A researcher investigated how light intensity changes with water depth in a freshwater lake, and how this affects the growth of green algae.
The table below shows the results of the investigation:
| Depth (m) | Mean light intensity in lux | Mean mass of algae in g/m3\text{g/m}^3g/m3 |
|---|---|---|
| 0.5 | 3200 | 15.4 |
| 1.0 | 2100 | 10.2 |
| 1.5 | 1300 | 6.5 |
| 2.0 | 750 | 3.1 |
| 2.5 | 300 | 1.1 |
| 3.0 | 80 | 0.2 |
State one variable the researcher should have controlled to make sure the light intensity measurements could be compared.
