Do Frozen Ponds = Frozen Fish?
Mississippi does not get much snow and ice, so last month's "Snowpocalypse" was quite the adventure for us Southerners. I saw something on Facebook that said, "WEATHER WARNING: Southerners are urged NOT to travel unless ABSOLUTELY NECESSARY! Northerners - You will need to wear your big coat." This is so true for us in the South! Following that advice, I enjoyed working from home and watching the freezing changes to the environment around me. I woke up the morning of February 15th to the pond in my backyard being frozen over. It reminded me of being in Minnesota one year in January and seeing the great Mississippi River frozen over. Being from the south, both of these occurrences amazed me and made me think about the living habitat underneath the ice. What happens to the water? How do the fish survive? Where does their oxygen come from beneath the frozen surface?
Lakes and ponds transition from season to season as nature's way of refreshing nutrients and oxygen. In the spring, the water surface warms causing the temperature of the top and bottom layers of the lake to equalize. With the help of strong winds, this new equilibrium breaks the thermal stratification. Oxygen from the surface mixes with the bottom, while nutrients trapped near the bottom are free to mix throughout. This is why, sometimes, in the spring and fall the pond or lake can smell unpleasant. Decomposing organic materials are churned up from the bottom of the lake, bringing with them a signature sign of lake turnover.
In the summer months, most ponds and lakes are stratified (formed or arranged into strata or layers). This is caused by the sun warming the top layer, while the lower layers are colder. Warm water is less dense, so it sits on top of the colder, more dense water. As summer transitions to fall, the upper layer cools and breaks down the density difference. Eventually, the water cools to 39.2°F which is the temperature at which water is the most dense, and the water at the surface settles to the bottom, pushing the warmer water at the bottom back to the surface. This process continues until the surface water cools below 39.2°F. Because fresh water is maximally dense at 39.2°F, the water at temperatures below that will rise to the top of the water column and eventually freeze, making the bottom layer the warmest for certain fish species to survive during the winter. The ice also acts as an insulating blanket, preventing deeper lakes from freezing completely solid.
Fish are cold-blooded, so their body's temperature matches their environment. Freshwater fish are divided into two categories: Warmwater and Coldwater. Warmwater fish include bass, bluegill bream, catfish, perch, crappies, etc. Coldwater fish include salmon, tuna, mackerel, herring, trout, anchovies, etc. The warmwater fish species seek out the warmest water they can find (usually around the bottom of the pond or lake). There, they make adjustments by decreasing movement which slows down their metabolism to conserve energy. By doing this, it also diminishes their need to hunt or forage for food. Some species of catfish will actually burrow into the soft silt down at the bottom of the water bed to stay warm. The coldwater fish species stay more active throughout the water column and continue preying on other organisms.
For students to visualize these different layers, first, ask them to think of diving down into a lake in the summer. The top layer of water is very warm. As they swim deeper, they notice a distinct and sudden drop in temperature. Now imagine it is winter and the lake is frozen over. As stated above, solid ice floats and stays on top of the lake because it is less dense than liquid water, despite the ice being colder than the water it floats on. The weather keeps the water near the surface cool, making it less dense than the warmer water deep in the lake.
True to Mississippi weather, we had snow and ice one week and temperatures in the 70's the next week. Now, we have now moved on to tornado season. The snow and ice was pretty while it lasted, but it never lasts long in the South! Share your photos with us on facebook.