The influence of floodplains on the composition and export of riverine dissolved organic matter (DOM) was investigated during 2009--2010 in the lower Mississippi River (MR) and its distributary, the Atchafalaya River (AR). The AR had significantly higher concentrations of dissolved organic carbon (DOC), chromophoric DOM (CDOM), total dissolved lignin phenols (TDLP), amino acids (TDAA), and neutral sugars (TDNS) than the MR. Syringyl to vanillyl phenol ratios in the AR were significantly lower, indicating greater contributions from gymnosperms (cypress) in AR floodplains. Higher DOC-normalized yields of CDOM and TDLP in the AR indicated a strong floodplain source of DOM. Compositions and yields of TDAA and TDNS suggested DOM in the AR was more bioavailable than that in the MR. Yields of CDOM and TDLP in the AR were highest during maximal litter fall, when the depletion in syringyl phenols was greatest. Significantly lower concentrations of total dissolved nitrogen observed in the AR were in part due to nitrogen removal in floodplains. Temporal variability in DOM composition was more pronounced in the AR than in the MR. These results demonstrate that floodplains have a major influence on the composition and concentrations of riverine DOM, and thereby on the fates of riverine DOM in coastal oceans. In 2009, over 40% of the DOC, TDLP, TDAA, and TDNS exported from the Mississippi-Atchafalaya River system was carried by the AR, indicating a more important role of the AR in DOM export than previously thought. Therefore, the AR should be considered in future studies of river impacts on coastal processes, such as coastal productivity and hypoxia in the northern Gulf of Mexico. Long-term (1996--2010) average DOC export from the AR and MR (0.90 and 1.72 Tg yr -1, respectively) indicates the Mississippi-Atchafalaya River system exports 0.7--1.0% of global riverine DOC. New estimates of DOC export provided here are critical to improve carbon budgets in the Gulf of Mexico.
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机译：在2009--2010年期间，在密西西比河下游（MR）及其支流阿察法拉拉河（AR）中调查了洪泛区对河流溶解有机物（DOM）的组成和出口的影响。与MR相比，AR的溶解有机碳（DOC），发色DOM（CDOM），总溶解木质素酚（TDLP），氨基酸（TDAA）和中性糖（TDNS）的浓度明显更高。 AR中的丁香基与香草基苯酚比率显着较低，这表明AR泛滥平原中裸子植物（柏树）的贡献更大。在AR中，较高的DOC标准化CDOM和TDLP产量表明DOM是洪泛平原的强大来源。 TDAA和TDNS的组成和产量表明，AR中的DOM比MR中的生物利用度更高。在最大的凋落物减少期间，当丁香基酚的消耗最大时，AR中的CDOM和TDLP的产量最高。在AR中观察到的总溶解氮浓度明显较低，部分原因是洪泛区的氮被去除。与MR相比，AR中DOM成分的时间变化更为明显。这些结果表明，洪泛区对河流中DOM的组成和浓度具有重要影响，从而对沿海海洋中河流DOM的命运也有重大影响。 2009年，AR携带了密西西比-阿恰法拉雅河系统出口的DOC，TDLP，TDAA和TDNS的40％以上，这表明AR在DOM出口中的作用比以前想象的要重要。因此，在以后的河流对沿海过程影响的研究中应考虑到AR，例如墨西哥湾北部的沿海生产力和低氧。 AR和MR的长期（1996--2010）DOC平均出口（分别为0.90和1.72 Tg yr -1）表明，密西西比-阿查法拉雅河系统的出口占全球河流DOC的0.7--1.0％。此处提供的DOC出口新估算对于提高墨西哥湾的碳预算至关重要。
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