Marie-
I was starting to wonder about the pollen. I just checked the skimmer sock that I replaced yesterday and it was PACKED with what I'm assuming is pollen skimmed off the water. Pollen is an organic material...maybe that's contributing to the chlorine loss. It dropped 1 ppm overnight, but I've lost around 12 ppm in 24 hours. However, the water is crystal clear.

I read Ben's spring start up tips:
http://www.poolforum.com/pf2/showthread.php?t=190
Although I didn't lose all my cya over the winter, I did drop from 100ppm to 60ppm. I wonder if that is contributing to the radical loss of cl.