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# A freshwater lake's summertime ecology is significantly affected by a

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A freshwater lake's summertime ecology is significantly affected by a  [#permalink]

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Updated on: 20 Oct 2018, 01:05
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A freshwater lake's summertime ecology is significantly affected by a phenomenon known as the thermocline. In a typical deep-water impoundment in the southern United States, (5) the water temperature measures a uniform 38-42 degrees in late winter after any surface ice has melted. Wave action stirs oxygen into the water at the lake's surface, and the temperature uniformity allows distribution of this dissolved oxygen to all depths. With oxygen plentiful, many of the reservoir's fish species, both predator and forager, are found throughout the water column.
As the lake's surface temperature increases in early spring, that water expands. A layer of warmer water builds at the surface of the lake, resting like a pillow on the mass of colder water below. The pillow of warm surface water slowly increases in thickness, as heat is transferred into the depths by the limited stirring of wave action.
By early summer, a sharp boundary separates two independent bodies of water within the lake. The boundary is a temperature gradient called the thermocline, and it acts as a barricade to prevent any further mixing of oxygen into the chilly depths. The temperature barrier prevents oxygen from circulating downward from the surface, as it does in the winter, to replace the oxygen consumed by fish and dying zooplankton. In order to survive, fish are forced upward into the relatively narrow zone between the thermocline and the surface.
The cold nights of autumn cool the surface to the point that it is heavier than the water below the thermocline. In a process known as the turnover, a current of richly oxygenated water plunges to the bottom of the reservoir, forcing stagnant water back to the surface. The lake reaches equilibrium by early winter and remains there until the process repeats itself the following spring.
1. The passage suggests that which one of the following can be inferred about dissolved oxygen in a lake?
(A) The colder the water, the less dissolved oxygen it can hold.
(B) There is always more dissolved oxygen within six feet of the surface than at 60 feet beneath the surface.
(C) The formation of ice can completely block the supply of dissolved oxygen.
(D) Dissolved oxygen is not necessary for organisms other than fish and zooplankton.
(E) Wave action at the surface increases dissolved oxygen levels in the lake.

2. The passage suggests that the effects of temperature stratification
(A) would be relevant to the interests of fisheries managers
(B) on an individual lake cannot be predicted
(C) represent the most important factor influencing a lake's ecology
(D) become more pronounced as a lake gets older
(E) are most severe in the lakes of the southern United States

3. Which one of the following is not mentioned in the passage as a step in the yearly lake stratification process?
(A) Fish congregate in the water layer above the thermocline in the summer.
(B) Water temperatures achieve a general uniformity by late winter.
(C) The thermocline reaches its maximum depth by early summer.
(D) Oxygen levels decline in the layer beneath the thermocline.
(E) The fall turnover replaces stagnant water in the depths with oxygen-rich water.

4. The author is primarily interested in discussing
(A) the effect of fish and other aquatic organisms on a phenomenon known as the thermocline
(B) the relationship between a lake's ecology and water purity
(C) the contribution of the thermocline to overfishing in Southern lakes
(D) an effect of the seasonal warming and cooling of water in freshwater lakes
(E) the changes in a lake's water temperature caused by fluctuating oxygen levels

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Originally posted by GmatWizard on 19 Oct 2018, 23:42.
Last edited by GmatWizard on 20 Oct 2018, 01:05, edited 2 times in total.
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Re: A freshwater lake's summertime ecology is significantly affected by a  [#permalink]

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19 Oct 2018, 23:48
1

Lake Stratification.

What Makes It Difficult A common thread to most science passages is the employment of jargon—technical terms and phrases peppered throughout the passage that serve to scare off, or at least befuddle, the unsophisticated test taker. An 800 test taker cuts past jargon, returning to cumbersome technical details only when a question demands it.
This passage is no exception, introducing us to things such as the "thermocline" and a process called "the turnover"—heck, they even throw in deep-water impoundment as a synonym for the simple term lake. You have to ask: is that really necessary? But the jargon here isn't even as thick as in some of the others that follow. The problem in this passage is keeping track of the various processes at work. Lots of details, lots of mechanisms, lots of numbers to keep on top of—that's the challenge here. But if you break each part of the process down into simple ideas, it's not so hard to handle. The passage is, after all, mostly descriptive, so we simply need to follow the cycle described from winter through the rest of the seasons and back to winter again, bearing in mind that the author is interested, as stated early on, in the role this process plays in a lake's summertime ecology.

Key Points of the Passage Purpose and Main Idea:

The author's purpose is to explore the cyclical process of lake stratification and its effects on lake ecology. The main idea is that seasonal temperature differences within lake water cause the cyclical formation of water layers of various temperatures separated by the thermocline, which prevents oxygen from reaching the colder, deeper lake waters and periodically causes redistribution of fish within the lake.

Paragraph Structure:

Paragraph 1

introduces the thermocline phenomenon and describes its development using the example of freshwater lakes in the southern United States. The description of the cycle that the author will offer throughout the passage begins at the end of the paragraph with info about late winter, when lake water temperatures are uniform and fish species thrive at all depths of the lake.

Paragraph 2

describes the changes that occur in early spring: the water on the surface of the lake becomes warmer and lighter than the water below it. This warm water expands and forms a layer over the water below.

Paragraph 3

describes the development and results of the thermocline. In summer, the warm water on top of the lake becomes distinctly separated from the colder water below it by the thermocline, a temperature gradient that acts as a boundary between the two parts of the lake. The thermocline boundary prevents oxygen from reaching the lower depths of the lake, and this causes fish to swim up to the warmer surface of the lake to survive. The final paragraph describes the reversal of the cycle. In fall, the water at the surface above the thermocline cools and becomes heavier than the water below the thermocline. This allows oxygen to plunge to the depths of the lake, and the lake reaches equilibrium again by early winter.
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A freshwater lake's summertime ecology is significantly affected by a  [#permalink]

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19 Oct 2018, 23:52
1

ANSWERS AND EXPLANATIONS 1.E, 2.A, 3.С, 4.D

1.

(E) This Inference question asks about the subject of dissolved oxygen, which is raised many times in the passage. It's not easy here to pinpoint exactly which part of the passage we should review to answer this question, so the best tactic is to evaluate the answer choices, returning to the passage for verification when necessary.
An 800 test taker knows when and how to allow the answer choices to guide her to the relevant portions of the text when a question is fairly open-ended.
We're told in paragraph 1 that wave action stirs oxygen into the water, making (E) the best inference in this set.
(A)We aren't given information relating water temperature to oxygen capacity. True, during certain months the thermocline prevents oxygen from reaching the cooler depths, but for all we know, cold water can hold just as much dissolved oxygen as warm water given the chance. (B) On the contrary: the passage tells us in paragraph 1 that during late winter, oxygen is found plentifully throughout all of a lake's water levels.
(C) The only thing we're told about ice formation is that ice melts in late winter, which is not nearly enough information to allow us to infer the statement in (C).
(D) This choice misrepresents the author's claim in paragraph 3. We're told that fish and zooplankton consume oxygen below the surface, but not that they're the only organisms that do so.

2

. (A) Here we have another Inference question, denoted by the word suggests. What does the author suggest about the effects of temperature stratification? The most significant effect he discusses is how temperature stratification causes fish to move to the top layer of lakes during the summer. This might certainly be pertinent for fisheries, as choice (A) indicates.
(B) No, the passage shows that the effects of temperature stratification can be predicted to some degree, at least in terms of the general effect on lake ecology. It also shows that the general timing of these effects can be predicted as well.
(C) and (E) exaggerate the author's claims in the passage. Paragraph 1 states that the lake's ecology is "significantly affected" by the thermocline, but we don't know if the thermocline is the most important factor (C). It is also clear that temperature stratification affects southern U.S. lakes, but these effects aren't compared to those of other lakes, so we can't determine the relative severity of the effects (E).
An 800 test taker recognizes extreme wording that damages the plausibility of certain answer choices, and instinctively stays away from choices that include exaggerated or extreme ideas. (D) is outside the scope of the argument. The age of lakes is never brought up in the author's discussion, nor is anything implied regarding the way lake age might affect the stratification process described.

3

. (C) Here's a detail question asking us to look for something that was NOT mentioned in the passage, a bit harder than the traditional detail question. Fortunately, the wrong answers are all stated in the passage fairly clearly. (C) is the only choice that's not mentioned. In fact, it even tends to contradict the passage: Paragraph 3 tells us that the depth of the thermocline fluctuates with temperature and winds, so we can't be sure when the depth is at its maximum. All of the other answer choices repeat information from the passage. Paragraph 3 tells us that fish move above the thermocline in the summer, so this eliminates (A). We're told in paragraph 1 that water temperatures are uniform in late winter, eliminating (B). Paragraph 3 helps us eliminate (D), because it states that the thermocline shuts off the supply of oxygen to the lower depths. Finally, choice (E) can beeliminated by paragraph 4, which describes how the turnover brings oxygen to deeper, stagnant water.

4.

(D) The stem itself gives away the fact that the author's purpose is "to discuss" something, so there's no need to choose among various verbs (discuss, argue, refute, etc.)—but what is he interested in discussing? The author's main concern should be firmly planted in your mind by now; he's simply interested in discussing the process and effects of lake stratification. Choice (D) comes very close to this prephrase, merely substituting "seasonal warming and cooling of water..." for the process described.
(A) and (E) both reverse the direction of causality in the author's discussion. The author is interested in how the thermocline affects lake ecology, not vice versa as in (A). Similarly, he shows how changing water temperatures, by means of the thermocline, affect oxygen levels—not the other way around, as (E) would have it. An 800 test taker is ultra sensitive to cause-and-effect relationships, and is not likely to mistake one for the other.
(B) and (C) go beyond the argument's scope. The author mentions "stagnant" water in paragraph 4, but does not focus on water purity throughout the passage. And the phenomenon of overfishing, if it exists, is not discussed at all. Not so bad, huh? Well, of course, we're just getting started, and not surprisingly, things do get a bit worse. "Mitochondria," the next passage in our little group of nightmares, raises the bar, both in the density of the material and in technical jargon.

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Re: A freshwater lake's summertime ecology is significantly affected by a  [#permalink]

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08 Jan 2019, 01:10
5 minutes, 3/4 correct.
Can someone please explain Q2? Choice A could be the correct answer if 'would' is replace by 'could'. It could be helpful to fisheries managers. Help is greatly appreciated. Thanks!
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Re: A freshwater lake's summertime ecology is significantly affected by a  [#permalink]

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08 Jan 2019, 02:31
GmatWizard wrote:

ANSWERS AND EXPLANATIONS 1.E, 2.A, 3.С, 4.D

1.

(E) This Inference question asks about the subject of dissolved oxygen, which is raised many times in the passage. It's not easy here to pinpoint exactly which part of the passage we should review to answer this question, so the best tactic is to evaluate the answer choices, returning to the passage for verification when necessary.
An 800 test taker knows when and how to allow the answer choices to guide her to the relevant portions of the text when a question is fairly open-ended.
We're told in paragraph 1 that wave action stirs oxygen into the water, making (E) the best inference in this set.
(A)We aren't given information relating water temperature to oxygen capacity. True, during certain months the thermocline prevents oxygen from reaching the cooler depths, but for all we know, cold water can hold just as much dissolved oxygen as warm water given the chance. (B) On the contrary: the passage tells us in paragraph 1 that during late winter, oxygen is found plentifully throughout all of a lake's water levels.
(C) The only thing we're told about ice formation is that ice melts in late winter, which is not nearly enough information to allow us to infer the statement in (C).
(D) This choice misrepresents the author's claim in paragraph 3. We're told that fish and zooplankton consume oxygen below the surface, but not that they're the only organisms that do so.

2

. (A) Here we have another Inference question, denoted by the word suggests. What does the author suggest about the effects of temperature stratification? The most significant effect he discusses is how temperature stratification causes fish to move to the top layer of lakes during the summer. This might certainly be pertinent for fisheries, as choice (A) indicates.
(B) No, the passage shows that the effects of temperature stratification can be predicted to some degree, at least in terms of the general effect on lake ecology. It also shows that the general timing of these effects can be predicted as well.
(C) and (E) exaggerate the author's claims in the passage. Paragraph 1 states that the lake's ecology is "significantly affected" by the thermocline, but we don't know if the thermocline is the most important factor (C). It is also clear that temperature stratification affects southern U.S. lakes, but these effects aren't compared to those of other lakes, so we can't determine the relative severity of the effects (E).
An 800 test taker recognizes extreme wording that damages the plausibility of certain answer choices, and instinctively stays away from choices that include exaggerated or extreme ideas. (D) is outside the scope of the argument. The age of lakes is never brought up in the author's discussion, nor is anything implied regarding the way lake age might affect the stratification process described.

3

. (C) Here's a detail question asking us to look for something that was NOT mentioned in the passage, a bit harder than the traditional detail question. Fortunately, the wrong answers are all stated in the passage fairly clearly. (C) is the only choice that's not mentioned. In fact, it even tends to contradict the passage: Paragraph 3 tells us that the depth of the thermocline fluctuates with temperature and winds, so we can't be sure when the depth is at its maximum. All of the other answer choices repeat information from the passage. Paragraph 3 tells us that fish move above the thermocline in the summer, so this eliminates (A). We're told in paragraph 1 that water temperatures are uniform in late winter, eliminating (B). Paragraph 3 helps us eliminate (D), because it states that the thermocline shuts off the supply of oxygen to the lower depths. Finally, choice (E) can beeliminated by paragraph 4, which describes how the turnover brings oxygen to deeper, stagnant water.

4.

(D) The stem itself gives away the fact that the author's purpose is "to discuss" something, so there's no need to choose among various verbs (discuss, argue, refute, etc.)—but what is he interested in discussing? The author's main concern should be firmly planted in your mind by now; he's simply interested in discussing the process and effects of lake stratification. Choice (D) comes very close to this prephrase, merely substituting "seasonal warming and cooling of water..." for the process described.
(A) and (E) both reverse the direction of causality in the author's discussion. The author is interested in how the thermocline affects lake ecology, not vice versa as in (A). Similarly, he shows how changing water temperatures, by means of the thermocline, affect oxygen levels—not the other way around, as (E) would have it. An 800 test taker is ultra sensitive to cause-and-effect relationships, and is not likely to mistake one for the other.
(B) and (C) go beyond the argument's scope. The author mentions "stagnant" water in paragraph 4, but does not focus on water purity throughout the passage. And the phenomenon of overfishing, if it exists, is not discussed at all. Not so bad, huh? Well, of course, we're just getting started, and not surprisingly, things do get a bit worse. "Mitochondria," the next passage in our little group of nightmares, raises the bar, both in the density of the material and in technical jargon.

GittinGud have a look here
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Re: A freshwater lake's summertime ecology is significantly affected by a  [#permalink]

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08 Jan 2019, 03:00
5 mins 3 secs and all correct!

Suggests means inference type and we need an answer that is 100% supported by the information given in the passage
Q2: The passage suggests that the effects of temperature stratification

(A) would be relevant to the interests of fisheries managers Hold. Not cleary mentioned.
(B) on an individual lake cannot be predicted Opposite. The passage talks all about how the temperature stratification can be predicted. Discard.
(C) represent the most important factor influencing a lake's ecology Hold. Sounds familiar. But too extreme.
(D) become more pronounced as a lake gets older Discard. Not at all mentioned in the passage. Also irrelevant to what is being discussed. Discard.
(E) are most severe in the lakes of the southern United States Lake of southern USA is called "typical" and is used only as an example. Hence discard.

Let's look at (A) vs. (C)

The passage does mention that - "A freshwater lake's summertime ecology is significantly affected by a phenomenon known as the thermocline." but to call this the "most important" factor would be a jump. It sounds too extreme.

On the other hand, the passage does not mention at all about fisheries. However, given the last line of the passage - "the process repeats itself the following spring." shows the predictability of the process. Also one part of the process is predicting when the fish will all rise to the surface ( in search of oxygen) and hence there is a possibility that this information could benefit the fisheries.

Hence Option (A) seems the better choice.

Hope this helps, GittinGud ... With "could" the option would become water-proof but even without it, it is the single best answer among the five and that is all we are looking for.
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Re: A freshwater lake's summertime ecology is significantly affected by a   [#permalink] 08 Jan 2019, 03:00
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