total number of organisms in the jar, multiply the average per
drop by the total number of drops in the water (you will need to divide the total mL in the jar
by the total mL per drop to get the total number of drops.
3. Bring Table 1 with you to lab for the next three weeks to continue data collection on the
experiment.
Data Table 1: Sampling Data
Week 4
Week 3
Week 2
Week 1
Data Collected
1
2
3
Sample (how many of each species?)
Average
Drops
4
5
6
7
8
per drop
per jar
Estimated
total
Experimental
Control
Experimental
Control
Experimental
Control
Experimental
Control
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Discussion Questions
1. Bias refers to sampling methodologies that yield results that do not accurately reflect the
population. What are some things that can cause bias?
2. If we were trying to compare phytoplankton populations in two different ponds, what would
happen if we searched the two ponds for different amounts of time, or with different numbers
of people (thus taking different numbers of samples)? Could we compare our results with
any certainty?
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Jar of Pond Week 4-5 Continue Plankton Experiment
(Time needed – 20 minutes per lab session)
1. Accommodate any evaporation that may have taken place over the past week, so that your
calculations will be accurate and comparable from week to week.
1a. Before taking your sample, examine your beaker to determine the amount of water in the
beaker this week compared to the previous week. Subtract the amount in the beaker this
week from the amount denoted by your sharpie mark from last week.
1b. Use a graduated cylinder to measure the needed water into your beaker prior to taking
this week’s samples.
2. Record the number of individuals you found in your drop of water in this week’s Test row in
Data Table 1.
2a. Each person at your table will have counted the number of organisms in their drop of
water.
2b. Calculate the average number of organisms per drop.
2c. Each drop of water is approximately 0.125 mL. Your jar contains 50 mL of water. To
determine the approximate total number of organisms in the jar, multiply the average per
drop by the total number of drops in the water (you will need to divide the total mL in the jar
by the total mL per drop to get the total number of drops.
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Jar of Pond Week 6: Data Analysis and Population Modeling
(Time needed – 110 minutes)
We saw in lab 3 that the type of environment influenced the population of different beans and in
lab 4 that both random and non-random environmental factors could influence evolution. We
discovered that the water quality of different bodies of freshwater can vary a great deal due to
different abiotic factors. Our leaf pack experiment examined how macroinvertebrates are
influenced by different kinds of factors. But macroinvertebrates aren’t the only organisms found
in freshwater environments. You have also been examining a type of micro-organism found in
freshwater habitats, phytoplankton. Planktos is a Greek term meaning drifter. Plankton are
organisms that drift in the water column. Many of these small organisms include phytoplankton
or plant-like plankton that conduct photosynthesis and are an important element in the food
chain. Today we are completing our experiment to help us understand how these plankton are
influenced by their environment.
Estimating weekly population growth
Over the past three weeks we established a sampling protocol that we could use to estimate the
population size of the phytoplankton in our pond jars. Use your sampling results to complete the
mathematical procedures below for both the control and experimental jars and
1. Find the absolute change G1 between Week 1 of the experiment (N1) and the following week
(N2):
N2 – N1 = absolute change in population = G1
2. Find the rate of change from last week to this week (r):
G1 / N1 = rate of change (r)
3. Using the rate of change from last week to this week, calculate what you expect the
population will be next week, the predicted week 3 population N3:
3a. Use the rate of change to determine how much we expect the population should increase
or decrease by multiplying this week’s total (N2) by the rate of change (r):
(r * N2) = absolute change (G2)
3b. Add the absolute change (G2) to this week’s total (N2):
G2 + N1 = Prediction of Week 3’s population (N3)
4. Repeat the process to compare weekly growth in Weeks 3 and 4.
5. Complete Data Table 3 to compare the predicted and actual N2, N3, and N4.
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Data Table 2: Plankton Population Growth
N1
N2
Jar
Population
change
Experimental
Week 1-2
Control
Population
Change
Week 2-3
Jar
G1
(N2 – N1)
r
(G1 / N1)
G2
(r * N2)
Predicted N3
(G2 + N2)
N2
N3
G2
(N3 – N2)
r
(G2 / N2)
G3
(r * N3)
Predicted N4
(G3 + N3)
N3
N4
G3
(N4 – N3)
r
(G3 / N3)
G4
(r * N3)
Predicted N5
(G4 + N4)
Experimental
Control
Population
Change
Week 3-4
Jar
Experimental
Control
Data Table 3: Predicted vs. Actual Growth of Plankton
Week 2
Jar
Actual N2
Week 3
Predicted N3
Actual N3
Week 4
Predicted N4
Actual N4
Experimental
Control
You made a hypothesis about how an abiotic factor would influence the phytoplankton
population in your jar. An easy way to examine the data patterns is to produce a graph. You’ve
practiced this already when you did your lab write up on the bean natural selection simulation.
Develop a line graph to determine if there is variation between the control jar and your test jar
over the three weeks that we ran the experiment. To determine if that variation was stable,
increasing, or decreasing, you can mathematically compare the test to control populations for
each week by subtracting the control population from the test population.
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Abiotic Experiment Graph:
Hypothesis evaluation: Explain whether you think your hypothesis is supported and why you
reached that conclusion!
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If there was variation between your prediction and the actual population, there could be a variety
of reasons for that. One reason might be carrying capacity. This factor, represented by K in a
population growth equation represents the idea that resources are limited in an environment and
populations cannot continue to increase at an exponential (or ever increasing rate) indefinitely.
Thus, K represents the maximum sustainable population. If your actual population is lower than
your predicted population, we can calculate K in the following way:
1. Experimental data for a long-term control jar is provided below. Complete the mathematical
procedures below the jar.
A control jar started with a population of 250 Volvox. Based on data, predicted population
growth (Gp) was 580 plankton per mL. You will need to compare the predicted to the actual Ga
of 337 plankton per mL.
4. Divide Ga by Gp to get the percentage by which the predicted population growth rate was
reduced due to carrying capacity (K).
= Ga / Gp
5. Subtract the % reduction (as a decimal) from 1 to determine the available growth allowed in
your population under K.
Available growth = 1 - % reduction
6. Determine K by dividing last week’s population (N1) by the available growth you determined in
the previous step.
K = N0/ available growth
Jar
Gp
Ga
(N2 – N1)
% reduction
(Ga / Gp)
Available
growth
(1 - % reduct)
K
(N0/ av.
growth)
Control
Different lab groups gathered data on different abiotic factors. Each group should take a few
moments to share what they found about their phytoplankton populations with the class. How
did different abiotic factors influence the carrying capacity of phytoplankton populations?
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Discussion Questions:
1. The growth factor r is used to predict population growth over a period. What two aspects of
a population are included in r?
2. When you use the population growth equation G = rN, you are figure out how many
individuals are added to the population. What do you need to do if you want to figure out the
actual size of the population?
3. Imagine a population that continues to grow at the same growth rate r over several
iterations. What will happen to the population size- will it continue to grow steadily, at a
decreasing rate, or increasing rate? Why do you think so?
4. What is carrying capacity and how does it influence population growth?
5. What did you find out about the impact of abiotic factors on phytoplankton populations?
6. An ecosystem includes both biotic and abiotic factors. How does this help you explain your
leaf pack data?
7. How do graphs and other mathematical tools help you to evaluate your hypotheses?
8. What did you find out about the relationship between different abiotic factors and the
carrying capacity of plankton populations?
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9. Macroinvertebrates often eat phytoplankton and zooplankton. You found out about abiotic
factors and plankton today and about macroinvertebrates living in larger bodies of water last
week. Could you combine this information to develop a new hypothesis and experiment to
learn more about freshwater aquatic communities?
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