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浅海底的许多地区覆盖着茂盛的水生开花植物，这些植物适应在海水中生活。

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.

海草床受到多种物理因素的强烈影响。

Seagrass beds are strongly influenced by several physical factors.

最主要的是水运动：水流和波浪。

The most significant is water motion: currents and waves.

由于海草系统既存在于有遮蔽的区域，也存在于相对开阔的区域，它们会受到不同量的水运动的影响。

Since seagrass systems exist in both sheltered and relatively open areas, they are subject to differing amounts of water motion.

但是，对于任何给定的海草系统，水运动都是相对恒定的。

For any given seagrass system, however, the water motion is relatively constant.

在相对湍急的水域中的海草草甸倾向于形成单个土丘状嵌合体，而在相对平静的水域中的草甸趋于形成平坦而宽阔的毯状体。

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.

反过来，海草床会减弱波浪作用，尤其是当叶片到达水面时。

The seagrass beds, in turn, dampen wave action, particularly if the blades reach the water surface.

这种阻尼效应会很显著，以至于只要进入海草床一米，波浪运动就可以减少到零。

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.

波浪作用和洋流的减慢意味着海草床易于积聚沉积物。

The slowing of wave action and currents means that seagrass beds tend to accumulate sediment.

然而，这并不是普遍的，而是取决于河床所处的水流。

However, this is not universal and depends on the currents under which the bed exists.

在强流作用下的海草床层往往会将许多较轻的颗粒（包括海草碎屑）移出，而在弱流区的床层会堆积较轻的碎屑物质。

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.

有趣的是，温带海草床从床外的源头积累了沉积物，而热带海草床的大部分沉积物则来自内部。

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.

由于大多数海草系统都是沉积环境，因此它们最终会积聚有机物质，从而导致产生的细粒沉积物的有机物含量要高于周围无植被的地区。

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.

反过来，这种积聚又减少了水的流动和氧气的供应。

This accumulation, in turn, reduces the water movement and the oxygen supply.

沉积物中微生物的高代谢率（为生存而进行的能量处理）导致沉积物在最初几毫米以下是厌氧的（无氧的）。

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.

生态学家J.W.肯沃西认为，沉积物中微生物的厌氧过程是再生和循环营养物质和碳的重要机制，确保了在这些海草床中测量的高生产率（即有机物质的产生量）。

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.

与海洋中的其他生产力不同，海洋中的其他生产力仅限于各种藻类和细菌，取决于水柱中的营养浓度，而海草是从沉积物或底物中吸收养分的生根植物。

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.

因此，它们能够将养分循环到生态系统中，否则这些养分将被困在水底而无法利用。

They are, therefore, capable of recycling nutrients into the ecosystem that would otherwise be trapped in the bottom and rendered unavailable.

影响海草床的其他物理因素包括光照，温度和干燥（脱水）。

Other physical factors that have an effect on seagrass beds include light, temperature, and desiccation (drying out).

例如，水深和浊度（水中颗粒的密度）共同或分别控制植物可利用的光量以及海草可能延伸的深度。

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.

尽管海洋植物学家W.A.塞切尔很早就提出温度对鳗草的生长和繁殖至关重要，但后来的研究表明，这种分布特别广泛的海草在北极2到4摄氏度之间，和在美国东北海岸的温度高达28摄氏度时都能生长和繁殖。

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.

尽管如此，极端温度与其他因素一起可能会产生巨大的不利影响。

Still, extreme temperatures, in combination with other factors, may have dramatic detrimental effects.

例如，在寒冷的北大西洋地区，冬天可能会结冰。

For example, in areas of the cold North Atlantic, ice may form in winter.

研究人员罗伯逊和曼指出，当冰开始破裂时，风和潮汐可能会使冰四处移动，冲刷水底，将鳗草连根拔起。

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.

相比之下，在美国东南海岸鳗草山脉的南端，夏季气温超过30摄氏度会导致过度死亡。

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.

如果海草床暴露于过多的空气中，它们也会退化。

Seagrass beds also decline if they are subjected to too much exposure to the air.

干燥带来的影响通常很难与温度的影响分开。

The effect of desiccation is often difficult to separate from the effect of temperature.

大多数海草床似乎可以容忍盐分（盐度）的显著变化，可以在微咸（有点咸）的水中生存，也可以在纯盐水中生存。

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.

According to paragraph 1, which of the following is true about seagrasses in calm ocean waters?

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

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.

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

According to paragraph 3, which of the following does NOT accurately describe the sediments that collect in seagrass beds?

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

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

It can be inferred from paragraph 4 that the reason seagrasses do not grow in very deep water is that

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

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

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

The phrase “tolerant of” in the passage is closest in meaning to

Look at the four squares  that indicate where the following sentence could be added to the passage. Seagrasses grow together in dense patches, or beds, with as many as 4,000 blades per square meter.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

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.