Distribution of diatoms and silicoflagellates in surface sediments of the Yellow Sea and offshore from the Changjiang River, China

The spatial distribution of siliceous microfossils(diatoms and silicoflagellates) in the surface sediments was mapped at 113 sites in the Yellow Sea and sea areas adjacent to the Changjiang(Yangtze) River,China. In total,267 diatom taxa and two silicoflagellate species were identified from the sediments. The spatial variations in abundance and diversity were classified into three distinct geographic patterns using Q mode clustering: a south-north geographic pattern,a coastal-offshore pattern and a unique pattern in the Changjiang River mouth. The south-north geographic pattern was related to the spatial variations in sea temperature. C oscinodiscus oculatus,a warm-water species,indicated these variations by a gradual decrease in abundance from the south to the north. The coastal-offshore pattern was in response to the spatial variations in salinity. Cyclotella stylorum,Actinocyclus ehrenbergii and Dictyocha messanensis,the dominant brackish species in coastal waters,significantly decreased at the isobaths of approximately 30 m,where the salinity was higher than 31. Paralia sulcata and Podosira stelliger indicated the impact of the Yellow Sea Warm Current in the central Yellow Sea. The unique pattern in the Changjiang River mouth showed the highest species diversity but lower abundance,apparently because: freshwater input can significantly increase the proportion of brackish species; nutrients can supply the growth of phytoplankton; and high sedimentation rates can dilute the microfossil abundance in the sediments. Our results show that an integration of environmental factors(e.g.,nutrient levels,sedimentation rate,sea temperature,salinity and water depth) determined the spatial characteristics of the siliceous microfossils in the surface sediments.

The spatial distribution of siliceous microfossils （diatoms and silicoflagellates） in the surface sediments was mapped at 113 sites in the Yellow Sea and sea areas adjacent to the Chang^iang （Yangtze） River, China. In total, 267 diatom taxa and two silicoflagellate species were identified from the sediments. The spatial variations in abundance and diversity were classified into three distinct geographic patterns using Q mode clustering： a south-north geographic pattern, a coastal-offshore pattern and a unique pattern in the Changjiang River mouth. The south-north geographic pattern was related to the spatial variations in sea temperature. Coscinodiscus oculatus, a warm-water species, indicated these variations by a gradual decrease in abundance from the south to the north. The coastal-offshore pattern was in response to the spatiaJ variations in salinity. Cyclotella stylorum, Actinocyclus ehrenbergii and Dictyocha messanensis, the dominant brackish species in coastal waters, significantly decreased at the isobaths of approximately 30 m, where the salinity was higher than 31. Paralia sulcata and Podosira stelliger indicated the impact of the Yellow Sea Warm Current in the central Yellow Sea. The unique pattern in the Changjiang River mouth showed the highest species diversity but lower abundance, apparently because： freshwater input can significantly increase the proportion of brackish species; nutrients can supply the growth ofphytoplankton; and high sedimentation rates can dilute the mierofossil abundance in the sediments. Our results show that an integration of environmental factors （e.g., nutrient levels, sedimentation rate, sea temperature, salinity and water depth） determined the spatial characteristics of the siliceous micro fossils in the surface sediments.