What phenomenon occurs when the epilimnion cools to 4°C in the fall and leads to lake turnover?

When the epilimnion cools to 4°C in the fall, the phenomenon that occurs is called lake turnover. During this process, the temperature of the surface layer of the lake, or epilimnion, decreases and becomes denser than the water below it. As air temperatures drop, the surface water cools, and once it reaches 4°C, it sinks to the bottom of the lake. This density-driven movement of water allows the lower layers of the lake to move to the surface, promoting mixing throughout the aquatic environment.

Lake turnover is crucial because it helps to redistribute nutrients that have settled in the bottom waters back to the surface, thereby enhancing the overall health of the aquatic ecosystem. This mixing is vital for maintaining oxygen levels in deeper waters and ensuring that organisms relying on these resources continue to thrive. It is a natural process that can have significant impacts on the local aquatic environment, including the promotion of biological productivity.

While thermal stratification refers to the layering of water caused by differences in temperature, it is the cooling and subsequent density change that specifically drives the turnover process. Nutrient cycling and oxygen depletion may occur as a result of turnover, but they are not the primary phenomenon initiated by the cooling of the epilimnion