The table below shows measurements made at four sites along a river after a warm summer. Students used a probe in a sample tube to measure dissolved oxygen....
Question 1 Report
The table below shows measurements made at four sites along a river after a warm summer. Students used a probe in a sample tube to measure dissolved oxygen. They then waited for fish to move through a marked 20 m section before recording the number of trout. Trout use oxygen carried in water across their gill cells and into the blood. At site D, a factory outlet releases warm water into the river. All counts were made at the same time of day.
| Site | Water temperature / degrees C | Dissolved oxygen / mg dm-3 | Number of trout seen |
|---|---|---|---|
| A, upstream | 12 | 10.8 | 18 |
| B | 15 | 9.4 | 14 |
| C | 18 | 7.1 | 7 |
| D, near outlet | 23 | 4.8 | 1 |
(a) Describe the relationship between dissolved oxygen concentration and the number of trout seen. [2]
(b) Explain why very low dissolved oxygen can reduce the number of trout. [3]
(c) Suggest two variables, other than time of day, that the students should keep the same to make the fish comparison fair. [2]
Answer Details
(a) As dissolved oxygen concentration decreases, the number of trout seen decreases. For example, at 10.8 mg dm-3 oxygen there were 18 trout, but at 4.8 mg dm-3 there was only 1 trout. [2]
(b) With very low dissolved oxygen, less oxygen diffuses across the gills into trout blood. Less aerobic respiration can occur in body cells, so less energy is released for movement, growth and survival. This can reduce trout numbers. [3]
(c) Keep the length or area of river counted the same and use the same duration for each count. Other acceptable controls include the counting method, number of observers, weather conditions, water depth or flow rate. Any two are required. [2]
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