Sometimes in talking to a pond owner who’s kept their water feature open all winter, it comes as a surprise to them that algae can grow, even in the coldest conditions.
As I always do, if they ask how it should be treated, I suggest using a good quality pond bacteria, and one that is specifically designed for cold water, and see if that helps the cause of cleansing. Sometimes it does, but you can be sure that any warm water bacteria will not.
The normal tools, like your biofilter will simply not be operating normally in cold weather so other than perhaps filtering physical debris, it’s not going to do much for the water quality as it would in the warmer months. Many people may simply decide to bypass the filter or shut the flow of water down altogether, and that’s fine.
Keep in mind, it’s not that the cold weather kills off these good microbes, it’s just that they go dormant.
So the question comes up, “do I really need to add any bacteria over the winter?”