The Great Lakes—Huron, Ontario, Michigan, Erie, and Superior—form the largest freshwater system in the world. Each of the lakes tends to stratify, or form layers of warmer and colder water, depending on the season. This is called seasonal turnover. In winter, for example, the coldest water in the lake lies just below the surface ice. The water gets progressively warmer at deeper levels. In spring, the sun melts the ice, and the surface water warms. Because the surface water is still cooler than the layers below, the water at the surface sinks to the bottom of the lake, forcing the cooler water at the bottom of the lake to the surface.
This mixing, known as spring turnover, eliminates the temperature stratification that was established during the winter. In the absence of this thermal layering, wind continues to mix the water to a greater depth, bringing oxygen (O2) to the bottom of the lake and nutrients to the surface. This results in a relatively even distribution of O2 throughout the lake. When summer arrives, the lake again becomes stratified, with warm water at the surface, and cold water at the bottom. A narrow zone of water undergoing rapid temperature changes separates these layers. This zone is called the thermocline. Cool, fall temperatures cause the lake water to mix again, until the surface begins to freeze and the winter stratification is reestablished.
The stability of the lake’s stratification depends on several factors: the lake’s depth, shape, and size, as well as the wind and both the inflow and outflow of lake water. Lakes with a lot of water flowing into and out of them do not develop consistent and lasting thermal stratification.
Figure 1 shows an example of lake stratification during the summer.
1. The best answer is H.
Figure 1 shows that the temperatures near the bottom of the lake are lower than the temperatures near the surface of the lake. The thermocline is shown as a layer separating the surface water from the deeper water. This best supports answer choice H.
2. The best answer is A.
According to the passage, spring turnover “eliminates the temperature stratification that was established during the winter.” This turnover “results in a relatively even distribution of O2 throughout the lake.” The graph shown in answer choice A indicates an even distribution of O2 at every depth of the lake.
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