托福阅读真题Official 44 Passage 3（二）

托福阅读真题Official 44 Passage 3（二）

Seagrasses

Many areas of the shallow sea bottom are covered with a lush growth of aquatic flowering plants adapted to live submerged in seawater.These plants are collectively called seagrasses.Seagrass beds are strongly influenced by several physical factors.The most significant is water motion:currents and waves.Since seagrass systems exist in both sheltered and relatively open areas,they are subject to differing amounts of water motion.For any given seagrass system,however,the water motion is relatively constant.Seagrass meadows in relatively turbulent waters tend to form a mosaic of individual mounds,whereas meadows in relatively calm waters tend to form flat,extensive carpets.The seagrass beds,in turn,dampen wave action,particularly if the blades reach the water surface.This damping effect can be significant to the point where just one meter into a seagrass bed the wave motion can be reduced to zero.Currents are also slowed as they move into the bed.

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

Since most seagrass systems are depositional environments,they eventually accumulate organic material that leads to the creation of fine-grained sediments with a much higher organic content than that of the surrounding unvegetated areas.This accumulation,in turn,reduces the water movement and the oxygen supply.The high rate of metabolism(the processing of energy for survival)of the microorganisms in the sediments causes sediments to be anaerobic(without oxygen)below the first few millimeters.According to ecologist J.W.Kenworthy,anaerobic processes of the microorganisms in the sediment are an important mechanism for regenerating and recycling nutrients and carbon,ensuring the high rates of productivity—that is,the amount of organic material produced—that are measured in those beds.In contrast to other productivity in the ocean,which is confined to various species of algae and bacteria dependent on nutrient concentrations in the water column,seagrasses are rooted plants that absorb nutrients from the sediment or substrate.They are,therefore,capable of recycling nutrients into the ecosystem that would otherwise be trapped in the bottom and rendered unavailable.

Other physical factors that have an effect on seagrass beds include light,temperature,and desiccation(drying out).For example,water depth and turbidity(density of particles in the water)together or separately control the amount of light available to the plants and the depth to which the seagrasses may extend.Although marine botanist W.A.Setchell suggested early on that temperature was critical to the growth and reproduction of eelgrass,it has since been shown that this particularly widespread seagrass grows and reproduces at temperatures between 2 and 4 degrees Celsius in the Arctic and at temperatures up to 28 degrees Celsius on the northeastern coast of the United States.Still,extreme temperatures,in combination with other factors,may have 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 ice begins to break up,the wind and tides may move the ice around,scouring the bottom and uprooting the eelgrass.In contrast,at the southern end of the eelgrass range,on the southeastern coast of the United States,temperatures over 30 degrees Celsius in summer cause excessive mortality.Seagrass beds also decline if they are subjected to too much exposure to the air.The effect of desiccation is often difficult to separate from the effect of temperature.Most seagrass 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

Question 3 of 14

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

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

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

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

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

正确答案：A

题目详解

题型分类：简化句子题

原文分析：该题型我们从逻辑和主干信息上进行分析处理。首先句中whereas表示前后两部分为对比；其次我们分别处理两个部分的主干信息。前半部分的主干信息是在强水流的影响下海草床中的一些较轻的颗粒会被带走；后半部分对比在较弱的水流中，这些碎屑物质则会沉积下来。

选项分析：

A选项中的but对应原句的whereas，表达两种不同情况，逻辑正确；collect对应accumulate；washed out对应moved out，并且说明是by strong currents。内容正确。

B选项说海草床在strong currents中会accumulate，与原句意思相反，内容错误。

C选项说determines how quickly，原句中没有提及速度这方面的信息，无中生有。

D选项说moved from areas of strong currents to areas of weak currents，原句没说是从strong currents移到weak currents，与原句描述不符。

Question 4 of 14

The word“derive”in the passage is closest in meaning to

A.maintain

B.expel

C.obtain

D.enrich

正确答案：C

题目详解

题型分类：词汇题

题干分析：词汇所在句子It is interesting that temperate seagrass beds accumulate sediments from sources outside the beds,whereas tropical seagrass beds derive most of their sediments from within译为“有趣的是，温带海草床会沉积外来的物质，而热带海草床则是从内部获得沉积物”，derive获得，得到。

选项分析：

C选项obtain获得，得到。

A选项maintain维持。

B选项expel逐出。

D选项enrich使丰富。