Re: How can I measure phytoplankton concentration in water?

I realize a glitch in the answering system may have delayed this beyond 
usefulness, but here is your answer...

You can measure phytoplankton concentration using a spectrometer that 
measures absorbance of visible light.  Find out which wavelength would be 
suitable for green or red, and measure there.  Take a background reading of 
the water or growth medium alone and zero the instrument, then take a 
measurement of your phytoplankton culture.  The spectrometer we commonly 
use for this is called a Spec20, and most high schools have this or a 
similar piece of equipment.  You will need to transfer some of your culture 
to a test tube to perform the measurement (unless you have a flask with a 
test tube arm).  You can sterilize the test tube with alcohol beforehand to 
prevent contamination (70% ethanol in water - swirl around for 30 seconds, 
then dump and allow to dry).  This way, you can minimize contamination when 
you pour the culture back into the larger flask. 

Good luck!