Wisconsin is home to a stunning array of lakes, each with its own unique ecosystem. One fascinating natural event that occurs in these lakes is lake turnover, a seasonal process that plays a vital role in maintaining the health and balance of aquatic environments. Lake turnover refers to the mixing of water layers in a lake. During the warmer months, lakes in Wisconsin typically stratify into three different layers. The first layer is the Epilimnion layer, this is the warm, oxygen rich surface layer. The second layer is the Metalimnion (or Thermocline), which is the middle layer where temperature changes rapidly with depth. The last layer is the Hypolimnion, this bottom layer gets cold, and is also very oxygen poor. As the temps start to cool down here in the next month, the surface water loses heat and becomes denser. Eventually, it sinks, displacing the water below and causing the lake to mix thoroughly. This process is repeated in the spring when ice melts and surface water warms, creating another turnover. One thing to note is that water is most dense at around 39ºF (4ºC). There are a few impacts that are left behind though from lake turnover. One of these impacts is nutrient redistribution. During turnover, nutrients that have settled at the bottom of the lake are brought to the surface. This supports the growth of phytoplankton, which forms the base of the aquatic food chain. The second impact is oxygenation. The mixing of water layers helps distribute oxygen throughout the lake, ensuring that fish and other aquatic organisms can thrive at all depths. Last but not least is the fishing implications. For anglers, lake turnover can make fishing challenging due to the disruption of fish habitats. However, once lakes stabilize, it often leads to more productive fishing. One factor that is said to be impacting lake turnover though is climate change, which is altering the timing and intensity of lake turnover. Warmer temperatures can delay the cooling process in the fall or shorten the duration of ice cover in the winter, which may disrupt the natural mixing cycle. This could have long-term effects on the health of Wisconsin lakes, including changes in fish populations and water quality. Within the next month the signs of lake turnover will be here. The water may appear murkier than usual due to the mixing of sediments, the temps will drop, and the fishing conditions will temporarily change. However, this natural process is a reminder of the dynamic and interconnected systems that sustain life in these beautiful lakes.
Conservation Corner is a weekly article produced by the Forest County Land Conservation and Land Information Department. For more information contact Lucas O’Brien, Forest County Land Conservation Technician at 715-478-1387 or by e-mail at