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 computing¹ community grazing rates for field conditions　using the known population density²　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 bog³ 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 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 agrarian⁹ discontent that had been developing　steadily¹⁰ in the United States since about 1870 had been precipitated¹¹ by the　　closing of the internal frontier——that is，the depletion¹² 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。