By collecting surface sediment samples from 158 stations in the near-shore waters of eastern Guangdong, grain size analysis and grain size parameter calculations were performed to explore the characteristics and migration trends of surface sediments in the area. The analysis of the grain size results showed that the surface sediments in the nearshore waters of east Guangdong could be classified into nine sediment types, mainly including seven types of gravel sand ((g)S), gravel muddy sand ((g)mS), gravelly mud ((g)M), sand (S), silty sand (zS), sandy silt (sZ) and silt (Z). The relative percentages of gravel, sand, silt and mud were 0.7%, 40.56%, 46.7% and 12.04%, respectively. The average grain size varied from −2φ to 8φ, with an average of 4.94φ. The selection coefficient ranged from 0.44 to 3.78, with an average value of 1.8. The skewness distribution ranged from −0.34 to 0.67, with an average value of 0.07. By extracting and analyzing the spatial distribution information of grain size in the study area, using the Gao–Collins migration trend analysis method and incorporating dynamic factors such as tidal currents and waves, the transport direction and trend of surface sediments in the study area could be analyzed and inferred. The results show that the surface sediment migration trend was significant, migration on the north side of Nan’ao Island was in an east-to-west direction, and the sediment of Yifeng River was mainly deposited to the sand spout at the mouth of Lianyang River. After southward transport from the Houjiang waterway, the migration was mainly southeastward and the trend was quite significant until the 20 m isobath, where the trend gradually decreased. The sediments of the Rongjiang River were mainly deposited outside the mouth of Niutian Yang and Rongjiang River, and the surface sediments of Guang’ao Bay and Haimen Bay migrated in the northwest–southeast direction. After the 30 m isobath, the southeast corner of the study area migrated in the southeast–south direction. This sediment transport pattern revealed by the grain size migration trend is in good agreement with the physical and hydrodynamic conditions of the study area and provides an important reference for decisions regarding port dredging and waterway management in the area.

Based on the water quality monitoring results from 2002 to 2018 in the coastal waters of Shantou City, this study used eight methods, including the single-factor index method, eutrophication index method, nutrient status quality method, comprehensive index method, organic pollution index method, nitrogen–phosphorus ratio method, potential eutrophication evaluation, and dissolved oxygen saturation parameter method, to evaluate the eutrophication and organic pollution status of the area. The applicability of the first-generation eutrophication model was assessed from the perspective of red tide occurrence, and a comparison was made among the various evaluation methods. The results indicate that the coastal waters of the study area were mostly in a phosphorus-limited state, with a few periods of nutrient deficiency and a very small portion of time in a moderately nutritious state with both nitrogen and phosphorus limitations. The overall N/P ratio showed an increasing trend primarily due to the gradual increase in inorganic nitrogen content. In terms of evaluating eutrophication based on red tide symptoms, the comprehensive index method showed better performance in reflecting eutrophication conditions, while the eutrophication index method and dissolved oxygen saturation parameter method highlighted problematic areas but often exaggerated their extent. The nutrient status quality method and organic pollution index method did not provide distinctive indications and could not verify red tide symptoms. The nitrogen–phosphorus ratio method and potential eutrophication evaluation could reflect certain nutrient structures and eutrophication characteristics but also failed to verify red tide symptoms. Furthermore, this study found a certain correlation between the occurrence of red tide and the ratio of nitrate nitrogen to ammonia nitrogen, which consistently ranged from 1.15 to 1.94 during red tide events. Therefore, whether this ratio can serve as a characteristic value for red tide occurrence and be used for early warning and monitoring still requires further validation.

The intrusive bodies in the study area are categorized into two intrusive units, γπK₂ and ηoπK₂, based on rock structure characteristics and intrusive contact relationships. The primary lithologies are granite porphyry and quartz monzonite porphyry. Through LA-ICP-MS zircon U-Pb dating, the weighted mean ages of zircon ^206Pb/^238U for these two units were determined to be 84.09 ± 0.86 Ma and 86.10 ± 0.76 Ma, respectively. According to geochemical characteristics and Sr-Yb granite classification, the quartz monzonite porphyry is classified as Zhejiang-Fujian type granite, and the granite porphyry as Nanling type granite. The study explores the geodynamic significance of granites in the area based on Sr-Yb granite classification, identifying the lower crust metamorphic facies as amphibolite facies. The source rocks are primarily basic granulites and felsic granulites, reflecting a thermally active lower crust with poorly developed fluids. Additionally, Sr-Yb granite classification aids in determining the crustal thickness of the lower crust, which is crucial for understanding continental geological evolution. This paper discusses the optimal locations and conditions for mineralizing fluid precipitation, noting that tungsten-tin mineralization is associated with Nanling type granites and that the mineralization process generally occurs later than the diagenetic process under conditions of low pressure and low oxygen fugacity. Tungsten-tin prospecting should focus on areas where the surrounding rock temperature ranges from 300 to 500 °C, with particular attention to the contact zones of sedimentary rocks or near structural fault zones.