Spatial Distribution Features And Influencing Factors Of Biogenic Elements In Surface Sediment From The Yangtze River Estuary

Biogenic elements which included carbon,nitrogen,phosphorus and silicon,are essential nutritive elements for the growth of marine life.It is the basis of marine primary production process and food chain.As the world’s third largest river estuary,the Yangtze River Estuary environment is particularly complex.The bottom of the sediment is the important sources and sinks of biogenic elements,which is playing an important role in the material cycle,energy flow and nutrient remineralization of marine ecosystems all the time.Based on the ecological environment investigation data from the fourth cruises carried out in February,May,August and November in 2014,the spatial-temporal distributions of biogenic elements(including Total Organic Carbon,Total Nitrogen,Total Phosphorus and Biogenic Sillicon)in the surface sediment were analyzed as well as their correlations with the environmental factors.At the same time,the main sources and influencing factors of sediment biogenic elements in estuarine material circulation were discussed.The main results are as follows:The concentrations of Total Organic Carbon(TOC),Total Nitrogen(TN),Total Phosphorus(TP)and Biogenic Sillicon(BSi)in the surface sediment of the Yangtze River estuary and its adjacent area were 0.073~0.811%,0.005~0.214%,0.05~0.090% and 0.045~0.650%,respectively.And the average contents of TOC、TN、TP and BSi were 0.366%,0.055 %,0.066% and 0.327% respectively.The seasonal variation of TOC in the surface sediments was not significant,and the variation of TN in different seasons was significantly different.TOC is the highest in autumn and second highest in winter.TN is the highest in winter and followed by autumn.spring and summer at the Spring and summer are at a low level.The seasonal fluctuation of TP contents is not obvious,which is slightly higher in spring and lower in summer.The order of BSi contents from high to low was autumn,summer,winter and spring.The change of BSi contents in the southeastern area was the most significant.The spatial distribution of TOC in the surface sediments demonstrated that the higher values appeared in the near-shore zones and in the southern area,the lower TOC was in the off-shore areas and in the northern area,which had subtle difference between different seasons.The highest contents of TN occurred in southern coastal zone,while lower contents distributed in northwest offshore area.The distribution of TN had varied factors during different seasons.The distribution of TP decreased gradually from the inner mouth zones to the near-shore and off-shore areas with the increasing in distance.And also there was not obvious seasonal variations.Spatial distribution showed that BSi concentration decreased seaward and the highest value occurred in the coastal zones,with the higher content appearing in the south area while the lower in northern zones.The median size of the surface sediments in the Yangtze River estuary and its adjacent area was between 1.80 and 7.12φ.The surface sediments was mainly comprise of sand and silt,the proportion of clay is less.There were some differences of the surface sediments in different seasons.The surface sediments was mainly comprised of sand in the spring.In summer,the content of silt was decreased and sand conponent was increased.The content of silt reached maximum value in autumn,and the content of clay was also increased.In winter,the components of the surface sediments was similar to spring.The types of the sediments in the Yangtze River estuary were mainly characterized by the transition from the clay silt to silt and sandy from the estuarine to the offshore sea with the increase of the offshore distance.The types of clay silt was mainly concentrated in the Turbidity Maximum Zone,while the silt was distributed in the Turbidity Maximum Zone and the southeastern sea,and the sand was mainly concentrated in the northern areas.It had been recognized that The main factors influencing the content and distribution of TOC and TN in surface sediments included sediment grain size,hydrodynamic factors and some water environmental factors.The concentrations of TOC and TN had a strong enrichment capacity that existed in fine particles,the finer the particle size,the TOC and TN content is higher.Factors affecting the content and distribution of TP and BSi in surface sediments were different in different seasons.The distribution of the dilution water in the Yangtze River,which is controlled by hydrodynamic factors,with the warm current in Taiwan as well as the coastal flow in Zhejiang affect the distribution of biogenic elements.The influence of the water environmental factors to each biogenic elements from surface sediments in different seasons is different.Compared with 2007,the contents of TOC and TN in the surface sediments from the Yangtze River Estuary decreased in 2014.The concentrations of TOC decreased significantly in the Turbidity Maximum Zone,and the conent of TN in the southeast of the sea decreased more than the Turbidity Maximum Zone.Which was related to the grain size increased in the turbidity maximum zone in the Yangtze River estuary and the decrease of the runoff as well as the sediment delivery of the Yangtze River.The TOC/TN ratios ranged from 2.47 to 19.83 with an average 8.22 in the surface sediments in the Yangtze River estuary and its adjacent area.It had been indicated that TOC and TN were significantly influenced by terrestrial inputs and the marine autogenic materials.According to the C/N ratio estimation,the contribution of terrestrial inputs TOC was reduce 20% in 2014 than that in 2007,which was not dominant compared with the marine autogenic materials.TN mainly comes from the marine autogenic materials input.The main reason was that the Yangtze River land input reduced in recent years.