For no.2, there's multiple way to do this. The most simple way, but less accurate way, is to count the number of gas bubbles the plant produces in a period of time, then calculate the rate, like how many oxygen bubbles are produced in 1 minute.
The more accurate way is to use the downward displacement of water method to find the amount of gas bubbles produced in a period of time. This method requires us to put an inverted funnel inside the water, on top of the plant, and making sure no air is inside. Then just measure how many gas area appeared in the inverted funnel after a period of time. Again, then calculate the rate.
For no.3, some of the environmental factors include the light intensity, the temperature of water, the amount of carbon dioxide avaliable in the water etc.
