A Study On The Impact Of Hydrodynamic Process On The Water Environment And Mass Transport In The Sanya Bay

With the development of China’s marine economy,a large amount of land-based pollutants are discharged into the sea,causing deterioration of the coastal ecological environment.Sanya Bay is located in the northern part of the South China Sea and is a popular tourist destination for Sanya.It is also an important area for coral reef distribution.Land-based input is the main source of pollutants in Sanya Bay,and the substances entering the ocean are transported and diffused through hydrodynamic processes,but our understanding of this process is still limited.In this study,we first investigated the water environmental factors of Sanya Bay in winter and summer seasons,then built a high-resolution(175 m)threedimensional hydrodynamic model of Sanya Bay based on the Regional Ocean Modeling System(ROMS),and finally used the method of conservative substance tracing to explore the distribution and transport characteristics of the material input from the Sanya River to the sea,and analyzed the influence of tidal currents and wind fields on its transport in Sanya Bay.The main results are as follows:(1)The overall concentration of nutrients in seawater in Sanya Bay exhibits a temporal and spatial pattern of high winter and low summer,with higher concentrations at the surface and lower concentrations at the bottom.In terms of horizontal spatial distribution,nutrient concentrations are highest at the mouth of the Sanya River and gradually decrease from the mouth of the river to the inner part of Sanya Bay.The distribution of nutrients shows that the Sanya River is a key factor affecting the temporal and spatial distribution of nutrients in Sanya Bay,therefore,in the subsequent simulation,the Sanya River is set as the input source of passive mass.(2)The hydrodynamic simulation results show that the tides in Sanya Bay are dominated by the diurnal tides K1 and O1,followed by the semidiurnal tides M2 and S2.Influenced by seasonal wind fields,the surface residual flow shows obvious seasonal variation,while the bottom residual flow compensates in the opposite direction to the surface residual flow.The impact of freshwater from the Sanya River on the salinity of Sanya Bay is mainly concentrated in the sea area near the river mouth,and the extension direction of the 33 salinity isohaline expands southwestward in winter monsoon and northward along the coastline in summer monsoon.(3)Calculated with an input concentration of 100 at the river mouth,the average concentration of the substance in the sea area is 0.19,indicating a high dilution rate of the Sanya River input substance in Sanya Bay.The high concentration is mainly concentrated near the river mouth,accounting for about 20%-70% of the input concentration.The input substance from the Sanya River can be transported to the bay through two channels on the north and south sides of Fenghuang Island,with the north channel being the main route from March to August and the south channel being the main route from September to February of the following year.The substances in the bay are transported to the sea through three channels,with the east channel being the main route in summer and the west channel being the main route in winter.(4)When considering only the effect of tidal currents,the distribution of substance concentration in Sanya Bay is significantly affected by the ebb and flow of the tides.The rising tide weakens the transport of conservative substances to the open sea,while the falling tide lowers the concentration of pollutants in the bay.When considering the combined effect of wind and tidal currents,the exchange capacity of substances between the bay and the open sea is enhanced.The addition of wind promotes the transport and diffusion of conservative substances in the bay,leading to a decrease in substance concentration in the bay by about57% to 76% compared to calm conditions.In addition,the wind field regulates the seasonal variation of substance flux from the two channels of Fenghuang Island to Sanya Bay,which corresponds to the seasonal variation of nutrient distribution observed.The role of tides is relatively small.Studies have shown that the Sanya River is a key factor in the spatiotemporal distribution of water environmental factors in Sanya Bay.Monsoons and tides have important effects on the transport of materials into the sea and the exchange between Sanya Bay and the open sea,especially monsoons,which enhance the transport of materials from the bay to the open sea,reduce the concentration of pollutants in the bay,and improve the marine environment.This study provides new data for the study of the ecological and hydrodynamic environment of Sanya Bay,and provides scientific reference for the formulation and improvement of policies for the protection of coral reef ecosystems in Sanya Bay.It helps to improve the level of management and protection,promote the healthy development of coral reef ecosystems in Sanya Bay,and also provides a reference and inspiration for the environmental protection of similar marine areas.