The influence of environmental factors on the silicon concentration in the freshwater-seawater mixing plume at the Changjiang Estuary

In order to study the migration,transformation,and the dilution mode of nutrients in the freshwater-seawater mixing plume at the Changjiang Estuary,we simulated environmental factors in the laboratory,such as salinity( S),temperature( T),pH,and dissolved oxygen( DO),and explored their influences on the silicon( SiO3-Si) concentration in the mixing plume of the diluted Changjiang River water and the external seawater. Our results and their implications are:( 1) whenthe external seawater was mixed with either filtered or unfiltered diluted Changjiang River water,the concentration of SiO3-Si stabilized after 40 h.( 2) The SiO3-Si concentration in the freshwater-seawater mixing plume showed a clear negative correlation with the salinity,indicating that the SiO3-Si concentration at the Changjiang Estuary was mainly affected by the physical mixing dilution. In the mixture of the external seawater and the diluted Changjiang River water,the concentration of SiO3-Si showed no significant difference no matter whether the Changjiang River water was filtered,which suggested that the suspending particles in the water had no effect on the SiO3-Si concentration in the freshwater-seawater mixing plume.( 3) The concentration of SiO3-Si slightly increased as the temperature of mixed water rose. This implied that there may be a seasonal change in the flow of SiO3-Si from the diluted Changjiang River water to the offshore area of the East China Sea.( 4) The SiO3-Si concentration in the mixed water remained unchanged at pH 4- 8,but altered when pH was 10. Simple explanations were that higher pH enhanced the SiO3-Si desorption by suspending particles,or changes in the pH affected the form of SiO3-Si.( 5) The oxygen levels( anoxia or oxygen-enriched state) did not affect the SiO3-Si concentration.