Q: How does winter affect my pond? – Steve in Michigan
A: A Closer Look at Water in the Winter…
We field many questions about water and the way it works during the winter. With such a range of questions, here is a quick course on the physical properties of water and how they relate to your pond.
Here’s a review on density. On our planet, matter that is denser will always be pulled down (gravity) more strongly than matter of lesser density. Density is temperature dependant in that warmer matter is always less dense than cooler matter. A good example is a hot air balloon, which lifts up because the air inside the balloon is warmer than the air outside.
A water molecule (H20 – don’t worry that’s as technical as we’ll get) contracts as it gets colder, causing it to become more dense. As water closes in on the freezing point, we run into one of nature’s great exceptions. At 39ºF (4ºC) water actually starts becoming less dense as it prepares to shift from a liquid to a solid at the 32ºF mark. Why is this so important? This means that water at the bottom of your pond will always stay about 39ºF because it is the heaviest. We refer to this as inverse stratification, where a pond has a thermocline that separates warmer water on the bottom from cooler water on top (FYI, normal stratification occurs during the warmer months of the year when the cooler water is at the bottom and the warmer water is at the top).
How does this apply to your fish? Fish actually get quite sensitive to temperatures below 39ºF. This warmer 39ºF pocket on the bottom of the pond is where they tend to hang out during the winter. This is why it is recommended that an aeration system be installed a couple feet up from the bottom of the pond to keep the 39ºF water in tact. Should you worry if yours isn’t? No, not really, mortality due to water temperature is extremely rare and NOT aerating is way more risky. The more severe issue is the toxic gases under the ice. Continuing to aerate will vent these gasses while also infusing oxygen into the water keep your fish happy and healthy.