Phytoplanktonareminute, free-floatingaquaticplants. Inadditionto themarkedchanges inabundanceobserved inphytoplanktonover the course of a year, there is also amarkedchange inspeciescomposition. This change in thedominantspeciesfromseasontoseasonis calledseasonalsuccession, and it occurs in a widevarietyof locations. Underseasonalsuccession, one or morespeciesdominatethephytoplanktonfor a shorter or longer period of time and then are replaced by another set ofspecies. Thispatternis repeated yearly. Thissuccessionis different fromtypicalterrestrialecologicalsuccessionin which various plantsreplaceone another until finally aso-calledclimaxcommunitydevelops, which persists for many years.

What are the factors causing thisphenomenon? Considering thatseasonalsuccessionis most often and clearly seen intemperateseas, which have amarkedchange intemperatureduring a year,temperaturehas been suggested as a cause. This may be one of the factors, but it isunlikelyto be thesolecause because there arespeciesthat becomedominantspeciesat various temperatures. Furthermore,temperaturechanges rather slowly in seawater, and the replacement ofdominantspeciesoften is much more rapid.

Another suggestedreasonis the change innutrientlevelover the year, with differing concentrations favoring different phytoplanklonspecies. While thisfactormay alsocontribute, observations suggest thatphytoplanktonpopulations rise and fall much more quickly thannutrientconcentrations change.

Yet another explanation is thatspeciessuccessionis aconsequenceof changes in seawater brought about by thephytoplanktonliving in it. Eachspeciesofphytoplanktonsecretes or excretesorganicmolecules into the seawater. These metabolites can have an effect on the organisms living in the seawater, either inhibiting or promoting their growth. For any individualorganism, theamountof metabolite secreted is small. But the effect of secretions by all the individuals of thedominantspeciescan besignificantboth for themselves and for otherspecies.

Theseorganicmetabolites could, and probably do, include a number of different classes oforganiccompounds. Some are likely toxins, such as those released by the dinoflagellates (aspeciesofplankton) during red tides, whichinhibitgrowth of otherphotosyntheticorganisms. In such cases, thepopulationexplosionof dinoflagellates is so great that the water becomes brownish red in color from the billions of dinoflagellate cells. Although eachcellsecretes aminuteamountof toxin, themassivedinoflagellate numbers cause the toxin toreachconcentrations that kill many creatures. This toxin can be concentrated in such filter-feeding organisms as clams and mussels, rendering themtoxicto humans.

Another class of metabolite is the vitamins. It is now known that certainphytoplanktonspecieshave requirements for certain vitamins, and that there areconsiderabledifferences amongspeciesas to requirements. The B vitamins, especially vitamin B12, thiamine and biotin,seemto be the most generally required Somespeciesmay be unable tothriveuntil aparticularvitamin, or group of vitamins, is present in the water. These vitamins are produced only by another species: hence, asuccessionofspeciescouldoccurwherebyfirst the vitamin-producingspeciesis present and then the vitamin-requiringspeciesfollows.

Otherorganiccompounds that mayinhibitorpromotevariousspeciesincludeaminoacids, carbohydrates, and fatty acids. Although it is suspected that theseorganicmetabolites may have an important role inspeciessuccessionand it has been demonstrated in thelaboratorythatphytoplanktonspeciesvaryboth in theirabilityto produce necessary vitamins and in their requirements for such in order to grow,evidenceis still inadequate as to their real role in the sea.

There is alsoevidenceto suggest that grazers (animals that feed on plants orstationaryanimals), particularlyselectivegrazers, can influence thephytoplanktonspeciescomposition. Many copepods (small,herbivorouscrustaceans) andinvertebratelarvaepick out selectedphytoplanktonspeciesfrom mixed groups, changing thespeciescomposition.

A growing body ofevidencenow suggests that all of the factors considered here are operating simultaneously to producespeciessuccession. The importance of anyfactorwillvarywith theparticularphytoplanktonspeciesand theenvironmentalconditions.

Questions:
question_stem: The word "minute" in the passage is closest in meaning to
choices: tag: A
text: light

tag: B
text: common

tag: C
text: tiny

tag: D
text: simple
answer_key: C
explanation: 本题在文中对应的句子是 Phytoplankton are minute, free-floating aquatic plants. 意思是 Phytoplankton 是一种小小的自由漂浮的水生生物。minute 的意思是微小的，选项 A 是轻的，选项 B 是普通常见的，选项 C 是小的，选项 D 是简单的。C 最符合。

question_stem: According to paragraph 2, one reason temperature is not likely to be the sole cause of seasonal succession in phytoplankton is
choices: tag: A
text: the temperature of seawater changes too often.

tag: B
text: seasonal succession usually occurs only in temperate seas.

tag: C
text: the rate of replacement of dominant species is slower than that of temperature change.

tag: D
text: certain dominant species have been observed at different temperatures.
answer_key: D
explanation: 本题根据关键词 the sole cause 定位到这一句话：This may be one of the factors, but it is unlikely to be the sole cause because there are species that become dominant species at various temperatures. because 后面就是原因：在不同的温度下都会有物种成为优势物种。意思与选项 D 契合。选项 A 说海水温度变化太频繁，选项 B 说季节性的更替只在温和的海里进行，选项 C 说优势物种的更替速率比温度变化的速率快，这三个都是无中生有。选择 D。

question_stem: The word "differing" in the passage is closest in meaning to
choices: tag: A
text: increasing

tag: B
text: necessary

tag: C
text: varying

tag: D
text: high
answer_key: C
explanation: 本题在文中对应的句子是 Another suggested reason is the change in nutrient level over the year, with differing concentrations favoring different phytoplankton species. 另一个原因是营养水平在一年里是变化的，不同的营养物质的集中会导致不同浮游生物的爆发。differing 在这里的意识是不同的，选项 A 是增长的，选项 B 是必须的，选项 C 是变化的，选项 D 是高的。选项 C 最合适。

question_stem: Why does the author provide the information that "phytoplankton populations rise and fall much more quickly than nutrient concentrations change"?
choices: tag: A
text: To argue that the nutrient level not determine phytoplankton succession.

tag: B
text: To argue that the succession of phytoplankton is actually the cause of changes in the nutrient level.

tag: C
text: To indicate that phytoplankton populations are very sensitive to changes in the nutrient level.

tag: D
text: To imply that changes in the nutrient level explain the succession of phytoplankton.
answer_key: A
explanation: 本题需要定位到第三段，总体上看，第三段先是说了另一个猜测，即营养水平的变化决定了优势浮游生物的种类，后面接着 while 转折说还有一个因素可能是有用的，那就是浮游生物数量的变化是快于营养的变化的，对于前一个原因提出了质疑。选项 A 说是想说明营养水平并不决定浮游生物的更替，是对的。选项 B 说想证明的确是营养物质决定浮游生物的更替，选项 C 是说浮游生物对于营养水平很敏感，选项 D 是说营养水平的变化解释了浮游生物的更替，都不对。

question_stem: According to paragraph 4, all of the following are true about metabolites secreted by phytoplankton EXCEPT
choices: tag: A
text: They have a less significant effect on the dominant species of phytoplankton than they do on other species of phytoplankton.

tag: B
text: They affect the growth of phytoplankton living in seawater.

tag: C
text: They are secreted in small amounts by individual phytoplankton.

tag: D
text: They cause changes in seawater that result in species succession of phytoplankton.
answer_key: A
explanation: 本题需要看第四段。第四段主要讲了另一个对于浮游生物更替的解释：一种生物自己影响海水，进而影响自己的生存。每一种浮游生物都会分泌有机分子，影响水里有机物的生长。如果浮游生物数量少的话，这种物质的分泌也是少的，影响也小；但是一旦他们数量变多，这种分泌物的影响就会变大。B，C，D 都在这一段中被提到过，只有选项 A 说那些分泌的物质对浮游生物自己的影响小，对别的物种影响大，这在原文中没有提到，选 A。

question_stem: According to paragraph 5, all of the following statements are true about dinoflagellates EXCEPT
choices: tag: A
text: They produce toxins that inhibit the growth of other organisms.

tag: B
text: Each produces a very little amount of toxin.

tag: C
text: Their toxins can concentrate in clams and mussels.

tag: D
text: Their toxins are dangerous only to other photosynthetic organisms.
answer_key: D
explanation: 本题需要回到第五段。选项 A 对应 which inhibit growth of other photosynthetic organisms，正确；选项 B 对应 Although each cell secretes a minute amount of toxin，正确；选项 C 对应 This toxin can be concentrated in such filter-feeding organisms as clams and mussels,正确。选项 D 没有提到，错误。选择 D。

question_stem: The word "massive" in the passage is closest in meaning to
choices: tag: A
text: enormous

tag: B
text: total

tag: C
text: observed

tag: D
text: increasing
answer_key: A
explanation: 本题所考单词 massive 在原文中对应 Although each cell secretes a minute amount of toxin, the massive dinoflagellate numbers cause the toxin to reach concentrations that kill many creatures. 这一句大意是：尽管单个细胞分泌的毒物很少，但是很多细胞汇集起来，分析的毒物就可以杀死别的生物了。此处 massive 意为大量的。选项 A 意为巨大的，选项 B 意为全部的，选项 C 意为观察到的，选项 D 意为增长的。选项 A 最符合。

question_stem: The word "considerable" in the passage is closest in meaning to
choices: tag: A
text: often

tag: B
text: a few

tag: C
text: basic

tag: D
text: significant
answer_key: D
explanation: 本题所考单词 considerable 在原文中对应句子为 It is now known that certain phytoplankton species have requirements for certain vitamins, and that there are considerable differences among species as to requirements. 这句大意为某几种浮游生物需要特定的维他命，不同种类的浮游生物彼此间需要的维他命种类有很大不同。此处 considerable 意为巨大的。选项 A 经常，选项 B 几个，选项 C 基本的，选项 D 重要的，巨大的，选则 D。

question_stem: Paragraph 6 supports which of the following statements about a phytoplankton species that cannot produce thiamine?
choices: tag: A
text: It can be followed in a seasonal succession only by another phytoplankton species that cannot produce thiamine.

tag: B
text: It can never become the dominant species in a seasonal succession.

tag: C
text: It can be preceded in a seasonal succession only by a phytoplankton species that produces thiamine.

tag: D
text: It is able to produce B vitamins other than thiamine.
answer_key: C
explanation: 本题需要看第六段。第六段大致讲了这样一件事情：有的物种在没有某些特定维他命的时候是不能存活的，所以出现了这样一种现象，先出现一种可以生产特定维他命的物种，然后其他一种需要这些维他命的物种跟着产生。题目问一种不能自己生产 thiamine 的浮游生物会发生什么。选项 A 说它只能紧跟另一种也不能产生 thiamine 的物种产生，错；选项 B 说它永远不能成为优势物种，错；选项 C 说它可以紧跟产生 thiamine 的物种产生，对；选项 D 说它可以自己生产维生素 B，跟本段所说没关系。选项 C 正确。

question_stem: The copepods mentioned in paragraph 8 are
choices: tag: A
text: one type of phytoplankton

tag: B
text: stationary animals

tag: C
text: selective grazers

tag: D
text: invertebrate larva
answer_key: C
explanation: 本题涉及到原文第八段。第八段主要讲了有一些食草动物也会影响浮游生物的组成，举了 copepods 作为例子，说它和 invertebrate larvae 会从各种浮游生物里挑出某几种吃掉。选项 A 是一种浮游生物，选项 B 是不会动的动物，选项 C 是有选择性的草食者，选项 D 是 invertebrate larvae。选项 C 符合原文意思。

question_stem: The word "simultaneously" in the passage is closest in meaning to
choices: tag: A
text: without limits

tag: B
text: at the same time

tag: C
text: merely

tag: D
text: spontaneously
answer_key: B
explanation: 本题所考单词 simultaneously 对应到原文中这一句：A growing body of evidence now suggests that all of the factors considered here are operating simultaneously to produce species succession. 这一句大意为：越来越多的证据表明这些因素是一起影响物种更替的。simultaneously 意为同时的，一起的，选项 A 是无限制的，选项 B 是同时，选项 C 是仅仅，选项 D 是自发的。选项 B 最接近原意。

question_stem: Which of the following statements best describes how the passage is organized?
choices: tag: A
text: A description of a phenomenon is provided, forward hypotheses as to the phenomenon's cause.

tag: B
text: Two phenomena are introduced, and the rest of the passage provides reasons as to why the first is more common than the second.

tag: C
text: A group of species is discussed, and the rest of the passage explains the environmental factors threatening it.

tag: D
text: A phenomenon is described, and the rest of the passage discusses effects of that phenomenon on the environment.
answer_key: A
explanation: 本题问文章的组织结构。需要我们总览全文来分析。这种情况下为了节省时间，我们可以只看每段第一句来抓出结构。本文第一段介绍了 phytoplankton 的季节性更替，第二段开头提问更替的原因，从第二段开始，作者开始一一列举各种推测。本文总体上是一个总分的结构。选项 A 是说先给一个现象，接着提出各种假设，正确；选项 B 是说介绍了两种现象，后文则说明为什么第一个现象比第二个常见；选项 C 是说介绍了危及到一些物种的因素；选项 D 说先说一种现象，后文介绍其影响。B，C，D 都不正确。