Final answer:
The final phase of fall turnover equalizes lake temperatures, circulating oxygen and nutrients, crucial for aquatic life. This process differs between temperate and tropical lakes due to variations in freezing conditions and seasonal temperature changes.
Step-by-step explanation:
The final phase of fall turnover in freshwater lakes is a critical period where water temperature equalizes throughout the lake. As the air temperature drops, the surface water cools and reaches 4 °C, its maximum density. This dense water sinks, causing an overturn that circulates oxygen from the surface to the bottom, while nutrients from the bottom are brought to the surface. This process replenishes oxygen for organisms like fish and decomposers throughout the winter, and thanks to the transparency of ice, allows photosynthesis to continue, primarily by algae.
In temperate regions, this process differs from tropical regions because temperate lakes can freeze over, which alters the cycle of stratification and turnover. Whereas in the tropics, lakes often do not experience the extreme temperature variations that drive seasonal turnover processes as seen in temperate climates.