you may be correct about it not "growing" below 50 degrees, however, if some grew while the water was above 50, it can still be there as stagnant or hybernating algae, but it won't "grow" per say.
Otherwise, I'd have infestations by october/november for sure since the shock level of chlorine and any residual chlorine in my pool would be long gone by october.
Besides, deep ponds have algae in them all winter, it just isn't active - like your lawn, it stops growing, however, it's still green, meaning it's still alive, just hybernating.