Many people find their CYA level dropping to 0 over the winter. Though it is possible that algae eat it for food, what IS known is that there are anaerobic bacteria that normally live in soil that definitely consume CYA. It is very possible that at least for pools where the FC gets to 0 that these bacteria consume the CYA. If they do, then they produce ammonia as a by-product which would mean needing LOTS of chlorine to open a pool.

Some users report the CYA dropping over the winter even maintaining chlorine levels. This happened to me this winter going from 20 to 10 (approximately) and I had no dilution (I have an automated opaque pool cover), but in past winter's I hadn't seen this and I've maintained chlorine levels every winter.

Richard