ED50 represents the effective dose where 50% growth is inhibited in relation to control. ED50 of any chemical can be calculated by growing and measuring the growth of the algae or related organisms like Spirulina in the presence of graded concentrations of the chemicals.  It can be expressed in terms of percent inhibition in relation to control.

A good guidance to follow is OECD Test No 201 (http://www.oecd-ilibrary.org/environment/test-no-201-alga-growth-inhibition-test_9789264069923-en)

http://www.oecd-ilibrary.org/environment/test-no-201-alga-growth-inhibition-test_9789264069923-en