Reading Passage
Read the passage and answer Questions 1–14.
Many temperate lakes undergo predictable seasonal changes in the way their water is arranged and circulated. These changes are driven largely by temperature and by the effect temperature has on water density. Because water at different temperatures behaves differently, lake water does not always mix evenly from top to bottom. Instead, at certain times of year it may form layers, while at others it may circulate more completely. Understanding this annual cycle is important because it affects oxygen distribution, nutrient movement, and aquatic life.
In summer, many lakes stratify into distinct layers. The uppermost layer, warmed by sunlight, becomes lighter than the colder water below it. Since the warm surface water is less dense, it tends to remain above the deeper layer rather than mixing with it. Wind may stir the surface region, but it often cannot generate enough force to mix the entire lake. As a result, the lower water remains relatively isolated for long periods. This layered state can persist for weeks or months, depending on the lake and the climate.
When autumn arrives, air temperatures decline and the surface water cools. As it cools, it becomes denser and begins to sink. This process weakens the thermal differences that maintained summer stratification. Eventually, the lake may undergo what limnologists call turnover, a period during which water from different depths mixes more thoroughly. Oxygen can then move downward while nutrients that collected in deeper water are brought closer to the surface. Because of this redistribution, autumn turnover plays a major role in restoring chemical balance within the lake.
In some lakes, a similar mixing event occurs in spring, after winter ice melts and surface water once again reaches temperatures that allow circulation. Not all lakes, however, follow exactly the same pattern. Deep lakes in cool climates may show strong seasonal stratification, while shallow lakes may mix more frequently because wind can affect a greater proportion of their water. In very warm regions, some lakes may remain stratified for much longer intervals. [■] As a result, substances that accumulate in deep water during summer may later be redistributed throughout the lake.
Extended stagnation in deep water can have biological consequences. If oxygen is not replenished below the surface for a long period, organisms living at depth may face stressful conditions. At the same time, nutrients released from sediments may build up in those lower zones. When turnover finally occurs, those nutrients may support new biological activity near the surface. Thus, the annual cycle of mixing is not simply a physical process; it is closely connected to the ecological functioning of the lake.
Scientists study these patterns not only to understand natural systems but also to evaluate human impacts on lakes. Climate change, altered runoff, and nutrient pollution can all influence how lakes stratify and mix. Even small changes in the timing or intensity of seasonal turnover may affect fisheries, water quality, and the distribution of aquatic organisms. For this reason, lake circulation remains a central topic in freshwater science.
1. According to paragraph 1, what is the passage mainly about?
A) Why lakes gradually disappear over time B) How rivers influence water temperature in nearby lakes C) The role of seasonal temperature differences in controlling lake water movement D) Why fish live only near the surface of lakes
2. The word stratify in paragraph 2 is closest in meaning to:
A) circulate rapidly B) separate into distinct horizontal layers C) freeze near the edges D) become less transparent
3. According to paragraph 2, why does summer stratification often develop in temperate lakes?
A) Sunlight warms surface water without equally warming deeper water. B) Wind becomes too weak to affect the lake at all. C) Deep water absorbs heat more quickly than surface water. D) Winter ice remains below the surface through most of summer.
4. Why does the author mention density in paragraph 2?
A) To show that all lake water has nearly the same physical properties B) To explain why deep water becomes warmer than surface water C) To suggest that oxygen levels are controlled only by chemical reactions D) to explain why surface water does not immediately mix with deeper water in summer
5. The word diminished in paragraph 3 is closest in meaning to:
A) measured B) distributed C) reduced or weakened D) replaced completely
6. According to paragraph 3, why can autumn turnover occur?
A) because plants stop producing oxygen in shallow water B) because cooling surface water becomes dense enough to sink C) because winter ice melts and releases trapped gases D) because wind forces warm deep water to the top
7. Which of the following can be inferred from paragraph 3?
A) Lake turnover helps move oxygen and nutrients through the lake. B) Turnover happens only in lakes with no plant life. C) Autumn turnover permanently eliminates layering in all lakes. D) Surface water remains warmer than deep water throughout autumn.
8. The word stagnation in paragraph 4 is closest in meaning to:
A) brief turbulence near the shoreline B) rapid warming caused by sunlight C) a sudden increase in water clarity D) a long period when deeper water remains cut off from the surface
9. Why does the author discuss shallow lakes in paragraph 5?
A) To argue that shallow lakes are more biologically diverse than deep lakes B) It clarifies why certain lakes may not develop stable summer layers. C) To show that shallow lakes are unaffected by seasonal changes D) To explain why shallow lakes freeze more slowly in winter
10. The word contingent in paragraph 5 is closest in meaning to:
A) resistant to measurement B) unrelated to climate C) depends heavily on environmental conditions D) easy to observe directly
11. According to paragraph 6, why are lake mixing patterns important to scientists?
A) the movement of water in lakes has important biological effects B) they determine the exact age of lake sediments C) they show that deeper lakes always support more fish D) they prove that temperature matters more than nutrients
12. Look at the four squares [■] that indicate where the following sentence could be added to the passage.As a result, substances that accumulate in deep water during summer may later be redistributed throughout the lake.
A) Paragraph 2 B) Paragraph 3 C) Paragraph 5 D) Paragraph 4
13. Why does the author mention fish and microorganisms in paragraph 6?
A) to show that only large organisms are affected by turnover B) because fish and microorganisms depend on oxygen levels influenced by water movement C) to argue that biological research is more useful than physical measurement D) to explain why microorganisms prevent lakes from freezing
14. What is the main idea of the passage?
A) Deep lakes are healthier than shallow lakes because they mix less often. B) Temperature differences influence only the surface regions of lakes. C) Seasonal mixing patterns strongly influence physical and biological conditions in lakes. D) Most lakes follow exactly the same annual cycle regardless of climate.
