In the context of the passage poetic is closest in

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Unformatted text preview: 10 vation, 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 15 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 over20 looked 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 25 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 30 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. 35 The potential magnitude of grazing pressure on freshwater phytoplankton has only recently been determined empirically. Studies by Hargrave and Geen estimated natural community grazing rates by measuring feeding rates of individual zooplankton 40 species in the laboratory and then computing community grazing rates for field conditions using the known population density of grazers. The high esti mates of grazing pressure postulated by these 45 50 55 60 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 co mmunity 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. (461 words) 1. It can be inferred from the passage that the ―first theories‖ of grazer control mentioned in the first paragraph would have been more convincing if researchers had been able to (A) observe high phytoplankton numbers under natural lake conditions (B) discover negative correlations between algae and zooplankton numbers from their field research (C) understand the central importance of environmental...
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