A Simulation Study Of The Effect Of The Yangtze River Runoff On The Phytoplankton In The Yangtze Estuary

As the largest river in China,Yangtze River discharges plenty of freshwater to the East China Sea annually.The freshwater discharge brings high-concentration nutrients,such as nitrate,silicate and phosphate,from lands to the open sea around Yangtze River Estuary,inducing huge influence on ecological processes over Yangtze River Estuary and East China Sea.Due to the limited and incomplete observational data,numerical simulation is a powerful tool to deepen our understanding of how the freshwater discharge influences the seasonal and interannual variabilities of chlorophyll over Yangtze River Estuary.In this study,using statistical analyses and a physical-biogeochemical coupled model,we analyzed the seasonal and interannual diffusive processes of Yangtze River discharge,and systematically investigated the seasonal and interannual influences of nutrients carried by the discharge on chlorophyll and the El Nino-Southern Oscillation impact on chlorophyll over Yangtze River Estuary.Our results are as follows:(1)We firstly established a physical-biogeochemical coupled model(ROMS-Co Si NE),which is composed of the regional ocean model system(ROMS)and the Co Si NE biochemical model,and the observational data are used to verify the capability of simulation.From the verification results,the simulated sea surface temperature,sea surface salinity and sea surface nutrients are highly consistent with the observations.(2)Using SSS data from Soil Moisture Active Passive(SMAP)satellite,we analyzed the seasonal and interannual variabilities of SSS and pathways of freshwater diffusion for the period 2015-2019,and compared with the ROMS-Co Si NE simulation.Under the circumstance of southerly wind induced by summer monsoon,the Yangtze River freshwater diffuses northeastward.During strong discharge years,the low salinity region affected by the freshwater discharge can reach Jeju island of the Korea while the freshwater diffuses southward in the winter season.(3)In ROMS-Co Si NE model,we modified the growth rate function for phytoplankton to simulate the seasonal and interannual variabilities of phytoplankton.The results show that the new function by adding limitation of temperature for growth simulated the phytoplankton well,which is verified by the data from Sea-viewing Wide Field-of-view Sensor(Sea Wi FS).(4)In terms of seasonal change,the chlorophyll concentration shows two peaks in May and July,respectively.In terms of interannual variability,during strong years of Yangtze River discharge(larger than 48000 m~3/s),it shows positive chlorophyll anomalies over Yangtze River Estuary.During weak years,the chlorophyll anomalies over Yangtze River Estuary are negative.When not considering the nutrients in the model,the chlorophyll concentration is about 0-0.5 mg/m~3,much smaller than the chlorophyll concentration of CCI(Climate Change Initiative),indicating that the freshwater discharge with nutrients is an crucial factor for the chlorophyll concentration over Yangtze River.(5)We find that the April-August averaged chlorophyll concentration is highly correlated with the previous December-February averaged Ni(?)o3.4 index,which means that the El Ni(?)o-Southern Oscillation(ENSO)is a good precursor for chlorophyll change.Through the study of its physical mechanism,the temperature of the sections and discharge may be the possible physical mechanisms for the chlorophyll change.To further verify this,we conducted a sensitivity experiment by only including the seasonal variability of discharge while removing its interannual variability.The sensitivity run shows an opposite result,while the observation data shows the positive chlorophyll anomalies over Yangtze River Estuary between positive and negative ENSO phases.This indicates that the interannual variability of discharge is crucial for the chlorophyll change.When we just considered the interannual variability of discharge without nutrients,the magnitude of chlorophyll anomalies are much smaller than the observation between positive and negative ENSO phases.