hard · Enhanced ACT science
Researchers measured dissolved oxygen (DO, mg/L) at the surface of a lake at different times of day across two seasons:
| Time | Summer DO | Winter DO |
|---|---|---|
| 6 AM | 6.0 | 11.5 |
| 12 PM | 9.5 | 11.8 |
| 6 PM | 8.0 | 11.6 |
| 12 AM | 5.5 | 11.4 |
Photosynthetic algae are abundant in summer and nearly absent in winter (frozen surface, low light).
The daily DO swing in summer (6.0 to 9.5 to 5.5) but near-constant DO in winter (11.4 to 11.8) is best explained by:
- Cold water simply holding more oxygen
- Summer algae cycling oxygen day and night
- Winter ice trapping a fixed oxygen level
- Instruments reading warm water less accurately
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