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Biogenic Silica (BSi) plays an essential role in availability of dissolved silicate (DSi) in aquatic ecosystems, but its role in silica cycling is still poorly characterized in the river and estuary systems. This study addressed the riverine BSi composition and reactive silica (RSi) export in the Yellow River, and estimated the contribution of terrestrial BSi dissolution to the estuary silica pool. Our results indicated that phytoliths comprised 1.3%–87% of the riverine BSi, while diatoms accounted for 13%–99% of the riverine BSi. The Yellow River transported 24 Gg yr −1 of BSi and 26 Gg yr −1 of DSi loadings, respectively; 63% of the riverine BSi and 54% of the riverine DSi fluxes were transported during the period from April to July. In total 46% of annual BSi loading was contributed by phytolith BSi in the Yellow River. The terrestrial BSi dissolution in the estuary would represent 15%–63% of riverine DSi loading on the basis of dissolution test, and low salinity could play an important role in enhancement of terrestrial BSi solubility, especially in the first few days during the estuarine mixing process. Although the terrestrial BSi represents a significant DSi contribution to the silica biogeochemical cycle in coastal waters, it tends to be refractory with the increasing of salinity. [ABSTRACT FROM AUTHOR]

Concentrations of suspended particle material (SPM), dissolved silicate (DSi), biogenic silica (BSi), phytoliths (plant produced siliceous microscopic structures), and other parameters were analyzed to examine the influence of both natural processes and human activities on silica delivery to the estuary of the Huanghe River (Yellow River). Our results indicate that the concentrations of DSi in the river decreased significantly since 1986. Approximately 34% of dissolved silica was trapped in the basin between 1986 and 2010 due to a reduction of soil erosion. Phytoliths comprised 67.2%-96.3% of BSi, with the smoothing bar type being the dominant form. Concentrations of BSi are significantly higher in the Huanghe River compared to other major rivers throughout the world due to its high sediment yield. We also found that the ratios of BSi/(BSi+DSi) and BSi/SPM were approximately 0.5 and 0.003 at Lijing near the river mouth, indicating that BSi carried in suspension by the Huanghe River was an important component of the rivers silica load. Significant amounts of BSi were also composed of phytoliths in Bohai Sea sediments near the Huanghe River estuary with the smoothing bar form again being the most abundant. The relatively high specific fluxes of BSi in the Huanghe River reflect its high turbidity and high erosion rates in the basin. The high sediment load originating on the Loess Plateau is likely responsible for the higher BSi flux, in agreement with a general trend of increasing BSi flux with increasing sediment flux in global river systems. This study demonstrates that BSi transported by rivers can be composed largely of phytoliths originating from the erosion of topsoils. The flux of phytoliths in river's suspended sediment load may therefore represent a significant contribution to the biogeochemical cycle of silica in coastal waters. [ABSTRACT FROM AUTHOR]