Institute of Physical Oceanography and Remote Sensing, Ocean College, Zhejiang University, Zhoushan 316021, PR China.
Institute of Physical Oceanography and Remote Sensing, Ocean College, Zhejiang University, Zhoushan 316021, PR China; Hainan Institute, Zhejiang University, Sanya 572025, PR China; Laoshan Laboratory, Qingdao 266237, PR China.
Water Res. 2024 Nov 15;266:122413. doi: 10.1016/j.watres.2024.122413. Epub 2024 Sep 7.
Estuaries are one of the most important ecosystems in the world, which are economically developed and densely populated. However, the intricate hydrodynamic environment and frequent human activities within estuaries have left the spatiotemporal variability of water properties in these areas inadequately understood. Recently, based on in situ observations and numerical simulations, we found significant spring-neap variability of water mass properties in the Yangtze River Estuary, which exhibited a bi-layered vertical structure. In the Yangtze River Estuary, salinity could decrease (increase) over 4 psu during spring (neap) tides in the upper layer, and satellite observations confirmed that both sea surface chlorophyll-a concentration and particulate organic carbon concentration also showed significant spring-neap variabilities. Decreasing salinity in the upper layer induced a shoreward pressure gradient force in the lower layer, which caused shoreward advection of high salinity water from the deep ocean and resulted in salinity increasing up to 2 psu in the lower layer of the Yangtze River Estuary. Dynamical diagnoses proved that spring-neap variability of water mass properties were caused by the asymmetry of tidal currents via modulating the ratio of freshwater to seawater. Similar situations also occurred in the Mississippi River Estuary. Furthermore, constructions of dams and other hydraulic projects in the watershed could greatly alter the locations with significant spring-neap water masses variability through reducing the riverine sediment flux and thus, leading to the erosion of the tidal flats in estuaries. The above results highlight the important roles of tidal asymmetry and human activities in affecting spring-neap variabilities of water mass properties in estuaries.
河口是世界上最重要的生态系统之一，经济发达，人口密集。然而，河口内部复杂的水动力环境和频繁的人类活动，使得这些地区的水质时空变化仍未得到充分理解。最近，基于现场观测和数值模拟，我们发现长江河口的水团性质具有显著的潮周期变化，呈现出双层垂直结构。在长江河口，上层盐度在涨潮时可降低（增加）超过 4 个盐度单位，卫星观测证实，海水表层叶绿素-a 浓度和颗粒有机碳浓度也表现出显著的潮周期变化。上层盐度降低导致下层产生向岸的压力梯度力，促使高盐度海水从深海向岸输送，导致长江河口下层盐度增加了 2 个盐度单位。动力诊断证明，水团性质的潮周期变化是由潮流的非对称性通过调节淡水与海水的比例引起的。密西西比河河口也存在类似的情况。此外，流域内水坝和其他水利工程的建设通过减少河流泥沙通量，极大地改变了具有显著潮周期水团变化的位置，从而导致河口滩涂的侵蚀。上述结果强调了潮汐非对称性和人类活动在影响河口水团性质潮周期变化方面的重要作用。
