Greetings from the Great White North!
Moving into the late fall and winter months, one must adjust our management strategies and prepare for drastically different conditions. Colder water has different chemical and physical characteristics that native fish and other aquatic organisms adjust to readily. It is way easier for a koi/goldfish or bass/bluegill to adjust to cooling water temperatures than the reverse in spring. This is due to higher oxygen levels in cold water and the mechanisms fish have evolved to slow their metabolism during cold water periods. Still, there are a few things you have to do to make sure your scaly friends survive until the warmth and sunshine return in spring.
Large waterbodies slide into winter fairly easy. If you have an aerator (and you should have an aeration system installed), it's time to shut it down for the winter. Another option is to reduce the system down to a single diffuser, which keeps a hole open in the ice. This negates the risk of winter kill due to de-oxygenation and/or the build up of toxic gasses under the ice.
I find that the best strategy is to move a single diffuser into shallow water near shore. This way, the hole created by the mixing water will be contiguous with the shoreline. This is helpful for two reasons:
1.) The pond does NOT need deep water mixing during these times. Freshwater at near freezing temps (33-40deg F) holds 10 times the oxygen as warm water. The hole allows bad gasses like nitrous oxide, methane, and sulfur dioxide to escape to the atmosphere. These gasses are mostly released from decaying sediments and are usually only an issue in old ponds with deep soft sediment (or muck) bottoms. The exposure to the atmosphere will also turn in abundant oxygen to prevent oxygen depletion from a high fish load or those same soft sediments.
2.) Open water during the winter months will lure in wildlife in droves. Liquid water is scarce in winter here in the North, and every creature that is wandering (of fluttering) about needs it to survive. So keeping the hole near shore will allow said wildlife to utilize the pond while remaining safe. If the critter has the misfortune of falling through the thin ice near the edge of the hole, the water is shallow enough for it to scamper out. (I have several experiences trying to fish out deceased critters from a hole open out in deep water.)
You can use aeration or de-icers/watermovers to eliminate the need to remove your dock/pier.
Oxygen depletion is a real concern for shallow garden ponds. The gasses released by decomposing organic material and respiring fish can accumulate rather quickly underneath an opaque layer of ice. Leaving a hole open in the ice is crucial for the survival of your fish.
There are several ways to accomplish this:
If you live in an era of the world where ice doesn't get to bad, go ahead and just run your system as normal. Beware of ice buildup on your waterfall/stream as ice-dams can appear and begin pumping your pond dry.
Notice how the ice is forming on the sides? This blockage can create a path for water out of the pond.... that is bad.... we want water in the pond.... where the fish are..... cuz fish need water.
Pond heaters come in two forms: floating heaters, and submersible tank heaters. Both of these products work very well.
These products use a fair amount of electricity to keep the desired open water.
Adding (or continuing to use) a small aerator during the winter will keep a hole open under most circumstances.
These systems are great! They use less electricity than the pond heaters and should be used during the summer months as well for the betterment of the pond. Be sure to move the diffuser to a planting shelf or other shallow area to reduce the risk of super-cooling the pond. Super cooling is when the temperature of the entire water column decreases to freezing (these are rare, unique events). Over mixing during extreme cold can super-cool the water, allowing ice crystals to form in your fish's gills (which is bad).
When conditions get super cold (below 0 deg F), an ice dome can form over the hole. This will usually subside in a day or two, or you can break the shell ice to keep the hole open until the temperature warms slightly. Aerating can increase evaporation loss as well, you may have to add water as the dry, cold air will be constantly stealing water from you.
Submerged water pump:
Simply use the pump you already have to circulate water to keep an area open in the pond. Either plumb the water from the skimmer area, or remove the fountain nozzle from your filter pump. This moving water is great for keeping the pond in good shape until spring. If your pump is too big for this operation, purchase a smaller filter pump to sit in the pond over winter.
There are a bunch of these to choose from. Pick the brand and size that will work best for you. If you need help picking one out, just let me know.
If you are not using your main pump through the winter, store it inside in a bucket of water so the seals don't dry out.
Keeping your fish alive and well throughout the winter is easy with a little bit of prep. So brace yourself for the cold weather and think warm thoughts.......
As always, feel free to ask questions in the comments section, or contact me directly.