Winter Water Chemistry: What Happens Under the Ice

When winter sets in and the pond freezes over, the surface seems calm, but what is below the ice is far from peaceful. The chemistry of the water below the ice is constantly changing, and it can have serious effects on fish health and the overall balance of the pond ecosystem. This can stress fish, reduce microbial activity, and influence nutrient cycles.

The Oxygen Problem

Proper aeration in the winter is vital for fish survival. When a pond freezes completely, it blocks the gas exchange between the water and the atmosphere. This means oxygen can’t enter, and harmful gases can't escape. Over time, oxygen levels drop while carbon dioxide, methane, hydrogen sulphide, and other toxic gases accumulate, creating a dangerous environment for aquatic life. Fish may become lethargic or stressed, and in extreme cases, winterkill can occur, leading to large-scale fish loss. Monitoring dissolved oxygen levels in the water can help prevent this. 

Plants and Oxygen 

During the warmer months, aquatic plants produce oxygen through photosynthesis. In winter, the amount of sunlight available for this process is reduced. Instead of producing oxygen, the plants begin to consume it through respiration, leaving even less available oxygen for fish and other organisms. Submerged plants and algae may also release stored nutrients into the water as they break down, contributing to nutrient loading and influencing algae blooms as temperatures rise.

Organic Matter and Decomposition

During fall and early winter, fallen leaves, dead algae, and decaying organisms accumulate at the bottom of the pond. Beneficial aerobic bacteria, such as those in our NaturalClear Pond and Lake Products, get to work breaking down organic matter, but they also consume oxygen in the process. In open water, the gases released during decomposition (such as carbon dioxide, methane, and hydrogen sulphide) escape harmlessly into the atmosphere. Anaerobic conditions can develop at the pond bottom, resulting in foul odours and toxic compounds that stress aquatic life further. Regularly removing excess organic debris before ice forms can significantly reduce these risks.

To mitigate this problem, we recommend aerating your pond, lake, or dugout. Our Can-Air Koi, ECO, SS and Mega Diffused Aeration Systems offer a range of diffusers for ponds, lakes, and dugouts of all shapes and sizes. Even partial aeration can improve water circulation, reduce ice thickness in targeted areas, and maintain critical oxygen levels throughout the winter. Reach out to us at sales@pondpro.ca or fill out our Aeration Quote Form if you are interested in aeration for your pond.

For additional winter protection, we also recommend using the Kasco Marine De-Icers (available in ½ HP, ¾ HP, and 1 HP in 120 and 240 V). These de-icers work by maintaining a small, open area in the ice to allow for continuous gas exchange between the water and the atmosphere. This opening helps prevent the buildup of harmful gases and supports circulation even during prolonged cold periods. When used alongside an aeration system, a de-icer ensures that your pond, lake, or dugout remains aerated throughout the winter. Our de-icers are energy-efficient, durable, and effective in extreme Canadian climates, offering peace of mind and reliable protection to your aquatic ecosystems.

Muck and Nutrient Buildup

Older ponds tend to accumulate more organic debris and sludge on the bottom. During winter, decomposition slows down due to cold temperatures. To prevent muck from accumulating, consider our Muck Pellets, Muck Pellets PLUS, and Muck Pellets OxyFizz. These pellets are designed to combat sludge accumulation in ponds and lakes. They contain billions of beneficial bacteria. Use our Pond Pro Calculator to determine the required dose for Muck Pellets and other products. For older, more problematic ponds, increasing the dosage and frequency will help achieve the desired results. 

Algae and Spring Recovery

Algae growth slows in cold water, but it doesn’t stop entirely. Many algae species survive winter in a dormant state, ready to bloom again once conditions improve. This is why it’s essential to treat algae preventively, not just when it appears. Early intervention helps maintain water clarity, reduces oxygen demand from decaying algae, and prevents excessive nutrient cycling that can destabilize your pond ecosystem.

Applying our ProClear Algaecide (or Green Pond Clarifier for smaller ponds) once water temperatures rise above 10°C helps control algae early, preventing the pond from being overtaken in the winter. Simply spray the recommended amount evenly over your pond’s surface for the best results. Use our Pond Pro Calculator to determine your required dose for ProClear Algaecide and other products. If you have fish in your pond, ensure you calculate the volume as accurately as you can. Pairing alagecide treatment with aeration and beneficial bacteria provides a comprehensive winter-to-spring pond management strategy, setting the stage for a healthy, balanced pond, all season long.