How would you investigate the effect of light intensity on the rate of photosynthesis in pondweed? [6]

Apparatus is set up by first adding the pondweed to a beaker filled with water. The beaker has been sealed and any gas produced by will be collected by a syringe. Light will be provided by a lamp and its light intensity can be controlled by moving the light source further away from the pondweed and following the inverse square law. For a given light intensity, the rate of photosynthesis will be measured by measuring the volume of gas displaced in a set amount of time.Using the same setup with the same pondweed, the light source will be moved back by a known amount, allowing time for the pond weed to acclimatise to the new light intensity and then measurements are taken. A set of pondweeds will be experimented on so the experimental and a measurement of the rate of gas displacement with no light source will also be measured as a control.