It’s 25F in the yard this morning headed for a high of just 27F (Minnesotans, don’t laugh), and the air temp shouldn’t reach freezing until a warm front comes through Tuesday, meaning five consecutive days below freezing for us. The water temp has fallen to 40.3, close to the turnover point.
Turnover occurs at 39 degrees, the point at which water is densest. Above turnover, fish find the warmest water at the top of the pond; below the turnover point, the warmest water lies at the bottom. This phenomenon allows aquatic life to winter, as the warmest water settles next to the warmer ground and the coldest water rises to meet the frigid air, where it freezes, forming a solar heating barrier. Light comes in, and heat is trapped. Algae lives, and predators are barred.
The one problem we ponders face, however, arises from our puddles’ status as closed systems. Because fresh water does not flow in and out of our water gardens, a complete ice-over traps noxious gases and deprives our fish, torpid though they be, of oxygen. Fortunately, a doughnut heater can keep a small vent in the surface ice without bankruptcy.
I prefer this alternative to putting an aerator in the pond, which many recommend. Airstones force freezing cold air through the water, cooling it down and causing temperature-dropping evaporation as well. It also breaks up the thermal stratification described above, disrupting the safe haven on the bottom. One pond store in our area recommends aeration, and the other finds it unnecessary. The first specializes in larger ponds, the other in smaller setups, so I follow the advice of the latter.
