1. Physical characteristics of aquatic environments at different depthssuch as salt level, light, inorganic nutrients, degree of acidity, and pressureall play key roles in the distribution of organisms. One of the most important physical features is thermal stratification.
2. When solar radiation strikes water, some is reflected, but most penetrates the surface and is ultimately absorbed. Although water may appear transparent, it is much denser than air and absorbs radiation rapidly. In clear water, 99 percent of the solar radiation is absorbed in the upper 50 to 100 meters. Longer wavelengths of light are absorbed first; the shorter wavelengths (which have more energy) penetrate farther, giving the depths their characteristic blue color.
3. This rapid absorption of sunlight by water has two important consequences. First, it means that photosynthesisthe process by which plants use the energy of sunlight to produce the organic carbon compounds necessary for lifecan only occur in surface waters where the light intensity is sufficiently high. Species that produce their own organic carbon compounds are called primary producers, and they are the base of the marine food web. Virtually all of the photosynthesis that supports the rich life of oceans and lakes comes from plants living in the upper 10 to 30 meters of water. Along shores and in very shallow bodies of water, some species such as kelp are rooted to the bottom. These plants may attain considerable size and structural complexity, and may support diverse communities of organisms. In the open waters that cover much of the globe, however, the primary producers of organic carbon are tiny, often one-celled algae (called phytoplankton), which are suspended in the water. Zooplankton, tiny invertebrates that feed on phytoplankton, migrate vertically on a daily cycle: up into the surface waters at night to feed and down into the dark, deeper waters during the day to escape predatory fish that rely on light to detect prey.
4. Second, the rapid absorption of sunlight by water means that only surface water is heated. The density of pure water is greatest at 4¡ãC and declines as the water's temperature rises above or falls below this point. When solar radiation heats the water surface above 4¡ãC, the warm surface water becomes lighter than the cool, deeper water, and so tends to remain on the surface, where it may be heated further and become even less dense. In tropical areas and in temperate climates during the summer, the surfaces of oceans and lakes are usually covered by a thin layer of warm water. Unless these bodies of water are shallow, the deep water below this layer is much colder (sometimes near 4¡ãC). The change in temperature between the warm surface water and the cold, deep water is called the thermocline. Mixing of the surface water by wave action determines the depth of the thermocline and maintains relatively constant temperatures in the water above it.
5. Tropical lakes and oceans show pronounced permanent stratification of their physical properties, with warm, well-oxygenated, and lighted surface water giving way to frigid, dark, deep water almost devoid of oxygen. Oxygen cannot be replenished at great depths where there are no photosynthetic organisms to produce it, and the stable thermal stratification prevents mixing and reoxygenation by surface water. Only relatively few organisms can live in such extreme conditions. The waste products and dead bodies of organisms living in the surface waters sink to the depths, taking their mineral nutrients with them. The lack of vertical circulation thus limits the supply of nutrients to the phytoplankton above. Consequently, deep tropical lakes are often relatively unproductive and depend on continued input from streams for the nutrients required to support life.
6. The situation is somewhat different in temperate and polar waters. Deep lakes, in particular,undergo dramatic seasonal changes: they develop warm surface temperatures and a pronounced thermocline in summer, but freeze over in winter. Twice each year, in spring and fall, the entire water column attains equal temperature and equal density; moderate winds may then generate waves that mix deep and shallow water, producing what is called overturn. This semiannual mixing carries oxygen downward and returns inorganic nutrients to the surface. Phosphorus and other nutrients may be depleted during the summer; overturn replenishes these nutrients by stimulating the growth of phytoplankton.
1..The word ultimately in the passage is closest in meaning to
2..According to paragraph 2, which of the following is true about solar radiation when it strikes water
A. Longer wavelengths travel farther.
B. Most of it is absorbed near the surface.
C. It is not absorbed by cloudy water.
D. More long wavelengths than short wavelengths are absorbed.
3..The word Virtually in the passage is closest in meaning to
4..According to paragraph 3, all of the following are true of kelp EXCEPT:
A. They are found at the bottom of shallow waters.
B. They are the primary producers of organic carbon.
C. They can grow very large.
D. They are a source of food for a variety of organisms.
5..The word suspended in the passage is closest in meaning to
6..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.
A. Zooplankton move up into surface waters at night to feed on phytoplankton, and down into deep, dark waters in daytime to avoid detection by predatory fish.
B. Zooplankton migrate vertically on a daily basis in order to feed on phytoplankton, following them up into surface waters by day and down into dark, deeper waters at night.
C. Unlike zooplankton, which migrate between surface waters and deeper waters, predatory fish must rely on the daytime light of surface waters to detect prey.
D. Zooplankton are so tiny that they can only be detected by predatory fish in the light conditions of surface water, where they feed on phytoplankton.
7..According to paragraph 4, what is a thermocline
A. The deep, cold layer of water below the surface of oceans and lakes
B. The thin layer of warm water on the surface of oceans and lakes
C. The change in density of surface water as it is continually heated
D. The point at which the temperature of water in oceans and lakes changes from warm to cold
8..The word replenished in the passage is closest in meaning to
9..According to paragraph 5, all of the following are true of the water at great depths in tropical lakes EXCEPT:
A. It has less oxygen than the surface water does.
B. It is very dark.
C. It contains relatively few living organisms.
D. It has low levels of mineral nutrients.
10..The word dramatic in the passage is closest in meaning to
11..According to paragraph 6, after overturn, deep water levels of lakes in temperate climates will contain increased amounts of
A. warm water
12..According to paragraphs 5 and 6, temperate lakes are in general more productive than tropical lakes because
A. temperate lakes receive more nutrients from incoming streams
B. the summer thermocline is relatively deep in temperate lakes
C. solar radiation penetrates more of the water in temperate lakes
D. temperate lakes experience vertical circulation of water
13..Look at the four squares that indicate where the following sentence could be added to the passage.
If the water is clouded by many microscopic organisms, absorption occurs even closer to the surface.
Where would the sentence best fit Click on a square to add the sentence to the passage.
14..Drag your choices to the spaces where they belong. To review the passage, click on View Text.
A. Sunlight is rapidly absorbed by the upper layer of water, which makes it warmer and consequently less dense than the water below.
B. Photosynthesizing plants, which provide the food and oxygen for most other forms of aquatic life, are limited to the levels of oceans and lakes where sunlight is available.
C. The diversity of plant life in shallow water makes it attractive to zooplankton.
D. As water becomes less dense and moves to the surface of the ocean or lake, it cools down and moderates the temperature of the surface.
E. Because more solar energy is absorbed by bodies of water in the tropics, the topmost layer of the thermocline is widest there.
F. In the tropics, permanent thermal stratification limits the amount of life supported in deep bodies of water; however, in temperate climates, seasonal thermal stratification allows vertical circulation and mixing of water, allowing the support of more life.