托福阅读真题Official 44 Passage 3（四）

托福阅读真题Official 44 Passage 3（四）

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 7 of 14

According to paragraph 3,how do seagrasses affect the nutrient supply in the ecosystem?

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

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

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

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

正确答案：C

题目详解

题型分类：事实信息题

原文定位：定位词：the nutrient supply in the ecosystem。利用定位词定位到第三段的最后一句。即海草能够将营养物质再循环进入生态系统，否则这些营养物质就被储存在海底而无法得以利用。

选项分析：

C选项take up,bring...back into use对应recycling...into...。

A选项提到high rate of metabolism对应的是第三段的第三句，说的是The high rate of metabolism of the microorganisms而不是指海草在生态系统中对营养的作用。张冠李戴。

B选项说海草attract various species of algae and bacteria在文中没有此信息，无中生有。

D选项说they produce a very nutrient-rich sediment，而最后一句说recycling，用词不准确，与原文描述不符。

Question 8 of 14

It can be inferred from paragraph 4 that the reason seagrasses do not 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

正确答案：C

题目详解

题型分类：推论题

原文定位：定位词：seagrasses do not grow in very deep water。利用定位词定位到第四段的前两句。第一句提出影响海草床的因素有光、温度和干燥。第二句举例说明水深和水中颗粒的密度会控制海草所获得的光以及海草生长所到的深度。根据此信息我们可以推论出光对于海草床生长很重要，而深的水域中光是不够的，也就会影响海草的生长。

选项分析：

C选项说would not get enough light对应第二句的water depth...together or separately control the amount of light available to the plants。

A选项说intense water pressure，水压在文中不是影响海草的因素，无中生有。

B选项说too cold。温度确实会影响海草，但根据第二句我们知道水深和光之间有关系，和温度没有关系，因此该选项与原文描述不符。

D选项说too salty。根据第四段最后一句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我们可以知道盐度并不会影响海草。