13.2.1 Nutrient chemistry of large rivers
Large rivers, especially large lowland rivers, often have their nutrient concentrations modified by industrial emissions, sewage and agricultural effluents, and they also are likely to have their flow regime regulated by dams. Because their catchments include a substantial land area, each large river has its individual climate and geology.
For these reasons it is difficult to generalize, and interested readers should consult studies of specific rivers (e.g. Whitton, 1975b; Davies and Walker, 1986; Dodge, 1989; and detailed descriptions of individual river systems in the Monographiae Biologicae series by W.Junk, The Hague). Even to summarize data for a given river is problematic, because values change greatly along a river’s length, seasonally, and between cleaner and polluted sections.
Meybeck (1982) provides a comprehensive review of the various forms (dissolved and particulate, organic and inorganic) of carbon, nitrogen and phosphorus in world rivers. Natural levels are estimated from small streams in the temperate zone and major rivers of the tropics and subarctic, where anthropogenic inputs are thought to be minimal. Nutrient concentrations from these systems are very similar to the average content of rain, supporting the view that they represent an unpolluted state. Natural levels of dissolved phosphorus are very low, around 0.01mg1-1 for PO4-3 and 0.025mg1-1 for total dissolved phosphate, which includes the organic form. Natural levels of DIN (including ammonium, nitrate and nitrite) are also low compared with rivers affected by human activities, about 0.12mg1-1,and nitrate is the major fraction. Ammonia averages15﹪, and nitrite about 1﹪, of DIN. BecauseNH4+ is usually the preferred form of inorganic nitrogen, data on nitrate alone or total inorganic nitrogen may be of limited usefulness.
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基於這些理由，它是困難地去歸納，有興趣的讀者應該查閱特定河流的研究(例如： Whitton, 1975b; Davies and Walker, 1986; Dodge, 1989; 以及專題生物系列中個別河流系統的詳細描述，由W.Junk所著, The Hague)。甚至對於一個給定的河流的摘要資料是有問題的，因為沿著河流全程、季節性地、及介於清潔與污染的河段的數值變化極大。
Meybeck (1982) 提供了一個各種形式(溶解的與微粒的、有機與無機的)在世界河流中的碳、氮與磷的廣泛評論。從溫帶的小溪流與熱帶及極帶的主要河流中評估自然標準，在那些地方中人為的輸入是被考慮到最小化。來自於這些系統的營養濃度與雨水的平均成份非常相似，這證實了它們代表著一個未污染狀態的論點。溶解磷的天然標準非常低，對於PO4-3大約0.01mg1-1及對於總溶解磷酸鹽是0.025mg1-1，包含了有機形式。DIN的天然標準(包含氨鹽基、硝酸鹽與亞硝酸鹽)與受到人類活動影響的河流相比也是低的，大約 0.12mg1-1，而硝酸鹽是主要的部份。氨平均15%、而亞硝酸鹽約DIN的1%。因為NH4+通常是無機氮的優先形式，單獨的硝酸鹽或總體無機氮的數據資料可能是效用有限。參考資料： mE