Stoneflies, Plecoptera spp., lay eggs in freshwater streams and rivers. The eggs hatch into nymphs which live in the water for several years before changing to adults. Stonefly nymphs are known as good indicators of pollution since they are very sensitive to a decrease in the oxygen concentration of the water. A biologist wanted to find out whether stonefly nymphs would be suitable as an indicator of water hardness. She collected samples from 12 streams and obtained values of calcium carbonate concentration from the local water authority. The number of stonefly nymphs and the concentration of calcium carbonate for each of the 12 streams are in the table below.

a. Draw a scatter graph to see if there is a correlation between the number of stonefly nymphs and the hardness of the water.

b. State a null hypothesis for this investigation.

c. Use the data in the table to calculate the Spearman’s rank correlation, rs. Show your working. [4]

d. What can you conclude from your calculation? You will need to consult Table P2.8 on page 504 to help you answer this question.

Table P2.8

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Concentration of calcium Number of stonefly nymphs carbonate / arbitrary units Stream 1 42 17 20 22 2 40 30 4 7 28 12 42 6. 10 55 7 8. 55 75 3 80 10 7 90 11 5 145 12 2 145