Final answer:
The spring turnover refers to when warmer surface water in temperate lakes reaches 4 °C, becomes dense and sinks, causing nutrient-rich bottom water to rise.
Step-by-step explanation:
The spring turnover is a critical process in freshwater lakes, particularly in temperate regions. As winter ends and the air temperatures rise, the ice on the surface of lakes melts, and the surface water warms up. When it reaches approximately 4 °C, the water attains maximum density and starts to sink. This sinking water displaces water from the bottom, which is then forced upwards, bringing nutrients and sediment with it. This movement of water effectively mixes the lake, distributing oxygen and nutrients throughout.
In contrast, tropical lakes experience little to no stratification due to relatively uniform temperatures year-round. Thus, they do not undergo the same dramatic turnover process seen in temperate lakes, because there is no ice melt to initiate the cycle and the temperature variations between seasons are minimal.
During summer stratification, temperate lakes form layers with warmer water on top and colder, denser water below. This layering persists until the fall turnover, when cooling surface temperatures cause the layers to mix again.
The final phase of spring turnover occurs at the moment before the stratification cycle restarts. It is the culmination of water mixing, when the distribution of nutrients and oxygen is completed and the temperature of the lake water is relatively uniform throughout.