ML4-PRAC_Chlorophyll in lakes
Data Literacy Project
Chlorophyll in lakes
Freshwater algae grow naturally in Maine’s lakes. Scientists hypothesize that most
freshwater algae need phosphorus to grow. In lakes that are not surrounded by human
development, phosphorus is often limited. In these phosphorus-limited lakes, algae do not
tend to grow out of control, and the lake stays in balance.
Human activities around a lake often generate excessive amounts of phosphorus that can
run off into lakes. Scientists hypothesize that the increased phosphorus can cause algae to
grow out of control. When algae multiply, they can block light and cause a green scum that
may eventually deplete oxygen in the water and make the lake unhealthy for fish and other
aquatic life.
Many lakes in Maine are monitored every year to see if there are any changes in chemistry,
and to see if there are any potential problems with too much algae. The concentration of
algae is measured by the amount of chlorophyll-a found in the water.
The table below gives concentrations of chlorophyll-a and of total phosphorus measured in
several lakes from the Belgrade Lakes region. Concentrations of both chlorophyll-a and
phosphorus are measured in parts per billion (ppb).
Lake Name
Chamberlain Pond
Long Pond
McGrath Pond
Salmon Pond (Ellis Pond)
Messalonskee Lake
Great Pond
Joe Pond
Penny Pond
Wellman Pond
Total phosphorus
concentration
(ppb)
18
8
11
14
12
9
16
14
54
Chlorophyll-a
concentration (ppb)
12
5
3
6
4
5
8
2
23
1. Make a graph that shows whether or not there is a relationship between phosphorus
concentration and algae growth.
2. Explain how the graph supports or refutes the hypothesis that algae need phosphorus to
grow.
ML4-PRAC_Chlorophyll in lakes
ML4-PRAC_Chlorophyll in lakes
Teaching notes:
Chemistry, ecology
Correlation
Although the same relationship will appear with chlorophyll and phosphorus on either axis, in this
context total phosphorus is most likely the independent variable because the algae are probably
growing in response to more phosphorus being available as a nutrient.
Explanations might include the following points:
More chlorophyll-a means there are more algae in the water because algae contain
chlorophyll. (Chlorophyll-a is a proxy for algae growth).
There is a positive relationship between algae and phosphorus concentrations because
the line of best fit slopes upward to the right. That means the more phosphorus there is,
the move chlorophyll-a there is.
There is more variability at lower concentrations of phosphorus (ie. less than about 12
ppb). The low points are more scatted from the line of best fit.
One lake (Wellman Pond) had very high concentrations of both P and chlorophyll-a
compared with the other lakes. (Make sure students don’t throw out this “outlier”.)
For extended discussion:
1. What human activities can cause excess phosphorus to run off into lakes?