My pool water is 71*. I am adding 14oz of 10% bleach daily and maintaining FC of 2. My question is, at what temp will the possibility of algae stop and will I have to add chlorine even if the water is too cold for algae to grow?
During the winter, I add chlorine about once every few weeks to a month as the chlorine usage drops to < 1 ppm FC per week at colder temps. Though you can ignore the chlorine level when the temps are cold, it's a bit safer to check on it once in a while and boost the FC as needed if that's not hard to do -- at least when a pool isn't completely closed (i.e. it isn't drained down at all and the circulation pump is still used every day though for less than a turnover).
Figure 1 in this paper gives a rough idea of the maximal algae growth rates as a function of temperature. The generation time, which is how long it takes to double in population, is a little over 5 hours at 86ÂºF but drops down to a little over 12 hours at 59ÂºF. Algae can still grow even near freezing temps, but the growth rate is much slower (someone on The PoolForum saw algae slowly growing under their frozen-over pool!). Note that these numbers are maximums assuming ideal nutrients and sunlight. This link in Figure 3 shows growth rates are temperature-dependent in a very species-specific way.
In practice, when the water temp gets colder, the amount of light is also reduced as more days are cloudy and the sun is lower in the sky. This one-two punch significantly slows down algae growth rates. If one also covers the pool with an opaque cover, then that cuts down sunlight even more.
Of course, if there is any chlorine or algaecide (including borates) leftover in the pool, then that can inhibit algae growth as well.
I live in northern California. Although the night temperatures had been in the low 40's to mid 30's during the past 2 nights, we still had a lot of sunshine during the day. The chlorine consumption in my pool is still about 1.0 ppm per day for the past 2 days. It was about 1.5 to 2 ppm per day in the summer. I keep FC at a minimum of 7 because I had mustard algae twice this year. The question is: should I maintain FC at 7 in the winter or can I go lower? The pool stays open year round and I don't want to battle mustard algae again. The water temperature this morning was 60 degrees.
Though the growth rate of algae slows down as water gets colder, the reaction of chlorine killing algae also slows down. So lowering the FC level too much could be risky. The main advantage of the cooler water, especially if the sun isn't on the pool due to clouds or a cover, is the lower daily FC usage. I'd keep the FC at whatever level you normally need to do to prevent algae. Though it's possible that at some cold water temperature the algae growth plummets, why take the chance that it doesn't?