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

Abstract:

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) when the 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.