Many areas of the shallow sea bottom are covered with a lush growth of aquatic flowering plants adapted to live submerged in seawater. ■ A These plants are collectively called seagrasses. ■ B Seagrass beds are strongly influenced by several physical factors. ■ C The most significant is water motion: currents and waves. ■ D 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.

  因为大多数海草系统是沉积环境,所以它们最终会积累一些有机物。这些有机物会产生细小的颗粒沉淀物,其中的有机物含量要高于周围无植被的区域。这种积累方式反过来会减弱水流流动,减少氧气含量。沉淀物里的微生物有着高速的新陈代谢速率,会使得在表面下方几毫米内的空气被消耗殆尽。根据生态学家 J. W. Kenworth 所说,微生物的无氧过程是一种再生和回收营养物质以及碳的重要过程,保证了在这些海草带的高产 (产量指的是产生的有机物的总量)。相比于多种依赖水中营养物质的藻类和细菌所产生的有机物,海草的根能够从沉淀物或基质中吸收营养物质。所以,它们能够将营养物质回收回生态环境中,否则这些营养物质会被吸收到海底而变得不可利用。

  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.

  其他能够影响海草带的物理因素包括光照、温度和失水。比如,水深和浑浊度(水中颗粒的浓度)能够共同或分别的控制海草所能利用的光照量,从而控制海草所能生长的深度。虽然海洋植物学家 W. A. Setchell 很早就指出,温度对于鳗草的生长和繁殖非常关键,但是此后被揭示出的是,这种分布非常广泛的海草在 2-4 摄氏度的北极和高达 28 摄氏度的美国东北海岸都能够生长和繁殖。但仍然,极端的温度伴随着其他因素,可能存在显著的害处。比如说,在寒冷的北大西洋中,冬季会形成冰。研究者 Robertson 和 Mann 指出,当冰开始碎裂,风和潮汐可能使得冰来回移动,擦过海底,将鳗草连根拔起。相比之下,在鳗草生长范围的南端,美国的东南海岸,夏天 30 摄氏度以上的温度会导致极高的死亡率。如果它们过多的暴露在空气中,海草带同样会减少。失水的作用通常很难从温度的影响中分离出来。大多数海草带貌似能够忍耐很大程度上的盐度变化,所以在盐水(比如纯海水)中被发现。