Paleoenvironmental analyses of surface sediments from the Bohai Sea, China, using diatoms and silicoflagellates

Diatom and silicoflagellate fossils from surface sediments were analyzed at 63 sites in the Bohai Sea. Their spatial variations of the fossils were indicative of past environmental conditions, hydrodynamics, and the interaction of the Yellow Sea intrusion and Yellow River discharge into the Bohai Sea. In total, 162 taxa were identified. Higher species diversity and lower abundance occurred near the mouth of the Yellow River and Laizhou Bay, while lower species diversity and higher abundance occurred in the Central Bohai Sea and Bohai Strait. The spatial differences between fossil diversity and abundance are strongly related to the freshwater and sediment inputs from the Yellow River; lower salinity provides a diverse habitat for brackish and marine species, while a higher deposition rate can dilute the fossil concentrations. Three distinct biofacies representing the Laizhou Bay, Central Bohai Sea and Bohai Strait were classified after principal component analysis. The dominant species along the salinity gradient shifted from Paralia sulcata in the Bohai Strait to Cyclotella stylorum and Dictyocha messanensis in the Laizhou Bay, while a mixing zone existed in the Central Bohai Sea. The canonical correspondence analysis between major species and environmental factors indicated a relationship between P. sulcata and the Yellow Sea intrusion, with significant positive correlations being observed between salinity gradient, water depth and autumn temperature. In comparison, C. stylorum and D. messanensis were related to the discharge of the Yellow River, having significant positive correlations with the low-salinity and nutrient-rich environment.