Assessing dissolved carbon transport and transformation along an estuarine river with stable isotope analyses
Assessing dissolved carbon transport and transformation along an estuarine river with stable isotope analyses
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DOI：
10.1016/j.ecss.2017.08.024
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发表时间：
2017-10
影响因子：
2.8
通讯作者：
Songjie He;Y. Xu
中科院分区：
文献类型：
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作者：
Songjie He;Y. Xu
河口在从河流到沿海海洋的溶解碳动力学中起着重要作用。然而，我们对世界沿海河流混合带中溶解碳的运输和转化的认识仍然有限。研究了美国南部路易斯安那州Calcasieu河（盐度范围为0.02 ~ 21.92）河口长88 km的溶解无机碳（DIC）和溶解有机碳（DOC）浓度及其稳定同位素（δ 13cdic和δ13CDOC）的变化规律。该研究有望阐明哪些过程最有可能控制淡水-盐水混合系统中的碳动态，并评估该河口的净代谢。在2015年5月至2016年2月期间，从Calcasieu河上游到下游的六个地点进行了五次实地考察，收集了水样并对环境水条件进行了现场测量。Calcasieu河进入墨西哥湾北部（NGOM）。随着混合带盐度的增加，DIC浓度和δ 13cdic1迅速增大。最靠近NGOM的站点（站点6）的平均DIC浓度和δ 13dicc分别为1.31 mM和- 6.34‰，远高于最上游站点（站点1）的平均值（0.42 mM和- 20.83‰）。DIC浓度在很大程度上受保守混合的影响，而高水温可能由于呼吸和分解的增加而使DIC浓度偏离保守线。5月、6月和11月的δ 13cdicc值与保守混合模型的值接近，但低于7月和2月的δ 13cdicc值，表明河口河流可以从平衡系统波动到异养系统（即生产/呼吸(P/R) < 1）。与DIC的纵向趋势不同，河口DOC浓度由上游到下游呈下降趋势，但下降程度要小得多。DOC浓度始终显示出与保守混合模型的偏差，这可能是流内光合作用的结果。该河口的δ 13cdoc1值在- 30.56‰~ - 25.92‰范围内持续下降，表明混合带的DOC源高度来自陆源。然而，在这个相对较小的同位素范围内，δ 13cdoc1在区分河流-海洋连续体中水生光合作用产生的碳和陆地光合作用产生的碳方面存在局限性。
Estuaries play an important role in the dynamics of dissolved carbon from rivers to coastal oceans. However, our knowledge of dissolved carbon transport and transformation in mixing zones of the world's coastal rivers is still limited. This study aims to determine how dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations and stable isotopes (δ13CDICand δ13CDOC) change along an 88-km long estuarine river, the Calcasieu River in Louisiana, southern USA, with salinity ranging from 0.02 to 21.92. The study is expected to elucidate which processes most likely control carbon dynamics in a freshwater-saltwater mixing system, and to evaluate the net metabolism of this estuary. Between May 2015 and February 2016, water samples were collected and in-situ measurements on ambient water conditions were performed during five field trips at six sites from upstream to downstream of the Calcasieu River, which enters the Northern Gulf of Mexico (NGOM). The DIC concentration and δ13CDICincreased rapidly with increasing salinity in the mixing zone. The average DIC concentration and δ13CDICat the site closest to the NGOM (site 6) were 1.31 mM and −6.34‰, respectively, much higher than those at the site furthest upstream (site 1, 0.42 mM and −20.83‰). The DIC concentrations appeared to be largely influenced by conservative mixing, while high water temperature may have played a role in deviating DIC concentration from the conservative line due likely to increased respiration and decomposition. The δ13CDICvalues were close to those suggested by the conservative mixing model for May, June and November, but lower than those for July and February, suggesting that an estuarine river can fluctuate from a balanced to a heterotrophic system (i.e., production/respiration (P/R) < 1) seasonally. Unlike the DIC longitudinal trend, the DOC concentrations in the river estuary decreased from upstream to downstream, but to a much smaller degree. The DOC concentrations consistently showed a deviation from those suggested by the conservative mixing model, which may have been a consequence of in-stream photosynthesis. This river estuary consistently showed depleted δ13CDOCvalues (i.e., from −30.56‰ to −25.92‰), suggesting that the DOC source in the mixing zone was highly terrestrially derived. However, in this relatively small isotopic range, δ13CDOCalone has limitations in differentiating carbon produced by aquatic photosynthesis from carbon produced by terrestrial photosynthesis in a river-ocean continuum.