TPO44阅读文本及答案解析

3. Seagrasses

Many areas of the shallow sea bottom are covered with a lush growthof aquatic flowering plants adapted to live submerged in seawater. These plantsare collectively called seagrasses. Seagrass beds are strongly influenced byseveral physical factors. The most significant is water motion: currents andwaves. Since seagrass systems exist in both sheltered and relatively openareas, they are subject to differing amounts of water motion. For any givenseagrass system, however, the water motion is relatively constant. Seagrassmeadows in relatively turbulent waters tend to form a mosaic of individualmounds, whereas meadows in relatively calm waters tend to form flat, extensivecarpets. The seagrass beds, in turn, dampen wave action, particularly if theblades reach the water surface. This damping effect can be significant to thepoint where just one meter into a seagrass bed the wave motion can be reducedto zero. Currents are also slowed as they move into the bed.

The slowing of wave action and currents means that seagrass bedstend to accumulate sediment. However, this is not universal and depends on thecurrents under which the bed exists. Seagrass beds under the influence ofstrong currents tend to have many of the lighter particles, including seagrassdebris, moved out, whereas beds in weak current areas accumulate lighterdetrital material. It is interesting that temperate seagrass beds accumulatesediments from sources outside the beds, whereas tropical seagrass beds derivemost of their sediments from within.

Since most seagrass systems are depositional environments, theyeventually accumulate organic material that leads to the creation offine-grained sediments with a much higher organic content than that of thesurrounding unvegetated areas. This accumulation, in turn, reduces the watermovement and the oxygen supply. The high rate of metabolism (the processing ofenergy for survival) of the microorganisms in the sediments causes sediments tobe anaerobic (without oxygen) below the first few millimeters. According toecologist J. W. Kenworthy, anaerobic processes of the microorganisms in thesediment are an important mechanism for regenerating and recycling nutrientsand carbon, ensuring the high rates of productivity—that is, the amount oforganic material produced-that are measured in those beds. In contrast to otherproductivity in the ocean, which is confined to various species of algae andbacteria dependent on nutrient concentrations in the water column, seagrassesare rooted plants that absorb nutrients from the sediment or substrate. Theyare, therefore, capable of recycling nutrients into the ecosystem that wouldotherwise be trapped in the bottom and rendered unavailable.

Other physical factors that have an effect on seagrass beds includelight, temperature, and desiccation (drying out). For example, water depth andturbidity (density of particles in the water) together or separately controlthe amount of light available to the plants and the depth to which theseagrasses may extend. Although marine botanist W. A. Setchell suggested earlyon that temperature was critical to the growth and reproduction of eelgrass, ithas since been shown that this particularly widespread seagrass grows andreproduces at temperatures between 2 and 4 degrees Celsius in the Arctic and attemperatures up to 28 degrees Celsius on the northeastern coast of the UnitedStates. Still, extreme temperatures, in combination with other factors, mayhave dramatic detrimental effects. For example, in areas of the cold North Atlantic,ice may form in winter. Researchers Robertson and Mann note that when the icebegins to break up, the wind and tides may move the ice around, scouring thebottom and uprooting the eelgrass. In contrast, at the southern end of theeelgrass range, on the southeastern coast of the United States, temperaturesover 30 degrees Celsius in summer cause excessive mortality. Seagrass beds alsodecline if they are subjected to too much exposure to the air. The effect ofdesiccation is often difficult to separate from the effect of temperature. Mostseagrass beds seem tolerant of considerable changes in salinity (salt levels)and can be found in brackish (somewhat salty) waters as well as in full-strength seawater.

1. According to paragraph 1, which of the following is true aboutseagrasses in calm ocean waters? A. They will not survive for very long withoutthe nutrients brought In by fast-moving waters.

B. They tend to form beds covering large areas along the oceanfloor.

C. They usually are arranged in separate mounds.

D. They grow more slowly than do seagrasses in fast-moving waters.

2. According to paragraph 1, which of the following is MOST likelyto describe a bed in which seagrasses reach the surface of the water?

A. The water is almost completely still.

B. The bed often has major damage from strong waves or currents.

C. The bed is generally no more than one square meter in size.

D. Grasses form a mosaic of individual mounds.

3. Which of the sentences below best expresses the essentialinformation in the highlighted sentence in the passage? Incorrect choiceschange the meaning in important ways or leave out essential information.

A. Light particles and debris collect in some seagrass beds, but arewashed out of those affected by strong currents.

B. Seagrass beds under the influence of strong currents tend toaccumulate many of the lighter particles from other beds

C. The strength of the currents determines how quickly accumulatedseagrass debris is moved out of the beds.

D. Seagrass debris and other light particles are often moved fromareas of strong currents to areas of weak currents.

4. The word "derive" in the passage is closest in meaningto

A. maintain

B. expel

C. obtain

D. enrich

5. According to paragraph 3, which of the following does NOTaccurately describe the sediments that collect in seagrass beds?

A. Fine-grained

B. Only a few millimeters deep

C. Low in oxygen

D. Rich in organic matter

6. The word "confined" in the passage is closest inmeaning to

A. related

B. limited

C. relevant

D. helpful

7. According to paragraph 3，how doseagrasses affect the nutrient supply in the ecosystem?

A. Because of their high rate of metabolism, they consume a largepercentage of the available nutrients.

B. They attract various species of algae and bacteria that producehigh nutrient concentrations in the water column.

C. They take up carbon and other nutrients trapped on the sea bottomand bring them back into use.

D. Through anaerobic processes at their roots, they produce a verynutrient-rich sediment.

8. It can be inferred from paragraph 4 that the reason seagrasses donot grow in very deep water is that

A. they cannot handle intense water pressure

B. deep water is too cold

C. they would not get enough light

D. deep water is too salty

9. The word “detrimental'’ in the passage is closest in meaning to

A. harmful

B. significant

C. unexpected

D. distinct

10. The word “detrimental'’ in the passage is closest in meaning to

A. harmful

B. significant

C. unexpected

D. distinct

11. Paragraph 4 suggests that which of the following would be theLEAST likely to cause major damage to eelgrass and other common seagrasses?

A. Factors related to extreme temperatures

B. Exposure to air

C. Major changes in salinity

D. The movement of ice on the seafloor

12. The phrase “tolerant of’ in the passage is closest in meaning to

A. unused to

B. strongly affected by

C. protected from

D. able to withstand

13. Look at the four squares [■] that indicate where the followingsentence could be added to the passage.

Seagrasses grow together in dense patches, or beds, with as many as4,000 blades per square meter.

Where would the sentence best fit? Click on a square [■] to add thesentence to the passage

14. Look at the four squares [■] that indicate where the followingsentence could be added to the passage.

Seagrasses grow together in dense patches, or beds, with as many as4,000 blades per square meter.

Where would the sentence best fit? Click on a square [■] to add thesentence to the passage

Drag your answer choices to the spaces where they belong. To removean answer choice, click on it.

To review the passage, click VIEW TEXT

Seagrasses are aquatic flowering plants that grow in eithersheltered or open areas of the sea

A. Seagrass beds are influenced by several physical factors, themost significant being the stability of the sea bottom, which must anchor themagainst the currents.

B. Because they slow currents and waves, seagrass beds collectdeposits of rich organic sediments, which are home to many anaerobic microorganisms.

C. Unlike sea organisms that depend on the water column for theirproductivity, seagrasses ensure high rates of productivity by taking nutrientsfrom ocean floor sediment.

D. Sediments in seagrass beds vary by region, with temperate bedsaccumulating sediments from within, and tropical beds collecting sediments fromwithout.

E. Seagrasses under weak currents tend to have higher rates ofmetabolism than those under strong currents, perhaps because of differences inoxygen levels.

F. Although seagrasses survive in temperatures rangingfrom 2 to 28 degrees Celsius, more extreme temperatures can damage them, as candesiccation and lack of light.

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