## 2 Materials and Methods  
### Study Area and Data  
#### 2.1.1 Study Area  
Haizhou Bay is located on the eastern coast of Lianyungang City, Jiangsu Province (Figure 1), and adjacent to the southern coast of Rizhao City, Shandong Province. The bay mouth commences at Bergamot Tsui (35\({}^{\circ}\)05\({}^{\prime}\)55\" N, 119\({}^{\circ}\)21\({}^{\prime}\)53\" E) in Lanshan Town, Rizhao City in the north, and reaches Gaogong Island (34\({}^{\circ}\)45\({}^{\prime}\)25\" N, 119\({}^{\circ}\)29\({}^{\prime}\)45\" E) of Lianyungang City in the south, facing the Yellow Sea. It is 42 km wide, with a total coastline length of 86.81 km. The bay covers an area of 876.39 km\({}^{2}\), of which the area below 0 m is 687.90 km\({}^{2}\), the area below 5 m is 340.67 km\({}^{2}\), the area below 10 m is 63.01 km\({}^{2}\), and the maximum water depth is 12.20 m. The gulf coast is mainly silty silt coast, followed by bedrock coast, and sandy plain coast. The sea floor slopes gently from west to east. Haizhou Bay is bound by Laoyeding in the north, Yuntai Mountain in the south, and an alluvial marine plain in the west. The rivers entering the bay along the coast include Xiuzhen River, Longwang River, Qingkou River, Xinshu River and Qiangwei River; the latter two are called Linhong River after their convergence. Qinshan Island and the East-West Islands are located in Haizhou Bay, and Pingshan Island, Dashan Island and Cheniushan Island are located outside the bay. These islands are all bedrock islands. Transportation along the Gulf Coast is convenient: it is the eastern starting point of the Longhai railway, which crosses eastern China, and is the shortest sea exit for the nine provinces (autonomous regions) in the northwest and central plains. It is known as the \"bridgehead at the eastern end of the Eurasian Continental Bridge\", and is an important transportation hub linking north and south, and east and west, in China. Figure 1: Map of the study area, Haizhou Bay, in 2016. From 1985 to 2005, the area of tidal flat and wetland in Lianyungang City decreased by 25,000 hectares due to CR, accounting for 30% of the total coastal tidal flat area. In 2006, after Lianyungang City proposed the strategy of \"advancing eastward and embracing the sea\", the accumulated sea area of reclamation projects reached 473.8 km\({}^{2}\), accounting for 29% of the total tidal flat utilization area. From 2010 to 2012, a number of reclamation projects were implemented in the vicinity of Haizhou Bay, mainly in Ganyu County, Lianyun New Town, and other areas, adding 61.3 km\({}^{2}\) of land, including 31.2 km\({}^{2}\) of tidal flat and 17.67 km\({}^{2}\) of reclamation [29]. This behavior of indirectly changing wetland by CR caused damage of biological habitats and a decrease in the cultivation of commercial crops in wetlands, and had an extremely serious negative impact on fishery resources. For example, the number of natural fish species in Hailing Lake of Lianyungang decreased from 105 species in 1985, to 56 species in 2020. Since 2010, water pollution in the coastal waters of Haizhou Bay has become increasingly serious, and seawater quality has generally been deteriorating. As a result of seawater pollution, the amount of biological resources declined and nutrient salt pollution became more serious. Inorganic nitrogen and phosphorus were found to be the main seawater pollutants, and highly polluted areas were the near-shore area and estuary [30]. The main manifestations of the impact of the reclamation projects on the sea area of Haizhou Bay were the change of the suspended sediment concentration in the water body, and the change of the self-purification capacity of the water body [31]. During the construction of the west levee, the semi-exchange period of the water body changed from one tidal period before the project, to 6.5 tidal periods after the project, which greatly reduced the hydrodynamic movement of the bay and weakened the water exchange capacity inside and outside the bay.