## 1 Introduction  
Source, transport and accumulation of sediment in the ocean are components in the study of the \"source-sink\" process, and are also an important part of land-sea interaction research. Transportation of sediment in the ocean is the result of complex dynamic processes. Their flux and fate directly affect the evolution characteristics of seabed topography and are closely related to the stability of coast and service life of marine engineering, especially around Bohai Bay, where it is usually accompanied by intense marine development and human activities. Sediment transportation has always been one of the widely concerning oceanographic issues.  
Bohai Bay is one of the three major bays in the Bohai Sea (Figure 1). Due to the discharges of three large rivers (Yellow River, Luanhe River and Haithe River), Bohai Bay has a rich source of sediment, and large deltas have developed along much of the bay's shoreline. In recent years, the runoff and sediment input from the Yellow River have gradually decreased, due to the massive construction of dams, power stations, increased water consumption, climate change and other factors. Subsequently, the evolutionary state of the Yellow River underwater delta has changed from constructive to destructive [1]. Sediment eroding from the subaqueous Yellow River Delta (YRD) is causing the retreat of the adjacent coast [2] as well as shoaling offshore in southern and western Bohai Bay (Figure 1). These conditions reveal that sediment transport in Bohai Bay has changed significantly. Bohai Bay is famous for its well-developed harbor transportation and oil exploitation industries. There are several large ports and oil exploitation areas along the coast. Therefore, the sediment transport and the resulting siltation and erosion in Bohai Bay hold great scientific significance, not only to enhance research on the source and sink Earlier work by [PERSON] et al. [3], [PERSON] et al. [4] and [PERSON] et al. [5] from 1985 to 2015 provided a basis for the work described here, and an indication that sediment transport in Bohai Bay has changed significantly over the past 30 to 40 years. Detrital sediments from the Yellow River and Luanhe River can affect the central and eastern parts of the Bohai Sea [6]. About 88% of the Yellow River sediment is deposited on the front edge of the underwater delta [7], and there are three directions of diffusion: northeast, northwest and southeast [8]. Due to the existence of the shear front, the estuary sediment is confined to the shore side of the shear front and transported to the north and south with the current [9]. Sediment discharge from the Luan River is much smaller than that of the Yellow River, and most of it is deposited in the coastal area from the mouth of the Luanhe River west to Caofeidian (Figure 1). The sediment outside the Caofeidian Sandbar is transported from sea to land, which is believed to be related to the coastal sediment transport caused by winds in the NW, SW and SE directions [10]. The sediment distribution of the Bohai Sea is consistent with the circulation trend [11]. However, the study areas of earlier studies are all local parts of Bohai Bay, such as the Yellow River estuary waters and Caofeidian waters. Therefore, a clear understanding of sediment transport in Bohai Bay overall is still lacking.  
This study clarifies the sediment transport trend and its dynamic mechanism in Bohai Bay through a large-scale investigation on the grain size and element content of surface sediment. The results can provide guidance and reference for coastal disaster prevention and safety.  
Figure 1: Location, circulation structure and wave direction of Bohai Bay and distribution of sampling sites. Black arrows indicate prevailing wave direction, and red arrows indicate strongest wave direction.