Quote Originally Posted by sailork
Richard, I've never understood how we can tell how much chlorine is lost through direct action of sunlight and how much is consumed by algae. Given that algae photosythesize I've always assumed they consume most of their chlorine when the sun is shining...
I am basing my assumption from the experience of users in the algae threads where it appears that algae continue to consume chlorine overnight and that when this stops (which is some time after the water is no longer green and is either cloudy and getting filtered or is near clear), then this means the algae is all killed.

The algae continue to metabolilze (though possibly at a lower rate) at night -- it's just that they oxidize stored organics and "exhale" (so to speak) CO2. During the day, the algae use photosynthesis to build up the organics as "food" and take in CO2 in this process and "exhale" O2.

The oxidation of nitrogenous organics and ammonia also occurs both day and night. So the overnight rate may underestimate the non-sunlight sources of chlorine consumption due to lower chlorine usage from algae algae at night vs. day, but it will at least give one some minimum number to work with.

Richard