Transportation Of Silica In The Lower Reach Of Yangtze And Yellow Rivers

As the second most element of the earth crust, silicon not only has far-reaching impact on the material circulation in the earth, but also plays an important role in the ecology system stability. Study on the transportation of silicon from land to ocean and its effects on the marine environment have become the key point and foundation research in the domain of land-ocean interaction. The input of major biogenic nutrients from terrestrial systems to the ocean plays a central role in regulating global oceanic production. As an important materia, silicon is widely used on the light catalysis, drug delivery, biosensors and adsorption. Compared with diatomite, siliceous shell has the unique advantages, such as complete and less impurity, pore structure, is the new focus of biological chemical field research.Rivers are the primary source of silicon to the oceans. Two categories of potentially bioavailable Si are transported by rivers: dissolved silicate(DSi) and biogenic silica(BSi). Here we report the dissolved silica and the contribution of BSi to the suspended load of Yangtze and Yellow Rivers, and the role of phytoliths in the BSi pool. The purpose of this study was to explore the transfer and composition of riverine silicon in the Yangtze and Yellow Rivers, with an aim of improving our understanding of silica transportation in the large river system. The major conclusions are as follows:(1) The concentrations of DSi and BSi in the Yangtze River were measured systemically during May 2013 to May 2014. The concentration of DSi ranges from 89.3 μmol L-1 to 123.0 μmol L-1 with an average of 102.0 μmol L-1, while the concentration of BSi varies from 4.9 μmol L-1 to 47.0 μmol L-1 with an mean value of 21.2 μmol L-1. The average concentrations of DSi and BSi in Summer are higher than those in Winter. In the world, the concentrations of DSi and BSi were relatively lower in the Yangtze River than in other world rivers, and the BSi carried in suspension by the Yangtze River is an important component of the rivers silicon load, accounting for more than 16% of the sum of DSi and BSi. Our results also indicate that the concentrations of DSi in the Yangtze River have tended to decrease in the past fifty years.Three types of BSi are identified: phytoliths, diatoms, and sponges in the Yangtze River during May 2013 to May 2014. Approximately 43.1% of the BSi is composed of phytoliths, and 55.0% of the BSi is composed of diatoms, whereas sponges contributes 1.90% of the total BSi pool. The identified phytolith assemblages indicate that the Yangtze River BSi is produced mainly by herbaceous plants. In general, diatom BSi forms are composed mainly of Cyllotella and Coscinodiscus of Centricae and Diploneis, Gomphonema and Closterium lunula of Pennales.(2) The concentrations of DSi and BSi in the Yellow River were measured during May 2013 to January 2015. The concentration of DSi ranges from 31.0 μmol L-1 to 121.3 μmol L-1 with an average of 81.4 μmol L-1, while the concentration of BSi varies from 30.7 μmol L-1 to 158.2 μmol L-1 with an mean value of 70.0 μmol L-1. The average concentrations of DSi and BSi are high during the flood season and low in the dry season. And the concentrations of BSi are relatively higher in the Yellow River compared to other rivers throughout the world, and the BSi carried in suspension by the Yellow River is an important component of the rivers silicon load, accounting for more than 41%(25 ~ 58%) of the sum of DSi and BSi. The concentrations of DSi in the Yellow River have tended to decrease since 1986 as well, reflecting natural and human activity trapped silica in the basin. Approximately 29% of dissolved silicon was trapped in the basin during 1986–2013 due to a reduction in soil erosion.Similar to the Yangtze River, three types of BSi were identified: phytoliths, diatoms, and sponges in the Yellow River. Approximately 50.3% of the BSi is composed of phytoliths, and 49.8% of the BSi is composed of diatoms, whereas sponges contributes 0.26% of the total BSi pool. The identified phytolith assemblages indicate that the Yellow River BSi is produced mainly by herbaceous plants. In general, diatom BSi forms are composed mainly of Cyllotella and Coscinodiscus of Centricae and Diploneis, Gomphonema and Closterium lunula of Pennales.In conclusion, the structure and composition of BSi were very similar between the Yangtze River and the Yellow River. The concentrations and fluxes of DSi in the Yangtze River and the Yellow River decreased significantly in recent decades. The influence factors of transportation of silica in rivers are dominated by earth surface processes and human activities. In the future, the views of big basin and the river continuum theory should be used in the study of effects of earth surface processes and human activities on the transportation of riverine silica.