 bacteria
Bacteria (Nitrosomonas and Nitrobacter) dont die that quickly. But you can knock back your numbers by allowing a filter to sit without sufficient air supply. No probs for a few hours but several days not advisable. You wont kill everything but best to keep your filter running. The bacteria converting nitrite to nitrate tend to be most oxygen sensitive. Twenty four hours is unlikely to lead to any significant problems. Over the winter same thing happens. You will loose some bacteria from your system, but they quickly recover as the temperature rises and you begin to feed your koi again. Assuming you dont heat your pond of course. . Generally these types of bacteria also have an ability to go dormant to some extent which helps their survival, particularly when food source is low. | 
