Why does it happen?
Basically, there are two types of bacteria in a pond – aerobic (good) and anaerobic (bad). These two types of bacteria are in a constant battle to digest the organic debris that ends up in the pond (twigs, leaves, fish food etc). When the anaerobic (bad) bacteria digests it, it takes a lot longer and produces gases like nitrogen and phosphorus which, in turn, act as a food source for algae. When we add aerobic (good) bacteria to the pond, they overcome to the anaerobic (bad) bacteria and digest the organic debris a lot quicker and don’t create any harmful gases. This is why, when you use bacterial pond treatments you are getting to the source of the problem; rather than just eating the algae, you are starving it of its food source.
The problem is that anaerobic (bad) bacteria can digest organic matter as low as 6°C, whereas aerobic (good) bacteria treatments traditionally work around 10-12°C. This means that all winter your pond is losing the battle against algae and when spring comes, the algae starts to feed.