托福阅读真题Official 44 Passage 3（五）

托福阅读真题Official 44 Passage 3（五）

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 9 of 14

In paragraph 4,why does the author mention that eelgrass thrives in both the Arctic and in the northeastern United States?

A.To show that environments with extreme temperatures rarely have any effect on eelgrass

B.To identify the northern and southern limits of the range where eelgrass is found

C.To support the author’s statement that eelgrass is a particularly widespread kind of seagrass

D.To cite evidence tending to disprove one view about the importance of temperature to the growth of eelgrass

正确答案：D

题目详解

题型分类：修辞目的题

题干分析：定位词：eelgrass thrives in both the Arctic and in the northeastern United States。利用定位词定位到第四段的第三句。分析该句：逻辑上前后为转折关系，具体内容是说尽管温度对于eelgrass的生长至关重要，但是在一些温度（或高或低）的地方还是发现eelgrass都可以广泛生长，也就是证明温度的影响不是唯一的或绝对的。该题为句间关系的修辞目的题。

选项分析：

D选项说disprove one view about the importance of temperature to the growth of eelgrass，与定位句的逻辑和内容相符。

A选项说极端温度的环境对eelgrass很少起作用。不符合定位句的前半部分temperature was critical to the growth and reproduction of eelgrass。与原文描述不符。

B选项说确定eelgrass生长的南北范围界限。但定位句说的是温度的影响。无中生有。

C选项说支持eelgrass是一种特别广泛的海草类型这一观点，与定位句内容不符。

Question 10 of 14

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

A.harmful

B.significant

C.unexpected

D.distinct

正确答案：A

题目详解

题型分类：词汇题

题干分析：词汇所在句子Still,extreme temperatures,in combination with other factors,may have dramatic detrimental effects译为“当然，极端温度及其他因素可能会一起产生巨大的不良影响”，detrimental不良的。另外根据下文的举例，提到scouring the bottom and uprooting the eelgrass以及cause excessive mortality等，我们也可以得到线索detrimental是表示消极意义的词汇。

选项分析：

A选项harmful有害的。

B选项significant重要的。

C选项unexpected意料之外的。

D选项distinct独特。

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