The one major factor leading to reservoir turnover is caused by...

Reservoir turnover is a vital process in aquatic ecosystems, particularly in temperate climates, where temperature gradients within the water column can significantly impact turnover events. The correct answer centers on the principle of density differences in water based on temperature.

When the upper strata of a reservoir becomes cooler—often during seasonal transitions—it can increase in density relative to the warmer water below it. This density change causes the colder, denser water at the surface to sink. As the upper strata sinks, it displaces the warmer, less dense water from the lower strata, prompting a mixing of the layers. This circulation is crucial for distributing oxygen and nutrients throughout the water column, supporting aquatic life.

The other options present alternative processes, but they don’t accurately describe the primary factor leading to turnover. For example, the notion that lower strata can be warmer than the upper does not typically lead to turnover, as warmer water tends to stay above cooler water due to its lower density. The concept of the upper and lower strata reaching the same temperature may facilitate mixing, but it is not the primary driving force behind turnover. Lastly, while decaying organic matter can cause changes within the water body, its influence on density and subsequent mixing is not as direct or significant as temperature-induced density