What is happening under the ice during the winter months? Our fish slow down considerably, but what about the rest of the pond? Here in Wisconsin, we typically shut down our ponds later in November before the ice sets in. Certainly, some folks keep their water features running all winter, but the vast majority of pond owners that I know shut their features down just to make everything worry-free and easy over the winter. Because we typically have thick ice and at least some snow cover, it is a bit hard to understand what is going on below the ice.
Fish
First off, let’s talk about our fish. The majority of winter questions that I get are about my client’s beloved fish. We will talk only about goldfish and Koi, since the vast majority of people that I know have one or both of these in their pond. Koi and goldfish are related since they both originally come from carp. This means that they are similar in some respects, but they also have their differences since they have evolved into different species. From my experiences over the years, I can tell you that goldfish are much more hardy than koi and they tend to survive almost any and all conditions.
Both of these types of fish are cold-blooded, so as the water temperatures cool down, their entire system slows down. They like to go down to a low, quiet area of the pond and just float without much movement for the winter. There have been studies done that show that as the water temperature drops, goldfish slow down their breathing. In warm water, a goldfish may breath 99-120 times per minute, but in cold water, this slows down to more like 11-30 breaths per minute. I have not seen studies like this done on koi, but I would assume that they do the same. A goldfish in 77 degree water has a metabolic rates 140% greater than a goldfish in 60 degree water. This is why it is important that we don’t feed our fish when the water temperature drops.
As I stated earlier, I have found goldfish to be more hardy than koi. This could in part be due to their ability to survive without oxygen for long periods of time. It has been proven that goldfish can live in water with zero dissolved oxygen for up to 5 months. Most animals bodies switch to anaerobic respiration when oxygen is scarce, breaking down carbohydrates for energy. This produces lactic acid, which is dangerous in high concentrations. Goldfish can convert these carbs to ethanol, which they expel from their gills. This is what allows them to survive for so long in low oxygen environments.
To give our fish the greatest chance for survival in cold temperatures, we always install a bottom aeration diffuser on a middle height shelf near one end of the pond. This ensures that the fish have plenty of oxygen in the water, and it lets dangerous gassessuch as hydrogen sulfide escape from the water (gasses caused by decaying plant material). This is also why it is important to skim as much debris out of your pond as possible prior to winter. We used to recommend that people keep a hole in the ice with a heater or a bubbling pump, but those are problematic and prone to failure. Now, a bottom aeration system does the work very reliably and we don’t need to worry about a hole in the ice, because the air bubbles are constantly finding their way out of the pond and carrying the gasses with them whether there is a hole in the ice or not. Be aware that bottom aeration will always result in thin areas of ice, you should never go out onto the ice of a pond with bottom aeration.
Short warm spells during our winter months can be hard on our fish because the water may warm up a bit, which will cause the fish to become more active prematurely. This will cause more waste and cause them to use more energy. Also, without our ponds circulating, there could be increased ammonia levels in the pond which get worse as the water temperatures increase. It is better for the fish if the pond freezes and stays frozen until just before spring.
Frogs
The frogs in our pond will usually go down to the deepest part of the pond to a nice calm area and hang out for the winter, similar to the fish. Their bodies also slow way down when they get cold and they can take in oxygen through their skin from the water. Some frogs that live on land can actually freeze solid and then come back to life when they warm, but pond frogs will usually die if the find themselves too high up in the pond for winter. Each spring, we find dead frogs in skimmer boxes and waterfall boxes because they are just too shallow and the aquatic frogs that try to find shelter in them simply freeze. Do your frogs a favor and block off their access to these boxes prior to winter.
Plants
The plants in our pond are affected by the cold water also. Aquatic plants are similar to terrestrial plants in that you can find tropical varieties that will only survive the warm summer months and then will need to be replaced each spring (just like planting annual flowers in your flower beds). Or, you can plant hardy aquatic plants which will grow and spread each year (just like planting perennial plants in garden beds). Both water lilies and marginal aquatic plants come n both varieties, so be careful when choosing which plants to put in your pond. If you have hardy aquatic plants, they will go dormant for the winter and then spring back to life when the ice melts and the water warms up again. Unfortunately, the algae always responds more quickly in spring that the aquatic plants, so algae will be the first green that you see in your pond. If you put tropical aquatic plants in your pond, they will die in the cold water, so it is best to remove them before the pond ices over to avoid letting them decay in the pond water over the winter.
Bacteria and other Organisms
Certainly the cold water has an effect on the rest of our ecosystem as well. The bacterial community and other organisms and microorganisms that are hard at work consuming nutrients and decomposing the leaves, debris, algae and fish waste all summer long, slow down considerably as the water temperature drops. For every 5-degree drop in temperature, the activity of these bacteria is cut in half. There are some bacteria that thrive in colder water, but these bacteria are relatively scarce, to begin with, are less active, and proliferate very slowly. This is why we always suggest giving your pond a good dose of beneficial bacteria in spring, to replace any of the warm water bacteria that may have died off over the winter.
Muck
Because our water gets colder and most of the bacterial activity slows down, the decomposition of debris slows down drastically and all of these debris particles end up on the bottom of our pond, which will result in more pond muck than when the winter started. This means that the muck is getting thicker and more nutrient rich over the winter. These extra nutrients will act as fertilizer for the spring algae bloom that is surely coming our way. Once again, this is a very good reason to skim out leaves and debris late in the fall right before our ponds ice over.
Algae
Algae’s growth slows in cold water also, but since there is less competition for pond nutrients by the fish and bacteria, and the fish have stopped nibbling on the algae, the algae really has no competition. This means that although growth slows, algae will likely come out of winter with larger quantities than when winter started. Winter water still gets plenty of sunlight, carbon, nitrogen, and phosphorus below the ice. The combination of larger algae quantities, a more nutrient rich muck layer and aquatic plants that are slowly coming out of a deep freeze is the reason that we often get a bright green algae bloom first thing in spring. While your green pond may be the first thing in the yard to green-up, it is usually not the look that homeowners are hoping for in early spring.
Spring Time
Running an aeration system over the winter months will add a healthy amount of oxygen to the body of water and increase circulation. This can help prevent winter fish kills and will help to keep the bacteria and other organisms as active as possible to allow the decomposition process to continue over the winter. When spring finally comes to your part of the world, your fish, plants, bacteria and other organisms will all start to slowly come back to life as the water warms. Even if you think your fish might look hungry, do not feed them until you are certain that the water is above 60 degrees and will stay that way. One thing that you can do as a treat for your fish will be to add some new water to the pond. Oftentimes we get some heavy spring rain showers, which will help to increase oxygen levels and decrease ammonia levels, but adding some more, fresh water is always a good idea. Although, if you are on city water, you will need to be sure to add dechlorinator to the water or you will be doing more harm than good.