LESSON
NUTRIENTS IN WATER: TOO MUCH OF A GOOD THING?
OBJECTIVES
Students will understand how N and P enter into and affect aquatic
environments, and how their presence in local waterways is related to
streamflow and runoff. They’ll learn how to use data they and others have
collected to look for clues about sources of nutrient in streams, and how to
calculate weighted means to predicted expected nutrient levels.
METHODS
Students discuss nitrogen and phosphorus in the environment with the help of a
PowerPoint presentation. They then analyze data on flow and nutrients in
Morrell Creek, creating graphs in Excel and using those to answer questions
about the locations of nutrients entering the stream and how nutrient levels are
related to flow and turbidity.
MATERIALS
Grade level: 9-12
o Nutrients in Water PowerPoint
Subject Areas:
o Access to Microsoft Excel
Biology, Chemistry
o Morrell Creek data in Excel worksheet
Duration: 2-4
o Instructor and Student pages from the end of this lesson
class periods
Optional Extension/Experiment:
Setting:
o Glass jars (number depends on experiment, but probably
Classroom
want to have at least 2 for every student pair or group)
o Surface water from a local source
o Plant fertilizer(s) or other nutrient source. (The type(s) you
use will again depend on the experiment chosen; see Extension section for
details. Whatever fertilizer you use must dissolve rapidly in water.
Avoid any product that might turn the water a different color, which may
interfere with test results.)
o Phosphorus, Nitrogen, and Ammonia test kits or strips
BACKGROUND
Phosphorus and nitrogen are both elements commonly found in nature and
nutrients essential for plant growth. Although naturally occurring, they are also
produced by humans in fertilizer and waste products from agriculture,
manufacturing, and sewage. They can enter bodies of water during runoff.
Nutrients like N and P are necessary for plants to grow, but excessive amounts in
aquatic systems can cause eutrophication, or overgrowth of aquatic plants.
Eutrophication in turn can lead to clogged waterways and water systems;
decreased water clarity; low levels of dissolved oxygen; loss of sensitive species
with higher oxygen demands; declines in quality of drinking water and higher
water treatment costs; and even toxicity due to some forms of nitrogen, or to
secretions from microorganisms and cyanobacteria (blue-green algae).
Aquatic plants rapidly take up nutrients, and algae usually outcompete other
plants for available nutrients. Because algae are able to assimilate large amounts
of nutrients quickly, we often see an explosion of algal growth in water that has
recently had an increase in nutrients. Aquatic plants convert nutrients to their
organic forms. The nutrients are no longer detectable by tests for available
forms, although they are still present in the system. The concentration of
nutrients that are available for plant growth can vary dramatically in streams
and lakes during the year. It is thus important to collect nutrient samples at
different points in time to understand the levels that may be available before
they are taken up and converted to plant and animal biomass and other organic
matter.
Rough estimates of nutrient levels can be made with inexpensive tests, but
precise measurements needed to understand nutrient levels that can be
biologically important, are expensive, time-consuming, and require special
equipment and complex laboratory analyses. Other water quality
measurements, however, are often strongly correlated with nutrients levels,
meaning that as one increases, so does the other. Sometimes these measurements
can be used effectively as surrogates for nutrient measurements because they can
be measured continually in a relatively inexpensive and easy way.
PROCEDURE
PART 1
Go through the Nutrients in Water PowerPoint slide show with your students,
making sure to give students time to respond to prompts and questions as you
go along.
Questions for Discussion

What are sources of nitrogen and phosphorus pollution in freshwater?

What is the chemical formula for nitrate? For phosphate?

When might excess nitrates in the water not cause excessive algal growth?
(Hint: What else is needed for plant growth? Is there much sunlight in
deep water? Are temperatures important? If one nutrient is high can
others still be limiting?)

What organisms produce nitrates as part of the nitrogen cycle?

What do you think a test for nitrogen levels would show before and after
algal growth in a body of water, if there have been no new inputs to the
water? Why is this significant?

What are the concerns in Seeley Lake regarding contamination of ground
water with nitrate?
PART 2
In this section, your students will first come up with a question that can be
answered using the data on nutrient levels collected at the four sites along
Morrell Creek. They are asked to think of this as a “natural experiment” and to
develop a question/hypothesis that can be addressed with the data. They may
come up with a variety of questions (nutrient levels at different dates, which
nutrients are higher, etc.), but the one that would have real interest (why would
we care about finding the answer?) would be something like: Do P and N levels
vary from site to site in Morrell Creek? If so, how?
You may want to start this all together as a class, using the PowerPoint and/or
having students open the data file, and use class discussion to come up with the
questions and identify independent and dependent variables. Once they’ve done
this, the PowerPoint and their Student Pages explain how to create a column
(bar) chart to get them started. Have your students use the Excel file Morrell
Creek Nutrient Data to create graphs of nutrient levels found at the four Morrell
Creek sampling sites. They should end up with 12 different graphs, each
showing the levels of one nutrient on a given date and the four different sites.
You may want to have them write down their observations of the charts and any
explanations they come up with, as well as additional questions it might bring
up, and then discuss these in class.
Discussion
Describe any patterns you see for each of the nutrients at the different sites and at
different dates. What do these patterns suggest, if anything? What further
questions do you have about nutrients in Morrell Creek after looking at your
graphs?
They should reach a few conclusions:
Except for one case with orthophosphate Site 2 is always substantially higher
than the other sites. This suggests a source of nutrients entering the creek near
this site. Nutrient levels at Site 4 (the lowest or most downstream site) are almost
always higher than Site 3 (High School Site). This suggests additional nutrients
entering the creek somewhere between the High School and highway 83. Where
could these nutrients be coming from? At this point you may want to share the
locations of the sampling sites, and the fact that there is a groundwater seep at
Site 2.
PART 3
Are there any more clues to where nutrients in the lower creek are coming from?
Calculating the levels of nutrients entering into the creek from all the tributaries
can help figure out if they are coming from the tributaries or somewhere else.
Your students can use the second sheet in the Excel file to calculate the expected
weighted means of N for lower Morrell based on tributaries from the high school
to Trail Creek, and compare them to what was actually found. To calculate
weighted means: add the products of the N level and mean flow for each
tributary, and divide that result by the sum of the mean flows for all tributaries:
((Morrell N*Flow) + (Trail N*Flow) + (Mountain N*Flow) + (Blind Canyon
N*Flow)) / (Morrell Flow + Mountain Flow + Blind Canyon Flow)
You can use the PowerPoint to help them figure these out or to make sure
they’ve done things correctly. When they’ve calculated the expected N levels,
they should compare them to the levels actually found at Site 4.
PART 4
Finally, they will use scatterplots and regression equations to look for any
correlations between nutrients and other water quality measurements. Since
Seeley Swan High School students have been collecting several water quality
measurements, see if any of the other measurements can be used as an indicator
of nutrient levels. To do this, they’ll need to see if there are any correlations
between nutrients and other measurements. Do students understand what is
meant by a correlation between two things? If a change in one variable allows us
to predict how the other will change, there is a correlation. It can be positive or
negative.
Have them start by opening the sheet labeled Nutrients and Flow in the Morrell
Creek Nutrient Data file. Do they know what kind of graph would help them
look at relationships between two variables? A scatterplot shows how pairs of
data points vary in relation to one another. You can use the PowerPoint to go
through the example of plotting flow and turbidity to see if these two variables
are correlated, and discuss what the regression equation and R-squared value
means. When you feel they understand, have them create some more
scatterplots to look for relationships between other pairs of data.
Discussion
What did you find? Does this give you any ideas about future water quality
monitoring in Morrell Creek? Summarize your findings about nutrients in
Morrell Creek and propose a 10-year monitoring plan for Morrell Creek.
OPTIONAL EXPERIMENT
When you have finished, ask your students how they would design an
experiment to test the effects of N and P on plant growth in local surface water.
What very specific questions can they come up with related to this topic? If
necessary, prompt them to think about rates of growth, amounts of nutrients, etc.
Tell them they need to think of questions that they can answer by doing an
experiment in the classroom, using materials that would be readily available.
Potential questions they might try to answer with a simple classroom experiment
might include:
 Do aquatic plants grow at different rates when nutrients are added?
 How long does it take to see a change in growth after nutrients are added?
 Do different types of plants respond differently to added nutrients?
 Do different forms of N have different effects on plant growth?
 Do different amounts of nutrients affect plant growth?
 Do N and P have different effects on plant growth?
All of these questions could be addressed experimentally using very simple
setups and materials: glass or clear plastic jars, surface water from a local source,
and commercial fertilizers. You can purchase fertilizers that are N only, P only,
or combinations of both (Ace Hardware, local nurseries, and online sources).
Before performing their experiments, your students should be able to clearly
articulate their experimental methods (use Student Pages at the end of this lesson
and have them turn in their proposed experimental design to you for prior
approval/discussion). Depending on your students’ confidence and ability
levels, you may want to guide them through an initial experiment. The basic
experimental protocol can be:
1. Collect enough water from a single source to divide into equal portions of
approximately 2-4 cups (whatever will fit in the glass containers you
have).
2. Test the initial levels of N, P and Ammonia in the water.
3. Add nutrient source(s) to the water in the jars. Students could add
nutrients to one jar and not another; add N to one jar and P to another;
add nutrients from different sources to different jars; add different
amounts of nutrients to different jars, etc.
4. Monitor and record levels of algae in the containers. This may have to be
a somewhat subjective measure (estimating % of jar wall covered by algae,
estimating opacity using the High School turbidimeter, or by counting
algal cells under a microscope.
STUDENT PAGES
NUTRIENTS IN MORRELL CREEK
Science is all about solving mysteries. Often the mystery is like a puzzle; you
need to look at a number of pieces and fit them together until you begin to see
what the entire picture looks like. The data you have been collecting when you
sample Morrell Creek are like the pieces of a puzzle; they can help you solve
mysteries related to the health of Morrell and other creeks and lakes in the Seeley
area. One of those mysteries concerns the levels of nutrients in those bodies of
water. How high are they? Are they higher in some places than in others?
Where are they coming from? Are any of them high enough to influence the
condition of the streams and lakes or to pose a threat to the health of Clearwater
streams and lakes? What might cause the levels to increase or decrease in the
future? Let’s use data students at Seeley Swan High have collected to address
some of those questions.
Open the Excel file Morrell Creek Nutrient Data. Click on the tab at the bottom
left for Morrell Sites 1-4.
These data show the results of the nutrient sampling done at 4 different sites in
Morrell Creek. The sites are numbered 1-4, with Site 1 being the most upstream
site in the creek and Site 4 being the most downstream site. Samples were taken
on 4 different dates at each site. Levels of phosphate (in the form of dissolved
orthophosphate), total nitrogen, and nitrites and nitrates (NO2 and NO3) are
reported for each site on each date. (Where there is no value, the level of the
nutrient was too low to be detected by the test used.)
Can you think of a potentially important question you can address with these
data?
We can think of this as a “natural experiment”—one where we collect data from
different conditions (locations) in the environment rather than creating those
different conditions ourselves. Can you ask a question (or state a hypothesis) for
this experiment? In this experiment, what would be the independent variable(s)
and what would be the dependent variable(s)? What type(s) of graphs would
you use to look for patterns in these data that might help answer the question?
You can create a series of graphs using Excel to
answer these questions.
1. If you’re not familiar with graphing in Excel,
start with a sample graph. To look at the levels
of N at the four different sites on 3/4/2013, you’ll
create a bar chart.
2. Start by clicking on an empty cell in the datasheet, then click on the Insert tab,
then the Columns chart type. Select whatever type of shape you want your
columns to be from the drop-down menu.
This will open a blank graph rectangle in
your sheet.
3. Now click on Select Data from the chart
toolbar. (You can always get the Chart
toolbar by clicking on your chart rectangle.)
A new dialogue window will open. In this
window, click on the blue Add button in the
Legend Entries box. Another small window
will open. This will allow you to select the data for your
y-axis, or your dependent variable. In the Series name
box, you can type in the name of the data series (such as
N levels 3/4/2013), or you can click on the Nitrogen
heading and it will use that as the name. The Series
values box is where you will indicate the actual data to
use. If you click on the button to the right of that box,
the box contents will be highlighted and replaced with
whatever you select. Use your cursor to select the four
boxes that show the levels of Nitrogen found at the four
sites on 3/4/2013. Click on the button to the right again
and then click OK. You’ll be back to the first dialogue
window.
4. Now you’ll select the x-axis data, or independent variable. Click on the Edit
button and when the next window opens, use your cursor to select sites 1-4 in
column B. Click OK in this and the next dialogue window. Your bar graph
should now look something like this:
You can change your chart
design, layout, and format;
add axis titles, etc., using
the Chart Tools toolbar that
opens when you click on
the chart.
5.
Now you can create
more charts comparing
levels of different nutrients
at different sites or on different dates, to address the original question. When
you have done this, describe any patterns you see for each of the nutrients at
the different sites and at different dates. What do these patterns suggest, if
anything? What further questions do you have about nutrients in Morrell
Creek after looking at your graphs?
6. Site 4 is located on lower Morrell Creek, near Highway 83. What do you
notice about nutrient levels at this site? Are they higher or lower than those
at the site above it (Site 3), which is the high school sampling site? How does
site 2 (the seep) differ? What does this suggest about groundwater as a
source of nutrients? Do you think all ground water would be similar? Where
might nutrients be entering the stream between the high school and the
highway?
Nutrients in a stream can be the sum of inputs from its tributaries, or there can be
other sources of nutrients entering a stream as well. One way to estimate what
levels of nutrients you would expect to find at a certain point in a stream is to
calculate the amounts each tributary above that point contributes.
For lower Morrell Creek, this would include Trail, Blind Canyon, Mountain,
Swamp and Drew Creeks. Data on N levels in all of these streams except Swamp
and Drew have been collected on or near three of the dates for which we have
the Morrell Creek data (see map to see where these samples were collected).
What other kind of information would you need to know, in addition to the
amount of N per liter of water, to calculate how much N that stream is
contributing to the system?
Because different tributaries are different sizes, we need to take that into
consideration as well, weighting each amount of N per liter by how much water
is in the stream. In other words, if a tiny stream has a pretty high concentration
of N, it’s not going to add the same amount as a larger stream with the same
concentration of N, just because it’s only adding a small proportion of the overall
amount of water.
We can use the estimated mean flow and N concentrations in samples from each
stream last spring to see what we might expect if N levels in Morrell Creek at the
highway was simply the average of all its tributaries above lower Trail Creek.
Then we can compare that to what was actually collected at the lower site (#4).
Since we do not have N levels for Swamp Creek, at this point we can assume it
would be equal to the average of all other streams unless we have any reason to
think otherwise.
In the Excel file, click on the tab at the bottom left named Weighted Means.
First you’ll need to fill in the data that are missing: the N (ug/l) measured at
Morrell Creek site 3 on the given dates. When you’ve added those values, you
will need to figure out how to calculate the weighted means. What formula do
you think you’d need to use to estimate how much N all these streams together
contribute to lower Morrell Creek?
You can use Excel to do the calculations for you if you want. Click on the first
cell under the Weighted Mean column. You can type your formula in the cell,
entering values from the other cells simply by clicking on them. For example, if
you wanted to add the mean flow values for Morrell and Trail Creeks you would
do the following:
1. Type =
2. Click on the cell with the Morrell mean flow
3. Type +
4. Click on the cell with the Trail Cr mean flow (You should now see in the
new cell: =C3+E3.)
5. Hit Return and the result of the formula will show up in the cell.
As in a written equation, you’ll need to put parentheses around terms that need
to be performed first.
When you have done your calculations or had Excel do them, and the estimates
of the weighted means—the amount of N you would expect to find in lower
Morrell based on what’s coming in from those tributaries—are in the cells under
the Weighted Means column, you can enter the actual means—the levels of N
observed in the samples—in the last column for comparison.
What did you find? Are the observed samples lower or higher than expected?
What do you think this indicates? What additional questions does it raise?
Nutrients and other water measurements
Nutrient levels in water are often expensive, time-consuming, and difficult to
accurately measure, requiring special equipment and complex laboratory
analyses. Other water quality measurements, however, are often strongly
correlated with nutrients levels, meaning that as one increases, so does the other.
Sometimes these measurements can be used effectively as surrogates for nutrient
measurements because they can be measured continually in a relatively
inexpensive and easy way.
Since Seeley Swan High School students have been collecting several water
quality measurements, let’s see if you think you can use any of the other
measurements as an indicator of nutrient levels. To do this, you’ll need to see if
there are any correlations between nutrients and other measurements.
1. Open the sheet labeled Nutrients and Flow in the Morrell Creek Nutrient
Data file. You’ll see columns of values for Q (Flow), Turbidity, TSS (total
suspended solids), Total P, Total N, and NO2+NO3 that were collected on
the same dates throughout 2012 and 2013. What type of graph would you
use to look for correlations between these variables?
2. Let’s start with a scatterplot that will help us see if there seems to be a
relationship between flow and turbidity:
a. Under the Insert tab, select the Scatter with only Markers option
under the Scatter chart drop-down menu.
b. Click on Select Data to open the dialogue box. As before, use the
Add button to create a title for your graph and to select the x-axis
and y-axis data.
c. In this case, you’ll select the flow (Q) values for the X values and
the Turbidity values for the Y values.
d. What does your chart look like? Do you see any relationship
between flow and turbidity in these data?
e. Besides using our eyes to estimate, we can also create a trendline
and calculate the relationship mathematically with a regression
equation. Click on the Trendline box under the Chart Layout tab.
Select More Trendline Options at the bottom. In the window,
select Linear trend type and check the Display Equation and
Display R-squared value boxes at the bottom, then click Close.
f. The regression equation shows you how to find a y-value based on
the x value. The R-squared value tells you how strong the
correlation is. This value can range between 0 and 1, with zero as
no correlation and 1 as a perfect correlation (all the points would
fall directly in a straight line). Using this graph you’ve created,
what would you predict the Turbidity to be at a flow of 200 cfs?
How confident are you in that prediction based on the strength of
the correlation (the R-squared value)?
3. Now create some more scatterplots to look for relationships between other
pairs of data. You might also try fitting different kinds of trend lines
(power or exponential) to some of the graphs. What do you find? Does
this give you any ideas about future water quality monitoring in Morrell
Creek? Summarize your findings about nutrients in Morrell Creek and
propose a 10-year monitoring plan for Morrell Creek. How can you share
what you’ve found with others in the community?