21.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.

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例进行了估计，其手段是通过测量出实验室内单独的浮游动物种类的结食比例，然后利用已知的食草动物种群密度，计算出实地状况下的群落食草比例。

22.In the periods　of peak zooplankton abundance，that is，in the late spring and in the summer，Haney recorded maximum daily com-munity grazing rates，for nutrient-poor　lakes and bog lakes，respectively，of 6.6 percent and 114 percent of daily phytoplankton production.

在浮游动物数量激增的高峰期，亦即在春季后期以及夏季，哈尼记录了最大程度上的每日群落食草比率，对于营养物不充足的湖和沼泽湖而言，分别为每日浮游植物繁殖量的6.6%和114%.

23.The hydrologic cycle，a major topic in this science，is the complete cycle of phenomena through which water passes，beginning as atmospheric water vapor，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 evaporation and transpiration.

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，开始时是作为大气中的水蒸气，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，最终通过蒸发和散发作用再度回复到大气水蒸气的形态。

24.The historian Frederick J. Turner wrote in the 1890's that the agrarian discontent that had been developing　steadily in the United States since about 1870 had been precipitated by theclosing of the internal frontier——that is，the depletion of available new land　needed 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发展的农民不满，由于国内边远地区（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利用的新土地几近耗竭。