The basic would be to do cell counts. You can also use optical density or percent transmittance with a spectrophotometer with cell counts to calibrate.

Without the microscope or spectrophotometer, the last choice could be to measure the increase in dry weight of the algae. Do this by taking samples and filtering onto tared filters, drying, and weighing. The change in weight is relative to the cell number.

Hi Mr Smith

Many thanks for your answer