Distribution characteristics of nutrients in the Changjiang River Estuary under the watershed extreme drought in July 2023
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摘要: 长江入海径流量极大地控制着长江冲淡水在长江口的扩散范围，以及河口生源要素的生物地球化学行为。2023年7月，长江流域发生了历史罕见的洪季干旱事件。为了研究洪季长江特枯水情下长江口营养盐的分布特点，于当月在长江口海域进行了32个典型站位的样品采集，并将其结果与2016年和2020年洪季长江洪水到达长江口前后的采样结果进行对比。在2023年7月流域干旱条件下，河口盐淡水混合区域收缩，
${{\rm {NO}}_3^-} $ -N和${{\rm {SiO}}_3^{2-}} $ -Si产生不保守过程的时间和空间均受限，导致这两种营养盐与2016年和2020年洪季相比更加保守。同时，由于在长江特枯水情下河口表层低、中盐度（< 25）的海水被局限在水深较浅的区域，使得底部通过沉积物再悬浮释放的${{\rm {PO}}_4^{3-}} $ -P较易被输送至海水表层，从而促进了表层${{\rm {PO}}_4^{3-}} $ -P浓度的升高以及“源”效应的产生。而随着盐度的继续升高，表层浮游植物吸收施加的“汇”效应的影响逐步显现；并且随着深度增加，沉积物再悬浮释放的${{\rm {PO}}_4^{3-}} $ 也越来越难以贯穿至海水表层，两者共同作用导致${{\rm {PO}}_4^{3-}} $ -P浓度在高盐度（>25）海水表层迅速下降。由此，2023年7月表层${{\rm {PO}}_4^{3-}} $ -P浓度随盐度的变化表现出了特殊的“上凸”趋势。在目前长江径流量年际间较大变幅的情景下，本研究丰富和深化了对长江口生源要素生物地球化学过程的认识，也有助于进一步回答长江口等磷限制型河口中赤潮的触发机理等关键科学问题。Abstract: The freshwater discharges of the Changjiang (Yangtze) River into the sea largely control the extension range of the Changjiang Diluted Water in the estuary and the inherent biogeochemical behaviors of biogenic elements. In July 2023, a summer drought event occurred in the Changjiang River basin, with extremely lower river discharge. In order to study the distribution characteristics of nutrients in the Changjiang River Estuary in response to this drought, samples were collected from 32 stations in the Changjiang River Estuary that month, and the results were further compared with those obtained before and after the arrivals of the Changjiang River watershed floods occurring in the summers of 2016 and 2020. Under the drought condition in July 2023, the estuarine mixing area of freshwater and seawater greatly shrank, and the time and space for the occurrences of non-conservative processes of${{\rm {NO}}_3^-} $ and${{\rm {SiO}}_3^{2-}} $ were largely restricted, resulting in that these two nutrients being more conservative compared to those in the flood seasons of 2016 and 2020. At the same time, since the surface seawaters with low-to-medium salinity values (<25) under the drought condition were confined to areas with shallower water depths,${{\rm {PO}}_4^{3-}} $ released through sediment resuspension from the bottom was more easily transported to the surface, and the${{\rm {PO}}_4^{3-}} $ concentrations at surface increased and displayed a “source” mode. As the salinity continued to increase, the impact of the “sink” mode owing to surface phytoplankton assimilation gradually became dominant. In addition, with the increasing water depths, the${{\rm {PO}}_4^{3-}} $ released via bottom sediment resuspension could not penetrate into the surface layer anymore. The combined effect of the above two processes led to a rapid decrease in${{\rm {PO}}_4^{3-}} $ concentrations at surface when salinity was higher than 25. Thus in July 2023, the surface${{\rm {PO}}_4^{3-}} $ concentrations showed a special “concave” trend. This study enriches and deepens our understanding on the response and feedback mechanisms of biogeochemical processes in the Changjiang River Estuary, to the large inter-annual variability in the Changjiang River discharges. This study also helps to further answer those key scientific questions regarding the triggering mechanism of algal blooms in the Changjiang River Estuary, a typical${{\rm {PO}}_4^{3-}} $ -limited large-river estuary in the world.-
Key words:
- nutrient /
- flood season /
- drought /
- biogeochemistry /
- non-conservative behaviors /
- Changjiang (Yangtze) River Estuary
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图 1 2000年以来长江下游大通站月平均径流量的变化（a），以及2015年以来7–8月间大通站日平均径流量的变化（b）（数据下载于长江水文网：
http://www.cjh.com.cn/ ）Fig. 1 Variation in monthly average discharge at the Datong station in the lower Changjiang River since 2000 (a), and variations in daily average discharge at the Datong station from July to August since 2015 (b) (the data were downloaded from
http://www.cjh.com.cn/ )
图 2 2023年7月长江口航次的采样站位，以及（a）表层和（b）底层盐度和SPM的质量浓度的分布
Fig. 2 The sampling stations, as well as the distributions of salinity and SPM mass concentration at the (a) surface and (b) bottom layers, during the Changjiang River Estuary cruise in July 2023
图 3 2023年7月长江口航次营养盐浓度在表层的分布
Fig. 3 Distributions of nutrients concentration at the surface layer, during the Changjiang River Estuary cruise in July 2023
图 4 2023年7月长江口航次营养盐浓度在底层的分布
Fig. 4 Distributions of nutrients concentration at the bottom layer, during the Changjiang River Estuary cruise in July 2023
图 5 2023年7月长江口航次表层和底层营养盐浓度与盐度之间的相关关系和线性回归直线
Fig. 5 The correlations and linear regression lines between nutrients and salinity, at the surface and the bottom layers during the Changjiang River Estuary cruise in July 2023
图 6 2023年7月流域干旱条件下长江口航次表层4条盐度等值线位置与2016年和2020年洪水到达长江口前后4个航次相应等值线位置的比较（其中盐度分别为：a. 10、b. 15、c. 20、d. 25）
Fig. 6 Comparison of the positions of the four salinity contour lines at the surface of the Changjiang River Estuary (including salinity: a. 10, b. 15, c. 20, and d. 25), among the five cruises including the one during the watershed drought in July 2023 and the four before and after the arrivals of the watershed floods in 2016 and 2020
图 7 2023年7月流域干旱条件下长江口航次表层(e)和底层(f)在各个盐度区间的SPM平均质量浓度与2016年和2020年洪季洪水到达长江口前后4个航次结果的对比（其中分图a、b、c、d分别为2016年和2020年4个航次所使用数据的采样站位）
Fig. 7 Comparison of the average SPM mass concentrations at different salinity ranges in the surface (e) and the bottom layers (f), among the five cruises including the one during the watershed drought in July 2023 and the four cruises before and after the arrivals of the watershed floods in the summers of 2016 and 2020 (The panels a, b, c, and d show the sampling stations for the data used during the four cruises in 2016 and 2020, respectively)
图 8 2016年和2020年洪季长江口4个航次表层（红色表示）和底层（蓝色表示）营养盐浓度与盐度之间的相关关系和线性回归直线
Fig. 8 The correlations and linear regression lines between nutrients and salinity, at the surface (red color) and the bottom (blue color) layers during the four cruises in the summers of 2016 and 2020
图 9 2023年7月流域干旱条件下(a)，以及2016年7月(b)和2020年8月(c)洪水到达长江口后等3个长江口航次表层
${{\rm {PO}}_4^{3-}} $ -P浓度随盐度的上凸变化使用二次多项式函数拟合后的结果Fig. 9 The fitting results of the concave
${{\rm {PO}}_4^{3-}} $ -P trend with salinity using a quadratic polynomial function, in the surface layer during the three Changjiang River Estuary cruises (including the one during the watershed drought in July 2023 and the two after the arrivals of the watershed floods in 2016 and 2020)
图 10 2016年、2020年、2023年洪季长江口5个航次典型断面（这5个断面的位置分别见图7a–d以及图2a中紫色线段）盐度（等值线表示）和
${{\rm {PO}}_4^{3-}} $ -P浓度的变化Fig. 10 Variations in salinity (isoline representation) and
${{\rm {PO}}_4^{3-}} $ -P concentration along the typical transects of the Changjiang River Estuary during the five cruises in the summers of 2016, 2020, and 2023 (the positions of the five transects are shown by the purple lines in Figs 7a–d and 2a)表 1 2023年7月长江口航次营养盐浓度与盐度之间线性回归直线的斜率在表层和底层间的比较
Tab. 1 Comparison of the slopes of the regression lines between the surface and the bottom layers, during the Changjiang River Estuary cruise in July 2023
营养盐 层次 斜率/（μmol·L−1/Salinity） 斜率差值 差异显著性 $ {{\rm {NO}}_3^-} $-N 表层 3.75 ± 0.11 0.42 p < 0.001 底层 3.33 ± 0.17 $ {{\rm {SiO}}_3^{2-}} $-Si 表层 3.54 ± 0.09 0.32 p < 0.05 底层 3.22 ± 0.14 $ {{\rm {PO}}_4^{3-}} $-P 表层 0.045 ± 0.004 0.026 p < 0.001 底层 0.019 ± 0.003 $ {{\rm {NH}}_4^+} $-N 表层 0.36 ± 0.10 0.18 p > 0.05 底层 0.18 ± 0.04 
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