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Many theories have been formulated to explain the role of grazers such as zooplankton in controlling the amount of planktonic algae (phytoplankton) in lakes. The first theories of such grazer control were merely based on observations of negative correlations between algal and zooplankton numbers. A low number of algal cells in the presence of a high number of grazers suggested,but did not prove,that the grazers had removed most of the algae. The converse observation, of the absence of grazers in areas of high phytoplankton concentration, led Hardy to propose his principle of animal exclusion, which hypothesized that phytoplankton produced a repellent that excluded grazers from regions of high phytoplankton concentration. This was the first suggestion of algal defenses against grazing.
Perhaps the fact that many of these first studies considered only algae of a size that could be collected in a net (net phytoplankton), a practice that overlooked the smaller phytoplankton (nannoplankton) that we now know grazers are most likely to feed on, led to a de-emphasis of the role of grazers in subsequent research. Increasingly, as in the individual studies of Lund, Round, and Reynolds, researchers began to stress the importance of environmental factors such as temperature, light, and water movements in controlling algal numbers. These environmental factors were amenable to field monitoring and to simulation in the laboratory. Grazing was believed to have some effect on algal numbers, especially after phytoplankton growth rates declined at the end of bloom periods, but grazing was considered a minor component of models that predicted algal population dynamics.
The potential magnitude of grazing pressure on freshwater phytoplankton has only recently been determined empirically. Studies by Hargrave and Geen estimated natural community graz-ing rates by measuring feeding rates of individual zooplankton species in the laboratory and then computing community grazing rates for field conditions using the known population density of grazers. The high estimates of grazing pressure postulated by these researchers were not fully accepted, however, until the grazing rates of zooplankton were determined directly in the field, by means of new experimental techniques. Using a specially prepared feeding chamber, Haney was able to record zooplankton grazing rates in natural field conditions. In the periods of peak zooplankton abundance, that is, in the late spring and in the summer, Haney recorded maximum daily community grazing rates, for nutrient-poor lakes and bog lakes, respectively, of 6.6 percent and 114 percent of daily phytoplankton production. Cladocerans had higher grazing rates than copepods, usually accounting for 80 percent of the community grazing rate. These rates varied seasonally, reaching the lowest point in the winter and early spring.Haney's thorough research provides convincing field evidence that grazers can exert significant pressure on phytoplankton population.
[数学 ]【新版】冲分救命题 -11998
The integers a, b, and c are positive, and a>b>c. When a, b, and c are divided by 3, the remainders are 0, 1, and 1, respectively.
[数学 ]【新版】冲分救命题 -11997
For all positive even integers n. n* represents the product of all even integers from 2 to n, inclusive. For example, 12*=12×10×8×6×4×2. What is the greatest prime factor of 20*+22*?
[数学 ]【新版】冲分救命题 -11996
[数学 ]【新版】冲分救命题 -11995
[数学 ]【新版】冲分救命题 -11994
In a game, the cards in a certain deck are distributed to players one at a time until each player has the same number of cards and there are not enough cards left in the deck to distribute one more card to each player. The number of cards in the deck is between 60 and 100. If the cards in the deck are distributed o 5 players, 2 cards will be left in the deck. If the cards in the deck are distributed to 6 players, 4 cards will be left in the deck. If the cards in the deck are distributed to 7 players, how many cards will be left in the deck?
[数学 ]【新版】冲分救命题 -11993
Let a be the greatest integer such that is a factor of 1,500, and let b be the greatest integer such that is a factor of 33,333,333. Which of the following statements are true? Indicate all such statements.
[数学 ]【新版】冲分救命题 -11992
A set consists of all three-digit positive integers with the following properties.Each integer is of the form JKL,where J,K,and I are digits;all the digits are nonzero;and the two-digit integers JK and KL are each divisible by 9.How many integers are in the set?
[数学 ]【新版】冲分救命题 -11991
[数学 ]【新版】冲分救命题 -11990
The number of children in a certain family is a prime number less than 10. The number of boys in the family is greater than the number of girls, and the number of boys is a prime number. If at least 1 of the children in the family is a girl, which of the following could be the number of boys in the family?Indicate all such numbers.
[数学 ]【新版】冲分救命题 -11989
[数学 ]【新版】冲分救命题 -11988
x and y are integers.
[数学 ]【新版】冲分救命题 -11987
x and y are integers.
[数学 ]【新版】冲分救命题 -11986
a and b are distinct odd prime numbers.
[数学 ]【新版】冲分救命题 -11985
When the integer n is divided by 33, the remainder is 24. Which of the following must be a divisor of n?
[数学 ]【新版】冲分救命题 -11984
4.What is the length of an edge of the smallest solid cube that can be made by placing together solid rectangular blocks of size 7 by 6 by 3?
[数学 ]【新版】冲分救命题 -11983
Light travels at approximately 186,000 miles per second.Which of the following is closest to the number of miles that light travels in 3 hours.
[数学 ]【新版】冲分救命题 -11982
The number n is the least positive integer for which 108n is the square of an intege
[数学 ]【新版】冲分救命题 -11981
The number n is the least positive integer for which 108n is the square of an intege
[数学 ]【新版】冲分救命题 -11980
If x=2y+1, and y=2w, where w, x, and y are integers, which of the following must be an odd integer?
[数学 ]OG -11979
If x=2y+1, and y=2w, where w, x, and y are integers, which of the following must be an odd integer?
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