2019 年夏季长江口及邻近海域锋面控制下叶绿素a分布特征及其环境影响因素分析

Spatial distribution of chlorophyll a and its relationships with environmental factors influenced by front in the Changjiang River Estuary and its adjacent waters in summer 2019

中文关键词:  叶绿素 a  锋面  光限制  营养盐限制  长江口

英文关键词:chlorophyll a  front  light limitation  nutrient limitation  Changjiang River Estuary

中文摘要:

      长江口外潮汐混合和低盐度羽流形成的泥沙锋和羽状锋对浮游植物与环境因子的空间分布具有重要控制作用。本研究依据 2019 年夏季长江口及邻近海域典型断面叶绿素 a （Chl-a） 浓度和环境因子的调查结果，以锋面为边界，探讨了不同区域 Chl-a 浓度与环境因子的分布特征及相互关系，以期深入了解锋面的生态效应。结果表明，在泥沙锋以内的近岸区域，水体垂直混合均匀；受长江径流输入和泥沙锋“屏障”作用影响，总悬浮物 （TSM） 和营养盐浓度最高，其中TSM为 220.0± 275.3 mg/L，溶解无机氮 （DIN）、溶解无机磷 （DIP） 和溶解硅酸盐 （DSi） 分别可以达到 94.7±21.2 umol/L、 0.85±0.33umol/L 和 95.3±22.6 umol/L；高浓度 TSM 引起显著的光限制效应，导致 Chl-a 浓度较低 （1.7 ±0.5 ug/L）。在羽状锋以外的区域，出现垂直层化现象；表层海水的 TSM 和营养盐显著降低，其中 TSM 为 5.1 mg/L，DIN、DIP 和 DSi 分别为1.0 umol/L、0.03 umol/L 和 2.4 umol/L；Chl-a浓度受到营养盐供应不足的影响，浓度仅为 0.2ug/L。高浓度的 Chl-a （7.5±4.1±g/L） 主要出现在泥沙锋和羽状锋之间的过渡区域，该区域营养盐得到长江径流与上升流的补充；同时，由于大量 TSM在泥沙锋快速沉降，缓解了水体的光限制效应，有利于浮游植物的生长和积累。研究结果验证了泥沙锋和羽状锋对 TSM 与营养盐的重要控制作用，这对于理解长江口及邻近海域藻类灾害高发区的成因具有科学参考价值。

英文摘要:

      The sediment front and plume front formed by tidal mixing and low salinity plumes off the Changjiang River Estuary(CRE) play an important role in controlling the spatial distribution of phytoplankton and environmental factors. Based on the observational data of chlorophyll a(Chl-a) and environmental factors in the typical section of the CRE and its adjacent waters in summer 2019, this study analyzed the distribution of Chl-a and its relationship with environmental factors along the two frontal boundaries, in order to further understand the ecological effects of the front. In the region shoreward of the sediment front, the water mass was well mixed; under the influence of Changjiang River runoff and the barrier effect of sediment front, the concentrations of total suspended matters (TSM) and nutrients of dissolve inorganic nitrogen(DIN), dissolve inorganic phosphorus (DIP) and dissolve silicate(DSi) were the highest, which were 220.0± 275.3 mg/L, 94.7±21.2 umol/L, 0.85±0.33 umol/L and 95.3±22.6 umol/L, respectively. The high TSM caused a significant light limitation, resulting in a low concentration of Chl-a (1.7±0.5 ug/L). Shelf water mass beyond the plume front showed vertical stratification, and its surface layer was characterized by the lowest TSM and nutrients (DIN, DIP and DSi), which were 5.1 mg/L, 1.0 umol/L, 0.03 umol/L and 2.4 umol/L, respectively. The Chl-a concentration was only 0.2 ug/L due to insufficient nutrient supply. High concentration of Chl-a (7.5±4.1ug/L) mainly occurred in the transition zone between sediment and plume fronts, where nutrients were contributed by Changjiang River runoff and upwelling. Meanwhile, rapid deposition of TSM across the sediment front alleviated light limitation in this transition zone, which contribute to the phytoplankton growth and accumulation. The results show that sediment and plume fronts play an important role in controlling the distributions of TSM and nutrients, which is helpful for understanding the causes of dense algal blooms in the CRE and its adjacent waters.