A biologist has a small pond containing goldfish in her garden. On a sunny day, more pondweed is at the surface of the water but on a cloudy day, less pondweed is at the surface. Suggest an explanation for this observation

The intensity of light is greater on a sunny day, therefore the rate of photosynthesis in the pondweed is greater during these days. Hence, more oxygen is produced which thereby increases the bouyancy (makes the pondweed float to the surface of the pond) on the lighter days.