Quantifying the influence factors on water exchange capacity in a shallow coastal lagoon

Abstract

The water exchange capacity of a shallow coastal lagoon is sensitive to its environment and hydrodynamics. In this study, the responses of hydrology and climate change on the water exchange characteristics of such a shallow environment are investigated in the Qilihai lagoon. Water circulation and transport are simulated by a numerical model and the water exchange capacity is evaluated by the water flushing and residence time. Our analysis reveals that water exchange with seasonal characteristics is the strongest in summer due to large runoff and is the weakest in winter due to ice roughness. The shallow terrain variation influences water exchange, which is more obvious for the transverse rather than longitudinal direction. Without an ice cover, runoff is the most significant influence on water exchange. This effect is nonlinear and in accordance with a two-terms exponential formula. Water transport time can be significantly reduced by the presence of wind. Westerly winds accelerate water exchange the most. The influence of mean water level on water exchange can be separated into two phases dominated by bathymetry. Ice dominates water exchange and mainly influences the shallow area during winter. Furthermore, the presence of chlorophyll-a positively correlates with water exchange dominated by hydrodynamics. The distribution of sedimentary heavy metal contamination has no direct link with water exchange, which mainly occurs in tidal inlets and the river mouth, and is controlled by source loading. Our results support the current understanding of what mechanisms influence water exchange capacity and establish a theoretical foundation for environmental protection in global shallow coastal lagoon regions.