Temporal And Spatial Distribution Of Nutrients And Nutrients Limitation For Phytoplankton In The Changjiang River Estuary

The Yangtze River Estuary severely affected by human activities.In recent decades,the Yangtze River runoff has transported a large number of nutrients into the China east sea,which has caused the eutrophication of the water area to increase,and ecological disasters such as red tides have occurred frequently.Aiming at the problem of the spatial and temporal distribution of nutrients in the waters of the Yangtze River Estuary,this master's thesis has studied the four-season voyages of February,May,August,and November 2014,and the five springs investigations of 2014,2015,2016,2017,and 2018.The survey data of the voyage explored the seasonal and interannual changes of nutrients in the Yangtze River Estuary waters,combined with the environment of salinity(S),dissolved oxygen(DO),temperature(T),suspension(SPM),and chlorophyll-a parameters to further explore the influencing factors of nutrient migration process.This provides basic data for long-term observation of nutrients in the Yangtze River estuary and provides a reference for the study of the mechanism of harmful red tides and other disasters.(1)Seasonal changes of nutrients in the Yangtze River estuaryBased on four investigations conducted in February,May,August and November 2014,results show that NO₃-N,PO4-P and SiO3-Si were mainly affected by the Changjiang terrigenous input,consisting with the variation of Changjiang Diluted Water(CDW).In the east of 122°20?E,water stratification caused by the thermocline and halocline besides winter,and the vertical distribution of nutrients was stratified obviously.In contrast,the water in vertical direction mixed well in winter,as well as nutrients.In spring,the Changjiang terrigenous input supplying high-concentration NO₃-N,the 40?mol/L isoline extended northeastward as far as 123°E,and reaching the water depth of 10m.In autumn,the Changjiang terrigenous input supplying high-concentration SiO3-Si and PO4-P,40?mol/L and 0.6?mol/L isolines extended eastward to 123°E and 123°20?E,and reaching the water depth of 20m and 50m,respectively.The distribution of NO₂-N and NH4-N were complicated,affected by many factors such as bioabsorbable and nitrification,and the seasonal variations were not obvious.The theoretical dilution line was introduced to study the mixing behaviors of NO₃-N,SiO3-Si and PO4-P in the Changjiang River Estuary.The results show that nutrients consumed in spring and summer might be related to the primary production,and PO4-P showed positive deviation with respect to conservative mixing could be connected with the process of SPM releasing.(2)Interannual changes of nutrients and its limitation for phytoplankton in the Yangtze River estuaryUsing the nutrients,chlorophyll-a and hydrological data of the Yangtze River estuary in the past five years in the spring(2014-2018),the following conclusions have been drawn:Changjiang terrigenous input higher concentrations of DIN,DIP,and DSi in the past five years have been significantly reduced from 2014 to 2016,then it increased significantly to 2018,which was negatively related to runoff.In 2014 and 2018,where the runoff was smaller,Changjiang terrigenous input higher concentrations of DSi,DIN,and DIP,corresponding to higher concentrations of nutrients in the Yangtze River estuary.The limitation of phytoplankton by nutrients in the Yangtze River estuary is mainly the P limitation.The P limitation is closely related to Changjiang terrigenous input concentration of DIP.When Changjiang terrigenous input higher concentrations of DIP is low,the P limitation is more likely to occur,and the P limitation station is more concentrated at north of 31.5°N,and east of 123° E.The areas of high concentration of chlorophyll-a correspond to the areas of high N/P ratio and Si/P ratio.This agreement is more obvious in the years with higher chlorophyll-a(2015,2017,and 2018),which indicates that biological absorption consumes DIP in water.This process will cause and exacerbate the degree of P limitation.By studying the relationship between chlorophyll-a and log(DIN/DIP),the following conclusions are obtained:the concentration of chlorophyll-a at a site with log(DIN/DIP)greater than 2.5 is also higher(Chl a>10 ?g/L),and the chlorophyll-a The higher the concentration,the larger the ratio may be.The concentration of DIP in the surface layer of high-value chlorophyll-a sites is low.With the increase of water depth,organic matter is depleted by DO,and the high value of DIP appears in the 20m layer,and the lowest value of DO and its saturation appears accordingly.It can be seen that Yangtze terrigenous input,water dilution and biological absorption jointly drive the nutrients spatial and temporal distribution,and also,nutrients have different spatiotemporal mechanisms.