21.Studies by Hargrave and Geen estimated natural community graz-ing rates by measuring feeding rates ofinpidual zooplankton species in thelaboratory and then computing1 community grazing rates for field conditionsusing the known population density2of grazers.
由哈格雷夫（Hargrave）和吉恩（Geen）所进行的研究，对自然条件下的群落食草比率进行了估计，其方法是通过测量出实验室内单独的浮游动物类型的结食比率，然后借助已知的食草动物种群密度，计算出实地情况下的群落食草比率。
22.In the periodsof peak zooplankton abundance，that is，in the late spring and in the summer，Haney recorded maximum daily com-munity grazing rates，for nutrient-poorlakes and bog3 lakes，respectively，of 6.6 percent and 114 percent of daily phyTOPlankton production.
在浮游动物数目激增的高峰期，亦即在春天后期与夏天，哈尼记录了最大程度上的每天群落食草比率，对于营养物不充足的湖和沼泽湖而言，分别为每天浮游植物繁殖量的6.6%和114%.
23.The hydrologic cycle，a major TOPic in this science，is the complete cycle of phenomena4 through which water passes，beginning as atmospheric5 water vapor6，passing into liquid and solid form as precipitation，thence along and into the ground surface，and finally again returning to the form of atmospheric water vapor by means of evaporation7 and transpiration8.
水文循环（hydrologic cycle），作为该学科中的一个主要课题，指的是水所经过的诸现象的整个循环过程，开始时是作为大方中的水蒸气，转而作为雨、雪、露、雹一类的降水量经过液体和固体形态，由此而沿着地层表面分布或进入地层表面，最后通过蒸发和散发用途再度回复到大方水蒸气的形态。
24.The historian Frederick J. Turner wrote in the 1890's that the agrarian9 discontent that had been developingsteadily10 in the United States since about 1870 had been precipitated11 by theclosing of the internal frontierthat is，the depletion12 of available new landneeded for further expansion of the American farming system.
史学家弗雷德里克。杰。特纳（Frederick J.Turner）于十九世纪九十年代著述道，美国约自18世纪70年代以来一直在持续不断进步的农民不满，因为国内边远区域（internal frontier）的封闭而更趋加剧亦即是说，美国农业系统进一步扩展所必需的可资借助的新土地几近耗竭。