My cya falls to almost 0 over every winter in NE fl. With cold temps I don't do too much but maintain cl and pH levels, the cl stays high with little work, and the pool is relatively clear all winter, we dont' get enough rain to dilute it out that much . I usually end the swimming season with a cya level of about 50-60. I'm not sure where it goes, but I assume it's getting consumed either chemically or biologically. I usually wait until all the oak pollen is done and the oak pollen pods are done before adjusting. I have to use a lot of liquid cl to get the stain from them out of the water and don't want cya levels making it necessary for more cl than need. After things are done falling from trees, I jack the cya back to normal and things stay pretty good all summer and fall. I many need to add smaller amounts of cya over the summer but not start from 0 like in the early spring. I just consider it a fact of life with an outdoor pool and no screen enclosure.