If you are unfamiliar with the theory of densitometry, that’s is the term you want to look for. The Beer-Lamber (also Lambert-Beer) law links absorbance (=densitometry at a certain wavelength) to a concentration.
The global setup will be a light source behind an aquarium (or a cuvette, a beker with straight, parallel, walls) of which you measure the density/absorbance. The intensity of the light with an empty container will be your reference intensity I0. Without changing the intensity of the illumination, or the settings of your camera, you have your sample in a similar container (ie. same length of path of light from source to detector) and measure the reduction in amount of light, this will be your In. A divide operation gives you the percentage transmission of the sample (0-100%), which can be converted using the above mentioned law into an absorbance value A.
Finally, you need to obtain a molar extinction coefficient for your samples; a calibration standard, which probably you need to make one yourself using a dilution series of a known concentration range. ImageJ then can help you to fit a Rodbard function for obtaining quantified numbers from unknown samples (i.e. your experiment).
Measuring at different (monochromatic) wavelengths (using proper colour filters) on the one hand gives you more precise results and on the other hand might even allow you to distinguish between different populations of algae.
Importance of monochromatic light is mentioned in this article and this one goes over the theory with some formulas. Wikipedia also might be of further help. I have no conflict of interes with either publication.
(@EriRox: edited to indicate importance of constant illumination, and of camera setting which should be of constant location, aperture and exposure time)