Alexander RB, Smith RA, Schwarz GE
U.S. Geological Survey, Reston, Virginia 20192, USA.
Nature. 2000 Feb 17;403(6771):758-61. doi: 10.1038/35001562.
An increase in the flux of nitrogen from the Mississippi river during the latter half of the twentieth century has caused eutrophication and chronic seasonal hypoxia in the shallow waters of the Louisiana shelf in the northern Gulf of Mexico. This has led to reductions in species diversity, mortality of benthic communities and stress in fishery resources. There is evidence for a predominantly anthropogenic origin of the increased nitrogen flux, but the location of the most significant sources in the Mississippi basin responsible for the delivery of nitrogen to the Gulf of Mexico have not been clearly identified, because the parameters influencing nitrogen-loss rates in rivers are not well known. Here we present an analysis of data from 374 US monitor ing stations, including 123 along the six largest tributaries to the Mississippi, that shows a rapid decline in the average first-order rate of nitrogen loss with channel size--from 0.45 day (-1) in small streams to 0.005 day (-1) in the Mississippi river. Using stream depth as an explanatory variable, our estimates of nitrogen-loss rates agreed with values from earlier studies. We conclude that the proximity of sources to large streams and rivers is an important determinant of nitrogen delivery to the estuary in the Mississippi basin, and possibly also in other large river basins.
在20世纪后半叶，来自密西西比河的氮通量增加，导致了墨西哥湾北部路易斯安那陆架浅水区的富营养化和长期季节性缺氧。这已致使物种多样性减少、底栖生物群落死亡以及渔业资源面临压力。有证据表明氮通量增加主要源于人为因素，但密西西比河流域中负责向墨西哥湾输送氮的最重要源头位置尚未明确确定，因为影响河流中氮损失率的参数尚不为人所知。在此，我们对来自374个美国监测站的数据进行了分析，其中包括沿密西西比河六大支流的123个监测站的数据，分析结果显示，随着河道规模增大，氮损失的平均一级速率迅速下降——从小溪流中的0.45天⁻¹降至密西西比河中的0.005天⁻¹。以河流深度作为解释变量，我们对氮损失率的估算结果与早期研究的值相符。我们得出结论，源头与大型溪流和河流的距离是密西西比河流域乃至其他大型河流域向河口输送氮的一个重要决定因素。
