In this study, microplastics were sampled and analyzed from surface water and sediment samples from July to August in 2020, in the Beibu Gulf (the northern South China Sea [SCS]), a gulf with intensive fishery activities while the economy is less developed, compared with other coastal areas of China. The abundances of microplastics in seawater and sediment in the Beibu Gulf were 0.67 items/m³ and 4.33 items/kg of dry weight, respectively. In seawater, the fragments (92.38%) contributed the most, and polystyrene (PS) was the dominant polymer (53.23%). In sediment, the most abundant microplastics were fiber (82.93%) and rayon (RY; 39.54%). The abundances of remarkably higher microplastics were found in the seawater and sediment adjacent to the urban area. The abundances of microplastics in far coastal sediment were only slightly lower than that in the coastal sediment, indicating that microplastics are ready to transport and bury in open area sediment. Significant positive correlations between the microplastic abundance and population density and per capita gross domestic product (GDP) were found in Chinese coastal seawater, with low population density and less developed economy, and the microplastic pollution in the Beibu Gulf was at a low level. This study provides preliminary data of microplastics in the Beibu Gulf, supporting further investigation of transportation fate and management of this emerging pollutant from the coastal zone to the SCS.

In this study, we analyzed the clay mineralogy and geochemistry of surface and drill core samples from the northeastern Beibu Gulf in order to unravel the sediment provenance of, and factors controlling, the sedimentary environment. The main clay mineral assemblage in the surface sediment samples included kaolinite (27–72%), smectite (4–51%), illite (7–20%), and chlorite (8–17%). The study area comprises three major clay distribution zones (from the northeastern coastal area to central Beibu Gulf basin), i.e., the kaolinite-dominated, kaolinite–smectite, and smectite-dominated zones. The zoning of the clay mineralogy and major and trace elements indicated the mixing of coarse terrigenous sediments with distal fine sediments. Early Holocene sea level rise was documented in core B15-1, which had three sedimentary units (divided into Units 1–3 from top to bottom), as revealed by the changes in the dominant clay minerals and geochemical/oxide ratios (SiO₂/Al₂O₃, Rb/Sr, Sr/Ba, and Ti/Ca) in Unit 2. Unit 1 and Unit 3 were likely deposited in the continental and marine environments, respectively. The low sedimentation rate and hydrodynamic disturbance may have influenced the deposition process more than climatic fluctuations.

Heavy metal contaminations in the marine environment are of considerable attention because of their high potential ecological effects and public concern for human health. However, the influencing factors for the large-scale distributions of heavy metals in Beibu Gulf, a newly developing industry and port in South China, are still unclear due to the lack of large-scale investigation. Here, a total of 871 samples in the 127 stations in the seawater of Beibu Gulf during spring, summer, fall and winter in 2020-2021 were analyzed for dissolved heavy metal concentrations and physicochemical parameters. The concentrations of heavy metals in the Beibu Gulf ranked following the order of Zn > Cu > Cr > As > Pb > Hg > Cd. Compared to other regions, the concentrations of Hg were at relatively higher levels, which were mainly influenced by the input of the transportation of water masses from the local and other regions; whereas the other heavy metals were at relatively lower levels. Seasonally variations in the concentrations of heavy metals were observed in the gulf, which is mainly influenced by human activities (i.e., shipping and mariculture activities) and seasonally hydrological conditions. Seasonal changes in the spatial distribution of heavy metals have been found in the gulf. The higher concentrations of heavy metals mainly occurred in the coastal bays or areas in summer whereas the higher concentrations were observed in the offshore areas during the other three seasons. This is mainly related to the seasonal changes of the water masses that affect the seawater of Beibu Gulf, which exhibits the dominant contribution of coastal current from the northern Beibu Gulf in summer, and the dominant contribution of west-Guangdong coastal current and SCS water during the other three seasons. The potential ecological risk index revealed that Hg is the main ecological risk factor in the gulf, and the heavy metal contamination in the gulf seems to be noticeable. This study highlights the seasonal changes of the water masses that affect the seawater of Beibu Gulf greatly affecting the large-scale distributions of heavy metals in the gulf.