Temporal Variation of Water and Salt Exchange at Xiaoqinghe River Mouth, North of China

Abstract:

Estuaries are important components of coastal ecosystem and function as dominant pathways of material exchange at the land-sea interface. The transport of terrestrial input through river inflow is controlled by physical process including tides, waves, and fresh water discharge. This study investigates net water and salt flux within Xiaoqinghe River mouth, a mesotidal shallow estuarine system (water depth < 8 m) exports substantial amount of nutrients and pollutants to the adjacent Laizhou Bay. Profile velocity and salinity are measured using ADCP and CTD through complete tidal cycles (25hours) in April, July and September 2013. The instantaneous velocity and salinity data are decomposed into time-averaged means and time-varying components based on the improved Kjerfve (1986) method to quantify the contributions of various physical processes. The results show that the freshwater discharge and tidal pumping are dominant processes of salt transport during the wet season and dry season, respectively, while both factors are almost the same during the normal season. The advective flux also determined the direction of the net salt flux. The remaining terms, which are dependent on the deviations from time-average means have a limited role in salt transports. The vertical shear flux tended to very small. There is a distinguishable difference between the transport of salinity and water for all three surveys, and also obvious separation character of salinity and water’s long-term transport during all three surveys. An imbalance of the salt budget across the river mouth is also observed. Overall, tidal pumping is the underlying process of salt transport while river discharge dominates its temporal variation. This study will make addition to scientific foundation for management hazardous contamination and best time to release of environmental flows during difference seasons.