Distribution And Enrichment Mechanism Of Biogenic Silica In The Intertidal Wetland Of The Yangtze Estuary

Silica is an essential element for diatom growth, which is the principal contributor of primary productivity. Biogenic silica is produced in the euphotic zone of the ocean by siliceous plankton such as diatoms, whose accumulation in the sediment can reflect the spatial and temporal distribution of productivity, in addition, the preservation and dissolution of biogenic silica played an important role in influencing the occurrence of eutrophication problems in the coastal zone. Takeing the Yangtze estuarine and coastal ecosystem as an example, this article discussed the temporal and spatial distribution of biogenic silica in the sediments, and the transfer and accumulation effects of biogenic silica had also been analyzed. Meanwhile, the accumulation mechanism of salt marsh vegetation had also been discussed.The main results are as follows:The significant seasonal variations of biogenic silica contents in surface sediments were observed along the coastal zone of the Yangtze Estuary: The biogenic silica contents in the nearshore sediments varied from 0.46% to 1.61% and gradually increased from land to sea in February, while in August they were in the range of 0.39%～1.36% and had the opposite spatial distribution pattern to February; Biogenic silica contents in surface sediments from the shoals varied from 0.47% to 1.02%, and had the similar spatial distribution pattern with the nearshore sediment in February, besides, the significant differences in biogenic silica contents were observed in high, middle and low intertidal flats of different shoals. The relation analysis of the sediment physico-chemical property and biogenic silica announced that biogenic silica contents had close correlation with sediment types and organic matter. In general, biogenic silica was apt to accumulate in the fine fractions of sediments rich in organic matter. Additionally, it was found that benthic diatoms might be an important source of biogenic silica in surface sediments.The biogenic silica buried in the sediment cores were analyzed by a mild acid-mild alkaline sequential extraction method. Results showed that: biogenic silica extracted by mild acid (Si-HCl) was much lower than that extracted by mild alkaline (Si-Alk). Compared to previous study, mild acid pretreatment can markedly increase the biogenic silica content extracted from the core sediment. Sediments with different particle size had different vertical distributions. For example, the Si-HCl and Si-Alk contents in the sediments 125-250μm and 63-125μm gradually decreased with the depth increased, while the sediment with particle size <63μm had the opposite spatial distribution pattern, furthermore, their mean cotents reduced with the particle size decreasing. Under the influence of plant growth, the biogenic silica contents in the 30cm sediments of the high and middle intertidal flats were much higher than that of the low intertidal flat, and the highest contents were all found in August, in which the most biomass obtained. Meanwhile, different variations were found in different month. The relation analysis announced that both of Si-HCl and Si-Alk contents correlated closely with organic carbon and organic nitrogen, suggesting the close relationship between biogenic silica and organic matter during sedimentation process.Strong enrichment of biogenic silica were found in Phragmites astralis and Scirpus mariqueter during their growing seasons, especially for P. astralis reeds in which a linear cumulation was found, and the biogenic silica content reached 4.86% at The end of growing season. However, biogenic silica content in Spartina alterniflora was much lower and no cumulation was found during the growing seasons. As thus, compared to Phragmites astralis and Scirpus mariqueter, S. alterniflora had no competitive ability in silica uptake. In addition, biogenic silica capability of the three plants were also analyzed, result suggested that P. astralis had the highest capability and S. alterniflora takes the second place while S. mariqueter had the lowest capability. The biogenic silica storage of salt marsh plants in Chongming Dongtan was estimated to 406.18t, of which P. astralis accounted for 53%, making magnitude contribution for biogenic silica storage in the intertidal wetland of the Yangtze Estuary. The relation analysis of the biogenic silica and organic nitrogen contents in rhizosphere sediment with biogenic silica contents in plant suggested that sediment effected biogenic silica enrichment in plant, and furthermore, the significant correlation with C/N ratio in plant showed that high C/N ratio may accelerate biogenic silica accumulation in plant.