Official 44 Passage 3




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  • 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.

  • 浅海底部的许多地方都被茂密的水生开花植物所覆盖,这些植物已经完全适应了浸没于海水的生活。 这些植物统称为海草。 海草床深受几个物理因素的影响。 其中,最重要的因素是海水运动:海流和海浪。 因为海草不仅生存在隐蔽的水域,也生存在相对开放的水域,因此海草需要去适应各种不同程度的水流运动。 然而,对于特定的海草系统而言,海水运动是相对恒定的。 在相对动荡的水域,海草一般会形成一个个小丘;而在相对平静的水域则倾向于形成平坦广阔的草地。 反过来,海草床也会减少波浪的作用,特别是当叶片长至水面以上时。 海草床的阻碍作用很强,一般的波浪只要遇到一米高的海草床,波动就会完全停滞。 而海流遇到海草床时,速度也会变慢。

    海浪和海流的速度减缓,意味着海草床经常会有沉积。 然而,情况也非绝对如此,是否有沉积主要取决于海草床下水流的强度。 遇到较湍急的海流,海草床中一些较轻的颗粒物,比如海草残骸,就会被海水流带走,而遇到较缓的水流,这些较轻的碎屑物质就会沉积下来。 有趣的是,温带海草床通常会沉淀外来物,而热带海草床的沉积物通常来自海草床内部。

    由于绝大多数海草系统都处于沉积环境,它们最终会积累下有机物质,进而得到有机质含量远高于比周围无植被区的细颗粒状沉淀物。 这样的积累反过来也减少了海水的运动和氧气供应。 沉积物中微生物的高新陈代谢(为生存而进行的能量转化)率,导致数毫米以下的沉积物缺氧(没有氧气)。 根据生态学家J•W•肯沃西所说,沉积物中微生物的无氧代谢是一项重要的使得营养物质和碳再生和循环的机制,保证了有机物的高产出率——即测量到的海草床中产生的有机物的量。 海洋中其他物种的产出率受到各种藻类和细菌的限制,而藻类和细菌又依赖于水体营养物质的浓度,但海草与其不同,海草是根系植物,可以从沉积物或海洋基底当中吸收养分。 因此它们能够将海洋底部的营养物质回收进入生态系统,否则,这些营养将会永远困在海底,不可利用。

    其他影响海草床的物理因素包括光、温度、干化(干燥)。 例如,水的深度和浊度(水中颗粒的密度)共同或单独控制海草可获取的光照量和海草可生长的深度。 尽管海洋植物学家W•A•萨契尔早期曾提出温度是海草生长和繁殖的关键,但研究已经证明,从2到4摄氏度的北极,到28摄氏度的美国东北海岸,这些广泛分布的海草都可以生长和繁殖。 当然,极端温度及其他因素一起可能会对海草的生存产生巨大的不良影响。 例如,在寒冷的北大西洋地区,冬天海水可能会结冰。 研究人员罗伯森和曼指出,如果冰层破裂,风和潮汐可以将冰块四处移动,刮擦海洋底部,将大叶藻连根拔起。 相反,在大叶藻可以生存的南端——美国的东南部海岸,夏季超过30摄氏度的温度会造成大叶藻大量死亡。 如果过多的暴露在空气中,海草床也会枯萎。 干化的影响通常难于和温度的影响分开。 大多数的海草床都能适应各种盐度(盐含量)的变化,在半咸水(微咸)海域和纯咸水海域都能生存。
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    原文定位:词汇所在句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.”即“海洋中其他物种的产出率受到各种藻类和细菌的限制,而藻类和细菌又依赖于水体营养物质的浓度,但海草与其不同,海草是根系植物,可以从沉积物或海洋基底当中吸收养分。”,confined:有限的。