The question
Name: ______________________ Date: ____________ Dissolved oxygen downstream of farmland (9 points)
Data table
Nitrate and dawn dissolved oxygen at five stream sites
A stream flows through a forest (site A), then past fertilized cornfields (site B), and on through more forest (sites C to E). Students measured nitrate and dissolved oxygen (DO) at dawn at each site on five mornings in a dry week of July, and again on five mornings in July two weeks after a week of heavy rain. Water temperature was 18-19 °C at every site on every morning. Values are means ± 2 SE (n = 5 mornings).
| Site (distance along stream) | Nitrate, dry week | DO, dry week | Nitrate, after rain | DO, after rain |
|---|---|---|---|---|
| A: 2 km above farms | 0.6 | 8.6 ± 0.4 | 0.8 | 8.4 ± 0.4 |
| B: beside farms | 1.8 | 8.0 ± 0.5 | 4.5 | 6.9 ± 0.6 |
| C: 2 km below farms | 2.4 | 7.4 ± 0.5 | 6.2 | 4.1 ± 0.7 |
| D: 5 km below farms | 2.0 | 7.6 ± 0.4 | 5.0 | 3.2 ± 0.6 |
| E: 10 km below farms | 1.5 | 8.1 ± 0.4 | 3.1 | 6.0 ± 0.6 |
(a) Describe how nitrate washed from the cornfields can increase the growth of algae in the stream. [1 point]
(b) Construct an appropriately labeled graph that shows the mean dissolved oxygen at dawn at each site for both sampling periods, including error bars. [4 points]
(c) (i) Justify the students' choice of site A as part of the study.
(ii) Using the error bars, determine whether dissolved oxygen at site D differs between the dry week and after the rain, and justify your answer. [2 points](d) (i) Nitrate after the rain was highest at site C, but dawn DO was lowest at site D. Explain why the lowest oxygen occurs farther downstream than the highest nitrate.
(ii) The farmers plant 20-meter strips of grass and trees between the fields and the stream. Predict the effect on dawn DO at site D after heavy rain, and justify your prediction. [2 points]