Mechanism Of Phytoplankton Bloom In Northern South China Sea

Phytoplankton plays an important role in marine food chain,and their rapid growth causes blooms.Algal blooms affect fisheries,and harmful algal blooms are disasters closely related to human beings.Therefore,it is of great significance to study the related mechanism of algal blooms for the development of marine economy and the protection of human living environment.Previous studies have shown that blooms are often found in the northern South China Sea in winter,and the main formation mechanisms include mesoscale eddies and upwelling.In recent years,many scholars believe that the restratification process caused by baroclinic instability also plays an important role in the sea surface bloom.However,the effect and contribution of baroclinic instability on winter algal blooms in the northern South China Sea are still unclearTaking the northern South China Sea as the study area,this paper based on BioArgo data and model results with high resolution to analyze the mechanism of winter bloom in the northern South China Sea,especially the influence of baroclinic instability process on bloom.A high-resolution physical-ecological model is established based on ROMS.The model results are compared with remote sensing data,Bio-Argo data and reanalysis data,and it is found that the model can get good simulation results in terms of SST and chlorophyll distribution.Based on the data above,the spatial and temporal correlation between chlorophyll concentration and dynamic factors such as mixing layer depth and wind speed was discussed in this paper.Dynamic analysis including nutrient balance diagnosis and vertical velocity diagnosis of vorticity equation were carried out.The analysis results showed that:On the seasonal scale,both the measured data and the model results show that there will be high chlorophyll concentration in the surface of the northern South China Sea in winter,which is related to the deepening of the mixed layer in winter.During winter,the increase of wind speed and the negative surface net heat flux lead to deepening of the mixed layer.Partial correlation analysis between the mixed layer depth,wind speed and surface net heat flux shows that wind speed contributes more to the deepening of the mixed layer in the northern South China Sea in winter.The deepening of mixed layer causes the vertical transport of nutrient improved and promote the photosynthesis of phytoplankton in the surface layer.Therefore,the vertical distribution of chlorophyll is extremely different with which in summer.At synoptic scale,sub-mesoscale baroclinic instability contributes to the formation of high chlorophyll in the surface layer.Energy analysis results show that when an obvious front is formed,the background potential energy in the surface and near surface will be transformed into eddy kinetic energy,accompanied by strong secondary circulation,which will cause the sloping isopycnal surface collapsed,and the stratification of the water strengthened.The process of re-stratification to some extent restricts phytoplankton and nutrients in the surface layer and enables them to photosynthesize under sufficient light conditions,resulting in the increase of chlorophyll in the sea surface.The sensitivity experiments with a nudging of weaker horizontal density gradient show that baroclinic instability is not easy to happen while the front is weaker,and the sea surface chlorophyll concentration decreases.It can be concluded that sub-mesoscale baroclinic instability can promote the primary productivity of sea surface in the northern South China Sea in winter.This study further improves the understanding of the mechanism of bloom in the northern South China Sea in winter,and provides a reference for the establishment of a high-resolution ecological prediction model for the offshore China in the future.However,in this study,only the factors affecting surface chlorophyll concentration in the northern South China Sea in winter were qualitatively analyzed,and more work is needed to quantitatively explain the contribution of each influencing factor to blooms.